The next step is to make the gnomon. For this get a piece of the same zinc plate about six inches by eight. Along one of the shorter sides, about a quarter of an inch from the edge, draw a line A B, making it equal in length to C—60 on the scale of chords. With one leg of the compasses on the point A, and the other opened out to B, draw the arc B C as in Fig. 3. Now from the same scale of chords take the length of the latitude of the place, and mark it along the arc B C from B, join A C, from C draw a line at right angles to A B, cutting it in the point D (Fig. 3). The triangle A D C will be the gnomon, with the line A D for its base. Cut this triangle out carefully, making the edges quite square. Now you must get a tinman to solder this in its place on the dial-plate, the point A of the gnomon to be at the points A C on the plate, and the line A D along the two lines A H and C G. You must be very careful that the gnomon stands at right angles to the dial-plate.

The dial must be fixed in a sunny spot, if possible in the middle of a large lawn. The best way to do this is to fasten the dial-plate on a square board, which is fastened to a post driven into the ground. The post can be ornamented with rustic work. The dial must be quite level, and the gnomon pointing due north.

The line A C of the gnomon being made at the angle from the base equal to the latitude of the place will be parallel to the axis of the earth, and will show the hours correctly both on long and short days, as the sun’s course is at right angles to it. The dial can be made to show quarters or five minutes if you so divide the line of hours on the dial scale. The dial, if placed in an open, sunny spot, will show the hours from sunrise to sunset.

II.—THE EQUATORIAL DIAL.

The horizontal sundial would be suitable for all places north and south of the equator. But in southern latitudes the style must point due south instead of north, and the numbering must be done from right to left instead of from left to right. The method, however, already described would not do for any place situated exactly on the equator. The reason for this is—the style, or gnomon, being parallel to the axis of the earth, it would be horizontal at the equator, and perpendicular at the poles, and the shadow would be parallel to the style at the equator and perpendicular to it at the poles. The style must be regulated in height by the size of the dial-plate, and the length of the line of hours, in the scale, must be regulated by the height of the upper edge of the style from the dial-plate.

I will now explain the construction of the equatorial dial. The dial-plate is to be cut about eighteen inches long by twelve inches wide. The inner lines are to be drawn all round, about one inch from the edges, as in the dial already described. Divide the dial-plate into two equal parts by a line drawn from points bisecting the long sides, as in Fig. 1 in the accompanying illustrations. This line is the twelve-o’clock line. The two lines A B and C D are to be drawn parallel to this line, one on each side of and a sixteenth of an inch from it. Before the hour lines can be drawn the style must be made. This must be rectangular in shape, with the long sides equal in length to the twelve-o’clock lines between the inner lines of the plate, and must not, for this size of dial-plate, be more than two inches wide or high. The dialing scale must now be made. It consists only of the line of hours. Draw the two lines O A and O B at right angles to each other, and make each equal in length to the height of the gnomon, or style—viz., two inches. Draw B C parallel to O A, and make it about ten or twelve inches long. Describe the arc A B, with O for the centre, and O A and O B for radii, as in Fig. 2. Divide this arc into six equal parts, and draw lines from the point O through the points of division to cut the line B C in the points 1, 2, 3, 4, 5. The six-o’clock point will not be required. This is the line of hours. With your compasses mark on the side lines of the dial-plate, from the points A B and C D, the divisions of the line of hours. Join the corresponding points on each side of the twelve-o’clock line by lines drawn parallel to it. These lines will represent the hours, and are numbered 1 to 5 to the right and 11 to 7 to the left. Fig. 3 shows this dial.

This dial does not show the time before seven o’clock in the morning or after five o’clock in the evening. The reason of this is, the days and nights at the equator being equal—viz., twelve hours each—the sun rises and sets at six o’clock. At six o’clock, the sun being exactly on the horizon, any object placed in the middle of a perfectly horizontal plane would cast an indefinite or unlimited shadow, as the shadow of the upper part would be parallel to the plane, and of course could not meet it. The dial can be made to show any time after six in the morning or before six in the evening by lengthening the dial-plate. Fig. 4 will show how inconvenient it would be to have a plate to show the time before seven or after five o’clock. In Fig. 4 the hour from five to six is divided into quarters, and shows that for 5.30 the plate must be about double, and for 5.45 about four times, the length required to show the time between seven and five o’clock. So that a plate about six feet long would be required for a dial having a style two inches high.

