"The waters at Z, on the side of the earth, A, B, C, D, E, F, G, H, next the moon M, (Fig. 68) are more attracted by the moon than the central parts of the earth, O, and the central parts are more attracted by her than the waters on the opposite side of the earth at n; and therefore the distance between the earth's centre and the waters on its surface under and opposite to the moon will be increased. Let there be three bodies at H, O, and D; if they are all equally attracted by the body M, they will all move equally fast toward it, their mutual distance from each other continuing the same. If the attraction of M is unequal, then that body which is most strongly attracted will move most quickly and will increase its distance from the other body. M will attract H more strongly than does O, by which the distance between H and O will be increased, and a spectator on O will perceive H rising higher toward Z. In like manner, O being more strongly attracted than D, it will move farther toward M than D does; consequently the distance between O and D will be increased; and a spectator on O, not perceiving his own motion, will see D receding farther from him towards N; all effects and appearances being the same, whether D recedes from O, or O from D.

"Suppose now there is a number of bodies, as A, B, C, E, F, G, H, placed round O, so as to form a flexible or fluid ring; then, as the whole is attracted toward M, the parts at H and D will have their distance from O increased; whilst the parts at B and F being nearly at the same distance from M as O is, these parts will not recede from one another; but rather by the oblique attraction of M, they will approach near to O. Hence, the fluid ring will form itself into an ellipse Z, n, L, N, whose longer axis n, O, Z, produced will pass through M, and its shorter axis B, O, F, will terminate in B and F. Let the ring be filled with fluid particles, so as to form a sphere round O; then, as the whole moves toward M, the fluid sphere being lengthened at Z and n will assume an oblong or oval form. If M is the moon, O the earth's centre, A, B, C, D, E, F, G, H, the sea covering the earth's surface, it is evident, by the above reasoning, that whilst the earth by its gravity falls toward the moon, the water directly below at B will swell and rise gradually toward her; also the water at D will recede from the centre, (strictly speaking, the centre recedes from D) and rise on the opposite side of the earth; whilst the water at B and F is depressed, and falls below the former level. Hence as the earth turns round its axis from the moon to the moon again in 24³⁄₄ hours, there will be two tides of flood and two of ebb in that time, as we find by experience.

"That this doctrine may be still more clearly understood, let it be considered that, although the earth's diameter bears a considerable proportion to the distance of the earth from the moon, yet this diameter is almost nothing when compared to the distance of the earth from the sun. The difference of the sun's attraction, therefore, on the sides of the earth under and opposite to him, will be much less than the difference of the moon's attraction on the sides of the earth under and opposite to her; and, for this reason, the moon must raise the tides much higher than they can be raised by the sun. The effect of the sun's influence, in this case, is nearly three times less than that of the moon. The action of the sun alone would, therefore, be sufficient to produce a flow and ebb of the sea; but the elevations and depressions caused by this means would be about three times less than those produced by the moon.

"The tides, then, are not the sole production of the moon, but of the joint forces of the sun and moon together. Or, properly speaking, there are two tides, a solar one and a lunar one, which have a joint or opposite effect, according to the situation of the bodies which produce them. When the actions of the sun and moon conspire together, as at the time of new and full moon, the flow and ebb become more considerable; and these are then called the spring tides. But when one tends to elevate the waters while the other depresses them, as at the moon's first and third quarters, the effect will be exactly the contrary: the flow and ebb, instead of being augmented, as before, will now be diminished; and these are called the neap tides.

"To explain this more completely, let Fig. 69 represent the sun, Z, H, R, the earth, and F and C the moon at her full and change. Then, because the sun S, and the new moon C, are nearly in the same right line with the centre of the earth O, their actions will conspire together, and raise the water above the zenith Z, or the point immediately under them, to a greater height than if only one of these forces acted alone. But it has been shown that when the ocean is elevated to the zenith Z, it is also elevated to the opposite point, or nadir, at the same time; and therefore in this situation of the sun and moon, the tides will be augmented. And again, whilst the full moon F raises the waters at N and Z, directly under and opposite to her, the sun S, acting in the same right line, will also raise the waters at the same point Z and N, directly under and opposite to him. Therefore, in this situation also, the tides will be augmented; their joint effect being nearly the same at the change as at the full; and in both cases they occasion what are called the spring tides.

