Yet perhaps it is in sequences that this artist best displays himself. He loves to look at either side of a thing: as, for instance, when he shows us both sides of the wall—“Grace Inextinguishable” on the one side, with the devil vainly pouring buckets on the flame, and “The Oil of Grace” on the other, where the Holy Spirit, vessel in hand, still secretly supplies the fire. He loves, also, to show us the same event twice over, and to repeat his instantaneous photographs at the interval of but a moment. So we have, first, the whole troop of pilgrims coming up to Valiant, and Great-heart to the front, spear in hand and parleying; and next, the same cross-roads, from a more distant view, the convoy now scattered and looking safely and curiously on, and Valiant handing over for inspection his “right Jerusalem blade.” It is true that this designer has no great care after consistency: Apollyon’s spear is laid by, his quiver of darts will disappear, whenever they might hinder the designer’s freedom; and the fiend’s tail is blobbed or forked at his good pleasure. But this is not unsuitable to the illustration of the fervent Bunyan, breathing hurry and momentary inspiration. He, with his hot purpose, hunting sinners with a lasso, shall himself forget the things that he has written yesterday. He shall first slay Heedless in the Valley of the Shadow, and then take leave of him talking in his sleep, as if nothing had happened, in an arbour on the Enchanted Ground. And again, in his rhymed prologue, he shall assign some of the glory of the siege of Doubting Castle to his favourite Valiant-for-the-Truth, who did not meet with the besiegers till long after, at that dangerous corner by Deadman’s Lane. And, with all inconsistencies and freedoms, there is a power shown in these sequences of cuts: a power of joining on one action or one humour to another; a power of following out the moods, even of the dismal subterhuman fiends engendered by the artist’s fancy; a power of sustained continuous realisation, step by step, in nature’s order, that can tell a story, in all its ins and outs, its pauses and surprises, fully and figuratively, like the art of words.

One such sequence is the fight of Christian and Apollyon—six cuts, weird and fiery, like the text. The pilgrim is throughout a pale and stockish figure; but the devil covers a multitude of defects. There is no better devil of the conventional order than our artist’s Apollyon, with his mane, his wings, his bestial legs, his changing and terrifying expression, his infernal energy to slay. In cut the first you see him afar off, still obscure in form, but already formidable in suggestion. Cut the second, “The Fiend in Discourse,” represents him, not reasoning, railing rather, shaking his spear at the pilgrim, his shoulder advanced, his tail writhing in the air, his foot ready for a spring, while Christian stands back a little, timidly defensive. The third illustrates these magnificent words: “Then Apollyon straddled quite over the whole breadth of the way, and said, I am void of fear in this matter: prepare thyself to die; for I swear by my infernal den that thou shalt go no farther: here will I spill thy soul! And with that he threw a flaming dart at his breast.” In the cut he throws a dart with either hand, belching pointed flames out of his mouth, spreading his broad vans, and straddling the while across the path, as only a fiend can straddle who has just sworn by his infernal den. The defence will not be long against such vice, such flames, such red-hot nether energy. And in the fourth cut, to be sure, he has leaped bodily upon his victim, sped by foot and pinion, and roaring as he leaps. The fifth shows the climacteric of the battle; Christian has reached nimbly out and got his sword, and dealt that deadly home-thrust, the fiend still stretched upon him, but “giving back, as one that had received his mortal wound.” The raised head, the bellowing mouth, the paw clapped upon the sword, the one wing relaxed in agony, all realise vividly these words of the text. In the sixth and last, the trivial armed figure of the pilgrim is seen kneeling with clasped hands on the betrodden scene of contest and among the shivers of the darts; while just at the margin the hinder quarters and the tail of Apollyon are whisking off, indignant and discomfited.

In one point only do these pictures seem to be unworthy of the text, and that point is one rather of the difference of arts than the difference of artists. Throughout his best and worst, in his highest and most divine imaginations as in the narrowest sallies of his sectarianism, the human-hearted piety of Bunyan touches and ennobles, convinces, accuses the reader. Through no art beside the art of words can the kindness of a man’s affections be expressed. In the cuts you shall find faithfully parodied the quaintness and the power, the triviality and the surprising freshness of the author’s fancy; there you shall find him outstripped in ready symbolism and the art of bringing things essentially invisible before the eyes: but to feel the contact of essential goodness, to be made in love with piety, the book must be read and not the prints examined.

Farewell should not be taken with a grudge; nor can I dismiss in any other words than those of gratitude a series of pictures which have, to one at least, been the visible embodiment of Bunyan from childhood up, and shown him, through all his years, Great-heart lungeing at Giant Maul, and Apollyon breathing fire at Christian, and every turn and town along the road to the Celestial City, and that bright place itself, seen as to a stave of music, shining afar off upon the hill-top, the candle of the world.

(Facsimile of original Title-page)

AN APPEAL TO THE CLERGY OF
THE CHURCH OF SCOTLAND

WITH A NOTE FOR THE LAITY

Gentlemen,—The position of the Church of Scotland is now one of considerable difficulty; not only the credit of the Church, not only the credit of Christianity, but to some extent also that of the national character, is at stake. You have just gained a great victory, in spite of an opposition neither very logical nor very generous; you have succeeded in effecting, by quiet constitutional processes, a great reform which brings your Church somewhat nearer in character to what is required by your Dissenting brethren. It remains to be seen whether you can prove yourselves as generous as you have been wise and patient. And the position, as I say, is one of difficulty. Many, doubtless, left the Church for a reason which is now removed; many have joined other sects who would rather have joined themselves with you, had you been then as you now are; and for these you are bound to render as easy as may be the way of reconciliation, and show, by some notable action, the reality of your own desire for Peace. But I am not unaware that there are others, and those possibly a majority, who hold very different opinions—who regard the old quarrel as still competent, or have found some new reason for dissent; and from these the Church, if she makes such an advance as she ought to make, in all loyalty and charity, may chance to meet that most sensible of insults—ridicule, in return for an honest offer of reconciliation. I am not unaware, also, that there is yet another ground of difficulty; and that those even who would be most ready to hold the cause of offence as now removed will find it hard to forget the past—will continue to think themselves unjustly used—will not be willing to come back, as though they were repentant offenders, among those who delayed the reform and quietly enjoyed their benefices, while they bore the heat and burthen of the day in a voluntary exile for the Truth’s sake.