The style is to be soldered to the dial-plate between the two lines A B and C D, and must be equal in thickness to the distance between them. The dial must be set up in a horizontal position with the gnomon directed due north and south.

Both these dials are horizontal. I will now explain the construction of vertical dials, or dials that are fixed in an upright position against a wall or house. The dialing scale, as already described, will be required for the construction of a vertical dial to be fixed on a wall facing the south.

Cut the zinc plate twelve inches square, and mark it with the inner square, the twelve-o’clock lines, and the six-o’clock line, as in the horizontal dial. From the line of latitudes, in the scale, take the length equal to the difference between 90 deg. and the latitude of the place (that is, not as in the horizontal dial, the latitude, but the complement of it).

Taking London as the place, take from the scale 38¹⁄₂ deg., which is the difference between 90 and 51¹⁄₂ deg. This distance must be marked off on the six-o’clock line from the points A and C. The rest of the construction is the same as for the horizontal dial, with the exception that the hours are limited to fourteen, viz. from five to seven o’clock, and are numbered backwards, or from left to right as in Fig. 5. The style is made as for the horizontal dial, but the angle C A B is to equal 38¹⁄₂ deg. in the case of London, or the complement of the latitude. The base A D is to equal the length of the twelve-o’clock lines, measuring from the six-o’clock line to the inside line at the lower edge, or the lines A B and C D. In fixing this dial care must be taken to let it face due south.

The east and west vertical dials are made something like the equatorial horizontal dial, with a rectangular style. The same scale is required for making the line of hours, the lines of which are regulated in length by the height of the style. Make the plate about eighteen inches long and twelve inches wide, and draw the double lines which in these dials represent the six-o’clock line, as in Figs. 7 and 8. These double lines are drawn, making an angle with the lower edge of dial-plate equal to the latitude of the place. The style is cut rectangular, with the long sides equal to the double six o’clock lines, and the short sides two inches long. Draw the scale making O A and O B two inches long, and mark the hour line. Before the points on the hour line can be marked in on the dial-plate, a plan (Fig. 9) must be made. Draw on a large piece of paper a plan of either dial-plate, and mark in the double lines. Through these draw the line A B perpendicular to them as in Fig. 9. On this line, on each side of the double lines, mark the points on the hour line, and through these points draw lines parallel to the double lines, and letting them cut the sides of the plan of the dial-plate. The points where the lines cut it can be transferred to the dial-plate with a pair of compasses, and the hour lines drawn in parallel to the six-o’clock line. The style must be fixed in its place, and will be parallel to the axis of the earth when the dial is fixed up with the long sides quite horizontal. The east dial is marked as in Fig. 7, and the west as in Fig. 8, if for the northern hemisphere. For the southern, the west dial would take the place of the east, and the east, the place of the west with the numbering reversed.

There are several other kinds of sundials, which may be used for any degree of latitude, a few of which I will describe.

The first of these is the globular (Fig. 10). This is a white globe (any size), supported on an axis which is fixed in a position parallel to the axis of the earth (or making an angle with the horizon equal to the latitude of the place, and pointing due north or south), in which position the globe is acted on by the sun exactly as the earth is. The globe is divided into twenty-four equal parts by lines running from pole to pole, and has an equator drawn around it, on which the hours are marked from 1 to 12 twice over. The axis is fixed in a stand so that one of the six-o’clock lines is in the zenith. The time is indicated by the edge of the shaded part (caused by the sun illuminating one-half of the globe, leaving the other in shade) passing over the hour lines. An ordinary globe answers very well for this dial, if it is rectified for the latitude, and placed so that the brass meridian is directed north and south.

Fig. 11 is another pattern; it is basin-shaped, and is made of a hollow hemisphere of metal, whitened inside, and has the inside divided into twelve equal parts by lines running from pole to pole, which are numbered on the equator from 6 to 6. A wire is stretched from pole to serve as the style, which casts a shadow on the line corresponding to the hour. The position of this dial is the same, as regards the axis of the earth, as Fig. 10.

Fig. 12 represents what I call the trough-shaped dial. It is made of metal-plate bent into the shape of a half-tube; the ends are closed with semicircular metal plates. The interior is divided into twelve equal parts by lines running parallel to the edges, and are numbered from 6 to 6. A wire is stretched from the centres of the semicircular end plates to serve for the style.