"On this theory, the tides ought to be highest directly under and opposite to the moon; that is, when the moon is due north and south; but we find that in open seas, where the water flows freely, the moon is generally past the north and south meridian of the place where it is high water. The reason is obvious; for though the moon's attraction were to cease altogether when she was past the meridian, the motion of ascent communicated to the water before that time would make it continue to rise for some time after; much more must it do so when the attraction is only diminished. A little impulse given to a moving ball will cause it still to move farther than otherwise it could have done; and experience shows that the day is hotter about three in the afternoon than when the sun is on the meridian, because of the increase made to the heat already imparted.

"Tides do not always answer to the same distance of the moon from the meridian at the same place, but are variously affected by the action of the sun, which brings them on sooner when the moon is in her first and second quarters, and keeps them back later when she is in her third and fourth; because, in the former case, the tide raised by the sun alone would be earlier than the tides raised by the moon; and in the latter case, later.

"The sea, being put in motion, would continue to ebb and flow for several times, even though the sun and moon were annihilated, and their influences at an end, on the same principle that if a basin of water is once agitated, the water will continue to move for some time after the basin is left to stand still. A pendulum, put in motion by the hand, continues to make several vibrations without any new impulse. When the moon is at the equator, the tides are equally high in both parts of the lunar day, or time of the moon's revolving from the meridian to the meridian again, which is 24 hours 50 minutes. But as the moon declines from the equator toward either pole, the tides are alternately higher and lower at places having north or south latitude. One of the highest elevations, which is that under the moon, follows her toward the pole to which she is nearest, and the other declines toward the opposite pole; each elevation describing parallels as far distant from the equator, on opposite sides, as the moon declines from it to either side; and consequently the parallels described by those elevations of the water are twice as many degrees from one another as the moon is from the equator; then increase their distance as the moon increases her declination, till it is at the greatest, when these parallels are, at a mean state, 47 degrees from one another; and on that day the tides are most unequal in their heights. As the moon returns toward the equator, the parallels described by the opposite elevations approach toward each other, until the moon comes to the equator, and then they coincide. As the moon declines toward the opposite pole, at equal distances, each elevation describes the same parallel in the other part of the lunar day which its opposite elevation described before. Whilst the moon has north declination, the great tides in the northern hemisphere are when she is above the horizon; and the reverse whilst her declination is south.

"In open seas, the tides rise to very small heights in proportion to what they do in wide-mouthed rivers, opening in the direction of the stream of tide. In channels growing narrower gradually, the water is accumulated by the opposition of the contracting bank—like a gentle wind, little felt on an open plain, but stronger and brisk in a street; especially if the wider end of the street is next the plain, and in the way of the wind.

"The tides are so retarded in their passage through different shoals and channels, and otherwise so variously affected by striking against capes and headlands, that in different places they happen at all distances of the moon from the meridian, consequently at all hours of the lunar day.

"There are no tides in lakes because they are generally so small that when the moon is vertical she attracts every part of them alike; and, therefore, by rendering all the waters equally light, no part of them can be raised higher than another. The Mediterranean and Baltic Seas suffer very small elevations, because the inlets by which they communicate with the ocean are so narrow that they cannot, in so short a time, receive or discharge enough to raise or sink their surface sensibly.

"Air being lighter than water and the surface of the atmosphere being nearer to the moon than the surface of the sea, it cannot be doubted that the moon raises much higher tides in the air than in the sea. Therefore many have wondered why the mercury does not sink in the barometer when the moon's action on the particles of air makes them lighter as she passes over the meridian. But we must consider, that as these particles are rendered lighter, a greater number of them are accumulated, until the deficiency of gravity is made up by the height of the column; and then there is an equilibrium, consequently an equal pressure upon the mercury as before; so that it cannot be affected by the aerial tides. It is probable, however, that stars seen through an aerial tide of this kind will have their light more refracted than those which are seen through the common depth of the atmosphere; and this may account for the supposed refractions of the lunar atmosphere that have been sometimes observed.

"You see now how the tides are caused; while there may be some influences at work other than those exerted by the sun and moon, the latter are the chief ones, so I will not attempt to explain any other.