In view of so many elements of difficulty, no intelligent person can be free from apprehension for the result; and you, gentlemen, may be perhaps more ready now to receive advice, to hear and weigh the opinion of one who is free, because he writes without name, than you would be at any juncture less critical. There is now a hope, at least, that some term may be put to our more clamorous dissensions. Those who are at all open to a feeling of national disgrace look eagerly forward to such a possibility; they have been witnesses already too long to the strife that has divided this small corner of Christendom; and they cannot remember without shame that there has been as much noise, as much recrimination, as much severance of friends, about mere logical abstractions in our remote island, as would have sufficed for the great dogmatic battles of the Continent. It would be difficult to exaggerate the pity that fills the heart at such a reflection; at the thought of how this neck of barren hills between two inclement seaways has echoed for three centuries with the uproar of sectarian battle; of how the east wind has carried out the sound of our shrill disputations into the desolate Atlantic, and the west wind has borne it over the German Ocean, as though it would make all Europe privy to how well we Scottish brethren abide together in unity. It is not a bright page in the annals of a small country: it is not a pleasant commentary on the Christianity that we profess; there is something in it pitiful, as I have said, for the pitiful man, but bitterly humorous for others. How much time we have lost, how much of the precious energy and patience of good men we have exhausted, on these trivial quarrels, it would be nauseous to consider; we know too much already when we know the facts in block; we know enough to make us hide our heads for shame, and grasp gladly at any present humiliation, if it would ensure a little more quiet, a little more charity, a little more brotherly love in the distant future.

And it is with this before your eyes that, as I feel certain, you are now addressing yourselves to the consideration of this important crisis. It is with a sense of the blackness of this discredit upon the national character and national Christianity that not you alone but many of other Churches are now setting themselves to square their future course with the exigencies of the new position of sects; and it is with you that the responsibility remains. The obligation lies ever on the victor; and just so surely as you have succeeded in the face of captious opposition in carrying forth the substance of a reform of which others had despaired, just as surely does it lie upon you as a duty to take such steps as shall make that reform available, not to you only, but to all your brethren who will consent to profit by it; not only to all the clergy, but to the cause of decency and peace, throughout your native land. It is earnestly hoped that you may show yourselves worthy of a great opportunity, and do more for the public minds by the example of one act of generosity and humility than you could do by an infinite series of sermons.

Without doubt, it is your intention, on the earliest public opportunity, to make some advance. Without doubt, it is your purpose to improve the advantage you have gained, and to press upon those who quitted your communion some thirty years ago your great desire to be once more united to them. This, at least, will find a place in the most unfriendly programme you can entertain; and if there are any in the Free Church (as I doubt not there are some) who seceded, not so much from any dislike to the just supremacy of the law, as from a belief that the law in these ecclesiastical matters was applied unjustly, I know well that you will be most eager to receive them back again; I know well that you will not let any petty vanity, any scruple of worldly dignity, stand between them and their honourable return. If, therefore, there were no more to be done than to display to these voluntary exiles the deep sense of your respect for their position, this appeal would be unnecessary, and you might be left to the guidance of your own good feeling.

But it seems to me that there is need of something more; it seems to me, and I think that it will seem so to you also, that you must go even further if you would be equal to the importance of the situation. If there are any among the Dissenters whose consciences are so far satisfied with the provisions of the recent Act that they could now return to your communion, to such, it must not be forgotten, you stand in a position of great delicacy. The conduct of these men you have so far justified; you have tacitly admitted that there was some ground for dissatisfaction with the former condition of the Church; and though you may still judge those to have been over-scrupulous who were moved by this imperfection to secede, instead of waiting patiently with you until it could be remedied by peaceful means, you must not forget that it is the strong stomach, according to St. Paul, that is to consider the weak, and should come forward to meet these brethren with something better than compliments upon your lips. Observe, I speak only of those who would now see their way back to your communion with a clear conscience; it is their conduct, and their conduct alone, that you have justified, and therefore it is only for them that your special generosity is here solicited. But towards them, if there are any such, your countrymen would desire to see you behave with all consideration. I do not pretend to lay before you any definite scheme of action; I wish only to let you understand what thoughts are busy in the heads of some outside your councils, so that you may take this also into consideration when you come to decide. And this, roughly, is how it appears to these: These good men have exposed themselves to the chance of hardship for the sake of their scruples, whilst you being of a stronger stomach, continued to enjoy the security of national endowments. Some of you occupy the very livings which they resigned for conscience’ sake. To others preferment has fallen which would have fallen to them had they been still eligible. If, then, any of them are now content to return, you are bound, if not in justice, then in honour, to do all that you can to testify your respect for brave conviction, and to repair to them such losses as they may have suffered, whether for their first secession or their second. You owe a special duty, not only to the courage that left the Church, but to the wisdom and moderation that now returns to it. And your sense of this duty will find a vent not only in word but in action. You will facilitate their return not only by considerate and brotherly language but by pecuniary aid; you will seek, by some new endowment scheme, to preserve for them their ecclesiastical status. That they have no claim will be their strongest claim on your consideration. Many of you, if not all, will set apart some share out of your slender livings for their assistance and support: you will give them what you can afford; and you will say to them, as you do so, what I dare say to you, that what you give is theirs—not only in honour but in justice.