This dial must be fixed, with regard to the position and direction of the style, as the other dials are. This is the one constant condition of all dials, that the edge of the style that is to cast the shadow must be parallel to the axis of the earth.

Fig. 13 is a very simple dial, and is the last I shall describe. It consists of a circular dial-plate divided into twenty-four equal parts, numbered from 1 to 12 twice over. The style is a perpendicular wire fixed in the centre of the plate. The plate is hinged to a stand, so that one of the twelve-o’clock lines runs directly from the top to the bottom.

From the construction of the dials 10, 11, 12, 13, they can be used in any latitude, as well as on the equator, but of course the numbering would have to be reversed for the southern hemisphere. They all have an arrangement by which the style can be fixed at the required angle to suit the latitude of the place.

TABLE OF MINUTES.
To be Added to or Subtracted from the Sundial for each Day in the Year.

The sun does not always point out the true time, as on some days it is behind time and sometimes before it. The table below gives the minutes to be added to or subtracted from the time pointed out by the sun for each day in the year:—

CHAPTER XXXVI.—THE CAMERA OBSCURA: HOW TO MAKE AND USE IT.
By Gordon Stables, C.M., M.D., R.N.

Did you never, reader, have a peep in beneath the black cloth where the artist hides his head while he is focusing a sitter for his photograph? I’m sure that many of you have. And what did you see? Why, a pretty little picture in colours of your friend sitting in the chair, laughing like a tramp at a twopenny roll, only upside down. And you have said to yourselves, What a pity it won’t come out in bright colours like that, and why in all the world should it be upside down?

Now I will answer this question before going any further, because it has a bearing on the subject before us—the making of a handy and cheap camera obscura, which cannot fail to be a source of amusement and pleasure, especially when the sun shines.

The reason why the object on the photographer’s ground-glass plate is seen upside down is easily explained. Light, as I need hardly remind older boys, proceeds in straight lines from any illuminated object. It is thus thrown upon the photographer’s plate. A glance at the accompanying diagram (Fig. 1) will suffice to show what I mean.

Let A B be the object—say, an arrow—under consideration, and C D a side view of the ground-glass plate on which the picture is seen. Passing, therefore, in straight lines, the light and colour from the point A will fall at a, will they not? and those from B at b, and so on from every portion of the arrow, so that the representation therefore on the object-glass will be upside down, or reversed. Q.E.D.

Now about the camera. No one can be said to have invented it, for it is constituted upon the firm and immutable laws of Nature. Roger Bacon is credited with having known this principle. Very likely he did, but he put it to no practical use, though over four hundred years after his time Giovanni Baptiste Porta did. But who knows that the ancients hundreds of years before the Flood were unacquainted with it? Here, for example, is a story a little bird told me one beautiful summer’s day while reclining on the greensward in my woodland study: I had been reading under the shade of my great oak-tree. The sun was very bright, and patches of its light penetrated even through the dark-green branches and fell on my face. Probably it was that which set me a-thinking about the laws of optics and the camera obscura and camera lucida. Suddenly close up above me a bird alighted—it was early in the season—and began pouring out the most charming notes.

‘Many people,’ I said to myself, ‘would take that bird to be the nightingale, but I know it is only a black-cap.’

The words were hardly out of my mouth when a saucy little head with a bright bead of an eye peeped round the corner of a twig at me.

‘Only a black-cap!’ said the owner of the head and the eye. ‘I’d have you know, sir, that we black-caps, as you call us, are of a far older family than the nightingales, and that they first learned their wild notes from us, and not we from them!’

‘You know a deal, I dare say,’ I replied. ‘Can you explain this, then? There is a streak of light creeping in from a point among the boughs up there, and falling on my foolscap, and whenever a pigeon, or hawk, or rook flies past away overhead, his image appears on the paper and crosses it, only in a contrary direction.’

‘Foolscap, indeed!’ replied the bird, ‘it is yourself that should be wearing one. The image on your paper is caused by the reflection of the luminous rays from the flying bird. Now,’ continued the black-cap, ‘I’ll tell you about the camera.

‘You know the ancient Egyptians understood everything!’

‘So they supposed,’ I grunted, ‘but——’

‘Don’t interrupt, please. The dungeons of that mysterious land were once upon a time small and dark and dismal in the extreme, and for a very little fault indeed people were thrown therein, perhaps never to leave them alive.’