For you know that the justice which should rule the dealings of Christians, how much more of Christian ministers, is not as the justice of courts of law or equity; and those who profess the morality of Jesus Christ have abjured, in that profession, all that can be urged by policy or worldly prudence. From them we can accept no half-hearted and calculating generosity; they must make haste to be liberal; they must catch with eagerness at all opportunities of service, and the mere whisper of an obligation should be to them more potent than the decree of a court to others who make profession of a less stringent code. And remember that it lies with you to show to the world that Christianity is something more than a verbal system. In the lapse of generations men grow weary of unsupported precept. They may wait long, and keep long in memory the bright doings of former days, but they will weary at the last; they will begin to trouble you for your credentials; if you cannot give them miracles, they will demand virtue; if you cannot heal the sick, they will call upon you for some practice of the Christian ethics. Thus people will knock often at a door if only it be opened to them now and again; but if the door remains closed too long, they will judge the house uninhabited and go elsewhere. And thus it is that a season of persecution, constantly endured, revives the fainting confidence of the people, and some centuries of prosperity may prepare a Church for ruin. You have here at your hand an opportunity to do more for the credit of your Christianity than ever you could do by visions, miracles, or prophecies. A sacrifice such as this would be better worth, as I said before, than many sermons; and there is a disposition in mankind that would ennoble it beyond much that is more ostentatious; for men, whether lay or clerical, suffer better the flame of the stake than a daily inconvenience or a pointed sneer, and will not readily be martyred without some external circumstance and a concourse looking on. And you need not fear that your virtue will be thrown away; the people of Scotland will be quick to understand, in default of visible fire and halter, that you have done a brave action for Christianity and the national weal; and if they are spared in the future any of the present ignoble jealousy of sect against sect, they will not forget that to that end you gave of your household comfort and stinted your children. Even if you fail—ay, and even if there were not found one to profit by your invitation—your virtue would still have its own reward. Your predecessors gave their lives for ends not always the most Christian; they were tempted, and slain with the sword; they wandered in deserts and in mountains, in caves and in dens of the earth. But your action will not be less illustrious; what you may have to suffer may be a small thing if the world will, but it will have been suffered for the cause of peace and brotherly love.

I have said that the people of Scotland will be quick to appreciate what you do. You know well that they will be quick also to follow your example. But the sign should come from you. It is more seemly that you should lead than follow in this matter. Your predecessors gave the word from their free pulpits which was to brace men for sectarian strife: it would be a pleasant sequel if the word came from you that was to bid them bury all jealousy, and forget the ugly and contentious past in a good hope of peace to come.

What is said in these few pages may be objected to as vague; it is no more vague than the position seemed to me to demand. Each man must judge for himself what it behoves him to do at this juncture, and the whole Church for herself. All that is intended in this appeal is to begin, in a tone of dignity and disinterestedness, the consideration of the question; for when such matters are much pulled about in public prints, and have been often discussed from many different, and not always from very high, points of view, there is ever a tendency that the decision of the parties may contract some taint of meanness from the spirit of their critics. All that is desired is to press upon you, as ministers of the Church of Scotland, some sense of the high expectation with which your country looks to you at this time; and how many reasons there are that you should show an example of signal disinterestedness and zeal in the encouragement that you give to returning brethren. For, first, it lies with you to clear the Church from the discredit of our miserable contentions; and surely you can never have a fairer opportunity to improve her claim to the style of a peacemaker. Again, it lies with you, as I have said, to take the first step, and prove your own true ardour for an honourable union; and how else are you to prove it? It lies with you, moreover, to justify in the eyes of the world the time you have been enjoying your benefices, while these others have voluntarily shut themselves out from all participation in their convenience; and how else are you to convince the world that there was not something of selfishness in your motives? It lies with you, lastly, to keep your example unspotted before your congregations; and I do not know how better you are to do that.

It is never a thankful office to offer advice; and advice is the more unpalatable, not only from the difficulty of the service recommended, but often from its very obviousness. We are fired with anger against those who make themselves the spokesmen of plain obligations; for they seem to insult us as they advise. In the present case I should have feared to waken some such feeling, had it not been that I was addressing myself to a body of special men on a very special occasion. I know too much of the history of ideas to imagine that the sentiments advocated in this appeal are peculiar to me and a few others. I am confident that your own minds are already busy with similar reflections. But I know at the same time how difficult it is for one man to speak to another in such a matter; how he is withheld by all manner of personal considerations, and dare not propose what he has nearest his heart, because the other has a larger family or a smaller stipend, or is older, more venerable, and more conscientious than himself; and it is in view of this that I have determined to profit by the freedom of an anonymous writer, and give utterance to what many of you would have uttered already, had they been (as I am) apart from the battle. It is easy to be virtuous when one’s own convenience is not affected; and it is no shame to any man to follow the advice of an outsider who owns that, while he sees which is the better part, he might not have the courage to profit himself by this opinion.

[Note for the Laity]

The foregoing pages have been in type since the beginning of last September. I have been advised to give them to the public; and it is only necessary to add that nothing of all that has taken place since they were written has made me modify an opinion or so much as change a word. The question is not one that can be altered by circumstances.

I need not tell the laity that with them this matter ultimately rests. Whether we regard it as a question of mere expense or as a question of good feeling against ill feeling, the solution must come from the Church members. The lay purse is the long one; and if the lay opinion does not speak from so high a place, it speaks all the week through and with innumerable voices. Trumpets and captains are all very well in their way; but if the trumpets were ever so clear, and the captains as bold as lions, it is still the army that must take the fort.