‘Well, it came to pass that a certain poor man had offended the king, and all his worldly goods were confiscated and he himself was thrown into a cell in a rock. It was not a large one, and its walls were smoothly cemented with a mixture of lime and sand, and some other ingredients known only to the ancient Egyptians. The cell was situated in the side of a hill, with a door at one side which was opened only once a week, to thrust in a pitcher of water and a bundle of cassava root, on which the poor man lived, and to have the cell cleaned out. The only aperture for light and air was a little round hole at the front of the room, too small for even a bird to get through, though bees and moths often entered and kept the prisoner company. But, lo! every day and all day, especially when the sun shone, the rays of light through that aperture brought with them a picture which they painted on the opposite wall. This picture was upside down, but that was but a small drawback, and everything that happened out of doors or in the city beneath was painted on the wall in a marvellous manner. But when the cell door was opened the picture faded away, so the gaoler never saw it.

‘One day the prisoner addressed the gaoler as follows: “Speak unto the king for me, O my son, and tell him, if all my trespasses are forgiven me, and I am taken up out of this loathsome den, I will build for him in his palace a dark room in which he can sit and see all that is going on in the city beneath spread out before him like a great moving picture.”

‘And the gaoler went and spoke to the king in the prisoner’s behalf, and the poor man was brought before the king and set to work in a room of the palace tower. With the aid of workmen he turned the room into a camera obscura, by means of well-placed steel mirrors casting the picture down upon a white concave table.

‘When the king saw it he was greatly astounded and delighted, and ever after that there was no guest about the palace so greatly honoured as the poor man he had but lately thrown into a dungeon.’

I began to rub my eyes after this, and I am hardly sure yet whether the black-cap had really been speaking, or whether I had dropped asleep and been dreaming.

However, this prisoner did nothing more than you could do. I slept, when a boy, in a little turret chamber, which I easily converted into a camera, a description of which I had read in an old book on ‘The Arts and Sciences.’ I had a white screen placed at the proper focus, and a tiny round hole in the shutter, that was all. It was a very primitive arrangement, but pleased me then.

And I believe that most of my readers who are over twelve can make a handy portable camera from the hints I shall now give.

Before you read any further, then, get an empty matchbox, and put it on the table, bottom upwards. Now draw out the drawer of it about half-way. That matchbox is your rough model for the portable camera. Simple, is it not?

The sketch I here append, however (Fig. 2), is not that of a matchbox, but of your portable camera itself, minus its dark shade. The size of this portable camera will depend upon, and be in the ratio of, your own ambition; the perfection of its make will depend upon your own ingenuity.

1. Well, then, you are to make or get made a small box of either very thin wood or very strong pasteboard, covered with thin cloth and painted some dark colour. Size, say, six inches high, six inches wide, and one foot long. This box is open only in front, and therein fits or slips the focusing drawer with its lens.

2. This drawer is also of the same material, and is open at the end that fits into the box, that is the end opposite the lens, and should work easily in or out, and admit no light except through the lens, the magnifying power of which need not be very great.

3. The whole interior of box and drawer is to be stained of a dull black colour.

4. Into the top of the box is let a piece of ground glass, occupying the whole breadth nearly of the top, and two-thirds of its length.

5. Into the box is fitted or let a mirror, which faces the drawer and lies at an angle of forty-five degrees.

Now turn your eyes to Fig. 2, and I will try to explain it. The dimensions of the box are marked in plain figures, as the drapers say. The length of the drawer is about five inches or less. This drawer is represented in the figure as pulled about half-way out. When shut up it will reach the letters C A. The lens is about an inch in diameter, and may be bought cheap at any optician’s, or even fitted for you there.

The piece of ground glass on the top occupies the position marked out by the letters F, E, B, G.

The mirror inside occupies the position indicated by the dotted line A B.

But your camera is not complete yet. You have your dark shade to slip on and fasten. This shade (vide Fig. 3) is a lid, open at both ends, that goes right over two-thirds of the whole box when closed, covering that portion of it seen in Fig. 2 between the squares F, E, G, B and E, H, B, I. This lid is fixed by means of a close-gummed cloth hinge to the box at the dotted line F, E. It is free every way else, so that, when lifted up to an angle of forty-five degrees, it keeps the light away from the ground-glass top, and permits you to see the picture thereon. This shade is, of course, also stained of a dark colour internally.