The laymen of the Church have here a question before them, on the answering of which, as I still think, many others attend. If the Established Church could throw off its lethargy, and give the Dissenters some speaking token of its zeal for union, I still think that union, to some extent, would be the result. There is a motion tabled (as I suppose all know) for the next meeting of the General Assembly; but something more than motions must be tabled, and something more must be given than votes. It lies practically with the laymen, by a new endowment scheme, to put the Church right with the world in two ways, so that those who left it more than thirty years ago, and who may now be willing to return, shall lose neither in money nor in ecclesiastical status. At the outside, what will they have to do? They will have to do for (say) ten years what the laymen of the Free Church have done cheerfully ever since 1843.

February 12th 1875.

THE CHARITY BAZAAR

THE LIGHT-KEEPER

ON A NEW FORM OF INTERMITTENT
LIGHT FOR LIGHTHOUSES

ON THE THERMAL INFLUENCE OF
FORESTS

THE CHARITY BAZAAR

AN ALLEGORICAL DIALOGUE

PERSONS OF THE DIALOGUE

The Tout, in an allegorical costume, holding a silver trumpet in his right hand, is discovered on the steps in front of the Bazaar. He sounds a preliminary flourish.

The Tout.—Ladies and Gentlemen, I have the honour to announce a sale of many interesting, beautiful, rare, quaint, comical, and necessary articles. Here you will find objects of taste, such as Babies’ Shoes, Children’s Petticoats, and Shetland Wool Cravats; objects of general usefulness, such as Tea-cosies, Bangles, Brahmin Beads, and Madras Baskets; and objects of imperious necessity, such as Pen-wipers, Indian Figures carefully repaired with glue, and Sealed Envelopes, containing a surprise. And all this is not to be sold by your common Shopkeepers, intent on small and legitimate profits, but by Ladies and Gentlemen, who would as soon think of picking your pocket of a cotton handkerchief as of selling a single one of these many interesting, beautiful, rare, quaint, comical, and necessary articles at less than twice its market value. (He sounds another flourish.)

The Wife.—This seems a very fair-spoken young man.

The Ingenuous Public (addressing the Tout).—Sir, I am a man of simple and untutored mind; but I apprehend that this sale, of which you give us so glowing a description, is neither more nor less than a Charity Bazaar?

The Tout.—Sir, your penetration has not deceived you.

The Ingenuous Public.—Into which you seek to entice unwary passengers?

The Tout.—Such is my office.

The Ingenuous Public.—But is not a Charity Bazaar, Sir, a place where, for ulterior purposes, amateur goods are sold at a price above their market value?

The Tout.—I perceive you are no novice. Let us sit down, all three, upon the doorsteps, and reason this matter at length. The position is a little conspicuous, but airy and convenient.

(The Tout seats himself on the second step, the Ingenuous Public and his Wife to right and left of him, one step below.)

The Tout.—Shopping is one of the dearest pleasures of the human heart.

The Wife.—Indeed, Sir, and that it is.

The Tout.—The choice of articles, apart from their usefulness, is an appetising occupation, and to exchange bald, uniform shillings for a fine big, figurative knick-knack, such as a windmill, a gross of green spectacles, or a cocked hat, gives us a direct and emphatic sense of gain. We have had many shillings before, as good as these; but this is the first time we have possessed a windmill. Upon these principles of human nature, Sir, is based the theory of the Charity Bazaar. People were doubtless charitably disposed. The problem was to make the exercise of charity entertaining in itself—you follow me, Madam?—and in the Charity Bazaar a satisfactory solution was attained. The act of giving away money for charitable purposes is, by this admirable invention, transformed into an amusement, and puts on the externals of profitable commerce. You play at shopping a while; and in order to keep up the illusion, sham goods do actually change hands. Thus, under the similitude of a game, I have seen children confronted with the horrors of arithmetic, and even taught to gargle.

The Ingenuous Public.—You expound this subject very magisterially, Sir. But tell me, would it not be possible to carry this element of play still further? and after I had remained a proper time in the Bazaar, and negotiated a sufficient number of sham bargains, would it not be possible to return me my money in the hall?

The Tout.—I question whether that would not impair the humour of the situation. And besides, my dear Sir, the pith of the whole device is to take that money from you.

The Ingenuous Public.—True. But at least the Bazaar might take back the tea-cosies and pen-wipers.

The Tout.—I have no doubt, if you were to ask it handsomely, that you would be so far accommodated. Still it is out of the theory. The sham goods, for which, believe me, I readily understand your disaffection—the sham goods are well adapted for their purpose. Your lady wife will lay these tea-cosies and pen-wipers aside in a safe place, until she is asked to contribute to another Charity Bazaar. There the tea-cosies and pen-wipers will be once more charitably sold. The new purchasers, in their turn, will accurately imitate the dispositions of your lady wife. In short, Sir, the whole affair is a cycle of operations. The tea-cosies and pen-wipers are merely counters; they come off and on again like a stage army; and year after year people pretend to buy and pretend to sell them, with a vivacity that seems to indicate a talent for the stage. But in the course of these illusory manœuvres, a great deal of money is given in charity, and that in a picturesque, bustling, and agreeable manner. If you have to travel somewhere on business, you would choose the prettiest route, and desire pleasant companions by the way. And why not show the same spirit in giving alms?

The Ingenuous Public.—Sir, I am profoundly indebted to you for all you have said. I am, Sir, your absolute convert.

The Wife.—Let us lose no time, but enter the Charity Bazaar.

The Ingenuous Public.—Yes; let us enter the Charity Bazaar.

Both (singing).—Let us enter, let us enter, let us enter, Let us enter the Charity Bazaar!

(An interval is supposed to elapse. The Ingenuous Public and his Wife are discovered issuing from the Charity Bazaar.)

The Wife.—How fortunate you should have brought your cheque-book!

The Ingenuous Public.—Well, fortunate in a sense. (Addressing the Tout.)—Sir, I shall send a van in the course of the afternoon for the little articles I have purchased. I shall not say good-bye; because I shall probably take a lift in the front seat, not from any solicitude, believe me, about the little articles, but as the last opportunity I may have for some time of enjoying the costly entertainment of a drive.

The Scene Closes

THE LIGHT-KEEPER

I

II

ON A NEW FORM OF INTERMITTENT
LIGHT FOR LIGHTHOUSES44

The necessity for marked characteristics in coast illumination increases with the number of lights. The late Mr. Robert Stevenson, my grandfather, contributed two distinctions, which he called respectively the intermittent and the flashing light. It is only to the former of these that I have to refer in the present paper. The intermittent light was first introduced at Tarbetness in 1830, and is already in use at eight stations on the coasts of the United Kingdom. As constructed originally, it was an arrangement by which a fixed light was alternately eclipsed and revealed. These recurrent occultations and revelations produce an effect totally different from that of the revolving light, which comes gradually into its full strength, and as gradually fades away. The changes in the intermittent, on the other hand, are immediate; a certain duration of darkness is followed at once and without the least gradation by a certain period of light. The arrangement employed by my grandfather to effect this object consisted of two opaque cylindric shades or extinguishers, one of which descended from the roof, while the other ascended from below to meet it, at a fixed interval. The light was thus entirely intercepted.

At a later period, at the harbour light of Troon, Mr. Wilson, C.E., produced an intermittent light by the use of gas, which leaves little to be desired, and which is still in use at Troon harbour. By a simple mechanical contrivance, the gas jet was suddenly lowered to the point of extinction, and, after a set period, as suddenly raised again. The chief superiority of this form of intermittent light is economy in the consumption of the gas. In the original design, of course, the oil continues uselessly to illuminate the interior of the screens during the period of occultation.

Mr. Wilson’s arrangement has been lately resuscitated by Mr. Wigham of Dublin, in connection with his new gas-burner.

Gas, however, is inapplicable to many situations; and it has occurred to me that the desired result might be effected with strict economy with oil lights, in the following manner:—

In Fig. 1, AAA represents in plan an ordinary Fresnel’s dioptric fixed light apparatus, and BB’ a hemispherical mirror (either metallic or dioptric on my father’s principle) which is made to revolve with uniform speed about the burner. This mirror, it is obvious, intercepts the rays of one hemisphere, and, returning them through the flame (less loss by absorption, etc.), spreads them equally over the other. In this way 180° of light pass regularly the eye of the seaman; and are followed at once by 180° of darkness. As the hemispherical mirror begins to open, the observer receives the full light, since the whole lit hemisphere is illuminated with strict equality; and as it closes again, he passes into darkness.

Other characteristics can be produced by different modifications of the above. In Fig. 2 the original hemispherical mirror is shown broken up into three different sectors, BB´, CC´, and DD´; so that with the same velocity of revolution the periods of light and darkness will be produced in quicker succession. In this figure (Fig. 2) the three sectors have been shown as subtending equal angles, but if one of them were increased in size and the other two diminished (as in Fig. 3), we should have one long steady illumination and two short flashes at each revolution. Again, the number of sectors may be increased; and by varying both their number and their relative size, a number of additional characteristics are attainable.

Colour may also be introduced as a means of distinction. Coloured glass may be set in the alternate spaces; but it is necessary to remark that these coloured sectors will be inferior in power to those which remain white. This objection is, however, obviated to a large extent (especially where the dioptric spherical mirror is used) by such an arrangement as is shown in Fig. 4; where the two sectors, WW, are left unassisted, while the two with the red screens are reinforced respectively by the two sectors of mirror, MM.

Another mode of holophotally producing the intermittent light has been suggested by my father, and is shown in Fig. 5. It consists of alternate and opposite sectors of dioptric spherical mirror, MM, and of Fresnel’s fixed light apparatus, AA. By the revolution of this composite frame about the burner, the same immediate alternation of light and darkness is produced, the first when the front of the fixed panel, and the second when the back of the mirror, is presented to the eye of the sailor.

One advantage of the method that I propose is this, that while we are able to produce a plain intermittent light; an intermittent light of variable period, ranging from a brief flash to a steady illumination of half the revolution; and finally, a light combining the immediate occultation of the intermittent with combination and change of colour, we can yet preserve comparative lightness in the revolving parts, and consequent economy in the driving machinery. It must, however, be noticed, that none of these last methods are applicable to cases where more than one radiant is employed: for these cases, either my grandfather’s or Mr. Wilson’s contrivance must be resorted to.

1871.

ON THE THERMAL INFLUENCE OF FORESTS45

The opportunity of an experiment on a comparatively large scale, and under conditions of comparative isolation, can occur but rarely in such a science as Meteorology. Hence Mr. Milne Home’s proposal for the plantation of Malta seemed to offer an exceptional opportunity for progress. Many of the conditions are favourable to the simplicity of the result; and it seemed natural that, if a searching and systematic series of observations were to be immediately set afoot, and continued during the course of the plantation and the growth of the wood, some light would be thrown on the still doubtful question of the climatic influence of forests.