Now your portable camera is complete. To use it, all you have to do is to hold it in your hand, with the lens turned towards the picture you want represented, and the shade raised, then to pull the focusing-drawer in and out till you have a clear, well-defined picture on your ground-glass top. When the sun is shining brightly the effect is charming; but you yourself and camera ought to be well in the shade.

Now, the instrument I describe is very simple, but its principles may be extended. You might have it on a stand with racks and pulleys for adjustment; and you may have a dark cloth over the shade: the picture will then be ever so much more bright. The great charm of a camera like this is to have a real and lifelike picture in natural colours spread out before you, to see still life as it stands, the trees waving in the wind, and flowers nodding in the sunshine, and every human being or animal that passes walking and moving on your plate.

SECTION VII.
THE BOY’S OWN WORKSHOP.

CHAPTER XXXVII.—CARDBOARD-MODELLING AND WOOD MODELLING.

I.—HOW THE REEDHAM BOYS MAKE THEIR CARDBOARD MODELS.
By the Head Master.

For some years it has been the custom for our boys to employ their leisure time during the winter months in the construction of cardboard models of locomotives, and some of the finished specimens exhibit a degree of skill and ingenuity which could hardly be expected. The work, to a great extent, is the result of the boys’ own observation and skill, added to from time to time by those more observant or more ingenious, and handed down traditionally from one generation of boys to another.

As it is a very interesting occupation, and a valuable means of educating both the eye and the hand, it occurred to me that, if it were possible to describe the processes on paper, it would prove acceptable to boys generally. I therefore gladly avail myself of the opportunity afforded me to lay before you, as clearly as I know how, all the steps necessary to produce a finished model.

To encourage those who may underrate their powers, and think it is useless for them to try, I would say that boys of nine, ten, and eleven often produce very creditable work, and that those who produce the very best are not always those most highly distinguished in class-work. A few more words only are necessary: take these three ‘P’s.’ as your helps, and I am sure you will succeed:

1. Be Precise; that is, endeavour carefully to carry out every detail.

2. Be Patient; that is, do not be too hasty in what you have to do.

3. Persevere; if you fail at first, ‘Try, try, try again.’

We will commence by making a list of the materials required, all of which may easily be obtained at a small cost. If two or three boys work together, the cost may be reduced, as the smallest quantity you can purchase of some of the things will suffice for more than one model.

MATERIALS.

1. Wood.—One-eighth of an inch thick, straight in the grain, and of width and length detailed afterwards. The backing of picture-frames, large cigar-boxes, boxes in which cocoa or blacking is kept at the grocers’ shops, all do capitally, and can generally be got for nothing.

2. Card.—(a) Thick, (b) medium, and (c) thin. The thick may be obtained from strong cardboard boxes; the thin can be bought at most stationers’ in penny sheets, about fourteen inches by ten; and for the medium obtain used postcards (white), which answer the purpose capitally.

3. Gum in solution.—Make your own, by purchasing two ounces of gum-arabic at one penny per ounce, and dissolving it with warm water till it is as thick as cream. Do not put too much water at first, you can easily add more if too thick. Have this always ready, for you require it constantly.

4. Water-Colours (Rowney’s or Reeve’s penny cakes).—Chrome green (2); vermilion (1); ivory-black (1); and a little Chinese white (the last not essential).

5. Camel-hair brushes.—Three or four of various sizes, halfpenny and penny each.

6. Elastic bands.—Six of various sizes.

7. Pins.—If you cannot beg from your mother or sister, one penny will buy what is necessary.

8. Sand-paper.—Coarse and fine (halfpenny per sheet).

9. Copper wire.—Such as is used for bell-hanging. Cost, one penny, sufficient for several.

10. Crystal varnish.—Cost, twopence.

11. Brass or steel chain.—A piece of a toy watch-chain does very well.

12. A few small pieces of coal from the coal-cellar, when required.

13. A knife, with sharp edge.—Boys need no advice on this point.

14. A cutting-board.—Any smooth piece of board about half an inch thick.

15. Cotton.—One reel of strong, any colour.

16. A few pieces of board for painting and drying gummed parts on. Such as described in No. 1 will do.

When all your materials are ready, select a piece of thin wood (marked 1 in the list of materials). Let it be free from knots and straight in the grain. The following dimensions are suitable for a medium-sized model; if larger or smaller models are desired, increase or decrease proportionately.

Length, fourteen inches; Width, two and three-quarter inches; Thickness, about one-eighth of an inch; this is called the Foundation.