Mr. Milne Home expects, as I gather, a threefold result:—1st, an increased and better regulated supply of available water; 2nd, an increased rainfall; and, 3rd, a more equable climate, with more temperate summer heat and winter cold.46 As to the first of these expectations, I suppose there can be no doubt that it is justified by facts; but it may not be unnecessary to guard against any confusion of the first with the second. Not only does the presence of growing timber increase and regulate the supply of running and spring water independently of any change in the amount of rainfall, but as Boussingault found at Marmato,47 denudation of forest is sufficient to decrease that supply, even when the rainfall has increased instead of diminished in amount. The second and third effects stand apart, therefore, from any question as to the utility of Mr. Milne Home’s important proposal; they are both, perhaps, worthy of discussion at the present time, but I wish to confine myself in the present paper to the examination of the third alone.

A wood, then, may be regarded either as a superficies or as a solid; that is, either as a part of the earth’s surface slightly elevated above the rest, or as a diffused and heterogeneous body displacing a certain portion of free and mobile atmosphere. It is primarily in the first character that it attracts our attention, as a radiating and absorbing surface, exposed to the sun and the currents of the air; such that, if we imagine a plateau of meadow-land or bare earth raised to the mean level of the forest’s exposed leaf-surface, we shall have an agent entirely similar in kind, although perhaps widely differing in the amount of action. Now, by comparing a tract of wood with such a plateau as we have just supposed, we shall arrive at a clear idea of the specialities of the former. In the first place, then, the mass of foliage may be expected to increase the radiating power of each tree. The upper leaves radiate freely towards the stars and the cold inter-stellar spaces, while the lower ones radiate to those above and receive less heat in return; consequently, during the absence of the sun, each tree cools gradually downward from top to bottom. Hence we must take into account not merely the area of leaf-surface actually exposed to the sky, but, to a greater or less extent, the surface of every leaf in the whole tree or the whole wood. This is evidently a point in which the action of the forest may be expected to differ from that of the meadow or naked earth; for though, of course, inferior strata tend to a certain extent to follow somewhat the same course as the mass of inferior leaves, they do so to a less degree—conduction, and the conduction of a very slow conductor, being substituted for radiation.

We come next, however, to a second point of difference. In the case of the meadow, the chilled air continues to lie upon the surface, the grass, as Humboldt says, remaining all night submerged in the stratum of lowest temperature; while in the case of trees, the coldest air is continually passing down to the space underneath the boughs, or what we may perhaps term the crypt of the forest. Here it is that the consideration of any piece of woodland conceived as a solid comes naturally in; for this solid contains a portion of the atmosphere, partially cut off from the rest, more or less excluded from the influence of wind, and lying upon a soil that is screened all day from isolation by the impending mass of foliage. In this way (and chiefly, I think, from the exclusion of winds), we have underneath the radiating leaf-surface a stratum of comparatively stagnant air, protected from many sudden variations of temperature, and tending only slowly to bring itself into equilibrium with the more general changes that take place in the free atmosphere.

Over and above what has been mentioned, thermal effects have been attributed to the vital activity of the leaves in the transudation of water, and even to the respiration and circulation of living wood. The whole actual amount of thermal influence, however, is so small that I may rest satisfied with mere mention. If these actions have any effect at all, it must be practically insensible; and the others that I have already stated are not only sufficient validly to account for all the observed differences, but would lead naturally to the expectation of differences very much larger and better marked. To these observations I proceed at once. Experience has been acquired upon the following three points:—1, The relation between the temperature of the trunk of a tree and the temperature of the surrounding atmosphere; 2, The relation between the temperature of the air under a wood and the temperature of the air outside; and, 3, The relation between the temperature of the air above a wood and the temperature of the air above cleared land.

As to the first question, there are several independent series of observations; and I may remark in passing, what applies to all, that allowance must be made throughout for some factor of specific heat. The results were as follows:—The seasonal and monthly means in the tree and in the air were not sensibly different. The variations in the tree, in M. Becquerel’s own observations, appear as considerably less than a fourth of those in the atmosphere, and he has calculated, from observations made at Geneva between 1796 and 1798, that the variations in the tree were less than a fifth of those in the air; but the tree in this case, besides being of a different species, was seven or eight inches thicker than the one experimented on by himself.48 The variations in the tree, therefore, are always less than those in the air, the ratio between the two depending apparently on the thickness of the tree in question and the rapidity with which the variations followed upon one another. The times of the maxima, moreover, were widely different: in the air, the maximum occurs at 2 P.M. in winter, and at 3 P.M. in summer; in the tree, it occurs in winter at 6 P.M., and in summer between 10 and 11 P.M. At nine in the morning in the month of June, the temperatures of the tree and of the air had come to an equilibrium. A similar difference of progression is visible in the means, which differ most in spring and autumn, and tend to equalise themselves in winter and in summer. But it appears most strikingly in the case of variations somewhat longer in period than the daily ranges. The following temperatures occurred during M. Becquerel’s observations in the Jardin des Plantes:—

A moment’s comparison of the two columns will make the principle apparent. The temperature of the air falls nearly fifteen degrees in five days; the temperature of the tree, sluggishly following, falls in the same time less than four degrees. Between the 19th and the 20th the temperature of the air has changed its direction of motion, and risen nearly a degree; but the temperature of the tree persists in its former course, and continues to fall nearly three degrees farther. On the 21st there comes a sudden increase of heat, a sudden thaw; the temperature of the air rises twenty-five and a half degrees; the change at last reaches the tree, but only raises its temperature by less than three degrees; and even two days afterwards, when the air is already twelve degrees above freezing point, the tree is still half a degree below it. Take, again, the following case:—

The same order reappears. From the 13th to the 19th the temperature of the air steadily falls, while the temperature of the tree continues apparently to follow the course of previous variations, and does not really begin to fall, is not really affected by the ebb of heat, until the 17th, three days at least after it had been operating in the air.49 Hence we may conclude that all variations of the temperature of the air, whatever be their period, from twenty-four hours up to twelve months, are followed in the same manner by variations in the temperature of the tree; and that those in the tree are always less in amount and considerably slower of occurrence than those in the air. This thermal sluggishness, so to speak, seems capable of explaining all the phenomena of the case without any hypothetical vital power of resisting temperatures below the freezing point, such as is hinted at even by Becquerel.

Réaumur, indeed, is said to have observed temperatures in slender trees nearly thirty degrees higher than the temperature of the air in the sun; but we are not informed as to the conditions under which this observation was made, and it is therefore impossible to assign to it its proper value. The sap of the ice-plant is said to be materially colder than the surrounding atmosphere; and there are several other somewhat incongruous facts, which tend, at first sight, to favour the view of some inherent power of resistance in some plants to high temperatures, and in others to low temperatures.50 But such a supposition seems in the meantime to be gratuitous. Keeping in view the thermal redispositions, which must be greatly favoured by the ascent of the sap, and the difference between the condition as to temperature of such parts as the root, the heart of the trunk, and the extreme foliage, and never forgetting the unknown factor of specific heat, we may still regard it as possible to account for all anomalies without the aid of any such hypothesis. We may, therefore, I think, disregard small exceptions, and state the result as follows:—

If, after every rise or fall, the temperature of the air remained stationary for a length of time proportional to the amount of the change, it seems probable—setting aside all question of vital heat—that the temperature of the tree would always finally equalise itself with the new temperature of the air, and that the range in tree and atmosphere would thus become the same. This pause, however, does not occur: the variations follow each other without interval; and the slow-conducting wood is never allowed enough time to overtake the rapid changes of the more sensitive air. Hence, so far as we can see at present, trees appear to be simply bad conductors, and to have no more influence upon the temperature of their surroundings than is fully accounted for by the consequent tardiness of their thermal variations.

Observations bearing on the second of the three points have been made by Becquerel in France, by La Cour in Jutland and Iceland, and by Rivoli at Posen. The results are perfectly congruous. Becquerel’s observations51 were made under wood, and about a hundred yards outside in open ground, at three stations in the district of Montargis, Loiret. There was a difference of more than one degree Fahrenheit between the mean annual temperatures in favour of the open ground. The mean summer temperature in the wood was from two to three degrees lower than the mean summer temperature outside. The mean maxima in the wood were also lower than those without by a little more than two degrees. Herr La Cour52 found the daily range consistently smaller inside the wood than outside. As far as regards the mean winter temperatures, there is an excess in favour of the forest, but so trifling in amount as to be unworthy of much consideration. Libri found that the minimum winter temperatures were not sensibly lower at Florence, after the Apennines had been denuded of forest, than they had been before.53 The disheartening contradictoriness of his observations on this subject led Herr Rivoli to the following ingenious and satisfactory comparison.54 Arranging his results according to the wind that blew on the day of observation, he set against each other the variation of the temperature under wood from that without, and the variation of the temperature of the wind from the local mean for the month:—

From this curious comparison, it becomes apparent that the variations of the difference in question depend upon the amount of variations of temperature which take place in the free air, and on the slowness with which such changes are communicated to the stagnant atmosphere of woods; in other words, as Herr Rivoli boldly formulates it, a forest is simply a bad conductor. But this is precisely the same conclusion as we have already arrived at with regard to individual trees; and in Herr Rivoli’s table, what we see is just another case of what we saw in M. Becquerel’s—the different progression of temperatures. It must be obvious, however, that the thermal condition of a single tree must be different in many ways from that of a combination of trees and more or less stagnant air, such as we call a forest. And accordingly we find, in the case of the latter, the following new feature: The mean yearly temperature of woods is lower than the mean yearly temperature of free air, while they are decidedly colder in summer, and very little, if at all, warmer in winter. Hence, on the whole, forests are colder than cleared lands. But this is just what might have been expected from the amount of evaporation, the continued descent of cold air, and its stagnation in the close and sunless crypt of a forest; and one can only wonder here, as elsewhere, that the resultant difference is so insignificant and doubtful.

We come now to the third point in question, the thermal influence of woods upon the air above them. It will be remembered that we have seen reason to believe their effect to be similar to that of certain other surfaces, except in so far as it may be altered, in the case of the forest, by the greater extent of effective radiating area, and by the possibility of generating a descending cold current as well as an ascending hot one. M. Becquerel is (so far as I can learn) the only observer who has taken up the elucidation of this subject. He placed his thermometers at three points:55 A and B were both about seventy feet above the surface of the ground; but A was at the summit of a chestnut tree, while B was in the free air, fifty feet away from the other. C was four or five feet above the ground, with a northern exposure; there was also a fourth station to the south, at the same level as this last, but its readings are very seldom referred to. After several years of observation, the mean temperature at A was found to be between one and two degrees higher than that at B. The order of progression of differences is as instructive here as in the two former investigations. The maximum difference in favour of station A occurred between three and five in the afternoon, later or sooner according as there had been more or less sunshine, and ranged sometimes as high as seven degrees. After this the difference kept declining until sunrise, when there was often a difference of a degree, or a degree and a half, upon the other side. On cloudy days the difference tended to a minimum. During a rainy month of April, for example, the difference in favour of station A was less than half a degree; the first fifteen days of May following, however, were sunny, and the difference rose to more than a degree and a half.56 It will be observed that I have omitted up to the present point all mention of station C. I do so because M. Becquerel’s language leaves it doubtful whether the observations made at this station are logically comparable with those made at the other two. If the end in view were to compare the progression of temperatures above the earth, above a tree, and in free air, removed from all such radiative and absorptive influences, it is plain that all three should have been equally exposed to the sun or kept equally in shadow. As the observations were made, they give us no notion of the relative action of earth-surface and forest-surface upon the temperature of the contiguous atmosphere; and this, as it seems to me, was just the crux of the problem. So far, however, as they go, they seem to justify the view that all these actions are the same in kind, however they may differ in degree. We find the forest heating the air during the day, and heating it more or less according as there has been more or less sunshine for it to absorb, and we find it also chilling it during the night; both of which are actions common to any radiating surface, and would be produced, if with differences of amount and time, by any other such surface raised to the mean level of the exposed foliage.

To recapitulate:

1st. We find that single trees appear to act simply as bad conductors.

2nd. We find that woods, regarded as solids, are, on the whole, slightly lower in temperature than the free air which they have displaced, and that they tend slowly to adapt themselves to the various thermal changes that take place without them.

3rd. We find forests regarded as surfaces acting like any other part of the earth’s surface, probably with more or less difference in amount and progression, which we still lack the information necessary to estimate.

All this done, I am afraid that there can be little doubt that the more general climatic investigations will be long and vexatious. Even in South America, with extremely favourable conditions, the result is far from being definite. Glancing over the table published by M. Becquerel in his book on climates, from the observations of Humboldt, Hall, Boussingault, and others, it becomes evident, I think, that nothing can be founded upon the comparisons therein instituted; that all reasoning, in the present state of our information, is premature and unreliable. Strong statements have certainly been made; and particular cases lend themselves to the formation of hasty judgments. “From the Bay of Cupica to the Gulf of Guayaquil,” says M. Boussingault, “the country is covered with immense forest and traversed by numerous rivers; it rains there almost ceaselessly; and the mean temperature of this moist district scarcely reaches 78.8° F.... At Payta commence the sandy deserts of Priura and Sechura; to the constant humidity of Choco succeeds almost at once an extreme of dryness; and the mean temperature of the coast increases at the same time by 1.8° F.”57 Even in this selected favourable instance it might be argued that the part performed in the change by the presence or absence of forest was comparatively small; there seems to have been, at the same time, an entire change of soil; and, in our present ignorance, it would be difficult to say by how much this of itself is able to affect the climate. Moreover, it is possible that the humidity of the one district is due to other causes besides the presence of wood, or even that the presence of wood is itself only an effect of some more general difference or combination of differences. Be that as it may, however, we have only to look a little longer at the table before referred to, to see how little weight can be laid on such special instances. Let us take five stations, all in this very district of Choco. Hacquita is eight hundred and twenty feet above Novita, and their mean temperatures are the same. Alto de Mombu, again, is five hundred feet higher than Hacquita, and the mean temperature has here fallen nearly two degrees. Go up another five hundred feet to Tambo de la Orquita, and again we find no fall in the mean temperature. Go up some five hundred further to Chami, and there is a fall in the mean temperature of nearly six degrees. Such numbers are evidently quite untrustworthy; and hence we may judge how much confidence can be placed in any generalisation from these South American mean temperatures.

The question is probably considered too simply—too much to the neglect of concurrent influences. Until we know, for example, somewhat more of the comparative radiant powers of different soils, we cannot expect any very definite result. A change of temperature would certainly be effected by the plantation of such a marshy district as the Sologne, because, if nothing else were done, the roots might pierce the impenetrable subsoil, allow the surface-water to drain itself off, and thus dry the country. But might not the change be quite different if the soil planted were a shifting sand, which, fixed by the roots of the trees, would become gradually covered with a vegetable earth, and be thus changed from dry to wet? Again, the complication and conflict of effects arises, not only from the soil, vegetation, and geographical position of the place of the experiment itself, but from the distribution of similar or different conditions in its immediate neighbourhood, and probably to great distances on every side. A forest, for example, as we know from Herr Rivoli’s comparison, would exercise a perfectly different influence in a cold country subject to warm winds, and in a warm country subject to cold winds; so that our question might meet with different solutions even on the east and west coasts of Great Britain.

The consideration of such a complexity points more and more to the plantation of Malta as an occasion of special importance; its insular position and the unity of its geological structure both tend to simplify the question. There are certain points about the existing climate, moreover, which seem specially calculated to throw the influence of woods into a strong relief. Thus, during four summer months, there is practically no rainfall. Thus, again, the northerly winds when stormy, and especially in winter, tend to depress the temperature very suddenly; and thus, too, the southerly and south-westerly winds, which raise the temperature during their prevalence to from eighty-eight to ninety-eight degrees, seldom last longer than a few hours; insomuch that “their disagreeable heat and dryness may be escaped by carefully closing the windows and doors of apartments at their onset.”58 Such sudden and short variations seem just what is wanted to accentuate the differences in question. Accordingly, the opportunity seems one not lightly to be lost, and the British Association or this Society itself might take the matter up and establish a series of observations, to be continued during the next few years. Such a combination of favourable circumstances may not occur again for years; and when the whole subject is at a standstill for want of facts, the present occasion ought not to go past unimproved.

Such observations might include the following:—

The observation of maximum and minimum thermometers in three different classes of situation—videlicet, in the areas selected for plantation themselves, at places in the immediate neighbourhood of those areas where the external influence might be expected to reach its maximum, and at places distant from those areas where the influence might be expected to be least.

The observation of rain-gauges and hygrometers at the same three descriptions of locality.

In addition to the ordinary hours of observation, special readings of the thermometers should be made as often as possible at a change of wind and throughout the course of the short hot breezes alluded to already, in order to admit of the recognition and extension of Herr Rivoli’s comparison.

Observation of the periods and forces of the land and sea breezes.

Gauging of the principal springs, both in the neighbourhood of the areas of plantation and at places far removed from those areas.