"Here, on the Passaic River, we do not have excessive tides, as the highest on the coast near us seldom rise over ten or twelve feet. As a rule, tides rise highest and strongest in those places that are narrowest. In the Black Sea and the Mediterranean, the tides are scarcely perceptible, while at the mouth of the Indus, in the Bay of Fundy, and other places, they rise thirty or more feet at times. The general rise, however, in mid-ocean, is from eleven to twelve feet.

"The diameter of our moon is nearly 2,200 miles, and her distance from the earth is about 240,000 miles; so you see it is not her size, but her proximity to the earth that gives her so much influence over the tides; for the sun, which is many times larger than the earth and moon combined, because of its being some ninety-three millions of miles away, exerts only one sixth of the attraction on the earth that the moon does.

"These facts, children, should be remembered, as you may often be called upon to make use of them.

"Oh, papa!" said Jessie "how many wonderful things there are in this world."

"But I have not told you all, my dear. There is much more to learn, but I hope the knowledge you have now acquired will act as an incentive, and cause you to pursue this study further."

"The best hygrometer of absorption is (according to Deschanel) that of De Saussure, consisting of a hair deprived of grease, which by its contractions moves a needle. When the hair relaxes, the needle is caused to move in the opposite direction by a weight which serves to keep the hair always tight as seen in the illustration, Fig. 70. The hair contracts as the humidity increases. In the accompanying illustration A A and B B represent the frame; e f, the scale; a, screw for tightening the hair; b, the hair; O, weight; H, thermometer.

"A neater hygrometer, and one on the same principle, may be made by taking an old tooth powder box (as deep a one as possible, since the longer the string, the more sensitive it is), and boring a hole through the centre of the top and bottom. Paste a kind of dial in paper on the top of the box; take a piece of catgut, or small fiddle string, and push it up through the hole in the bottom and out at the one in the lid. Glue the bottom end immovably, and let the top end move freely: make a small index of a strip of whalebone (Fig. 71); bore a hole in the centre, and fix it on the catgut with glue. Wet the catgut, see which way it turns, and mark 'wet' and 'dry', accordingly on the dial.

"So much for the hygrometer. Now about that curious thing, the boomerang. If the following directions are closely adhered to, and the proper shape followed, a regular Australian boomerang will result. It is not difficult to make. Take a piece of hard wood, the natural shape of one of the segments of an ordinary wheel felloe, or bend in the wood; let it be ¹⁄₄ inch thick, shaped as at Fig. 72, to be held in the right hand at A, which shows the way the edges of the side facing the left hand must be bevelled off. It requires a slight curve on the flat side; so that, if on a table, each end would turn about ¹⁄₈ inch. It is then a part of a very fine pitch screw, in motion similar to a piece of slate jerked into the air, the sole difference being due to the slight curve in the back, which gives the screw motion, in conjunction with the forward and rotatory motion given by the hand. Sheet-iron would not do, as there would not be thickness to show the bevelled edge. The boomerang was made in the form of a cross, with four legs of equal length, bevelled, but it does not work as well as the regular form. You must be careful in throwing it as it may strike you on return."

George asked his father to describe one and to explain its uses. Mr. Gregg told the boys that a boomerang, as used by the aborigines of Australia for a weapon or missile of war or in the chase, consisted of a flat piece of hard wood bent or curved in its own plane, and from 16 inches to 2 feet long. Generally, but not always, it is flatter on one side than on the other. In some cases the curve from end to end is nearly an arc of a circle; in others it is rather an obtuse angle than a curve, and in a few specimens there is a reverse curve toward each end. In the hand of a skilful thrower, the boomerang can be projected to a great distance, and made to ricochet almost at will. It can be thrown in a curved path, somewhat as a ball can be "screwed" or "twisted," and it can be made to return to the thrower, striking the ground behind him. It is capable of inflicting serious wounds.

"It is very good of you, father," said Fred, "to tell and show us all these things; I'd like very much to have a very common, every-day matter explained: the theory of the pump." The following questions also were asked by one or another on the same line: What is the greatest distance or height a pump of any type can be placed away from the water? Is there any limit to the length of the delivery pipe to the tank? What is the difference between a lift and a plunger or force pump? Is it the sucker of the pump that draws the water up, or does it flow because the air being drawn out of the pump barrel and forced on the water outside, causes it to flow into the pump?

Mr. Gregg started in at once to give them the facts desired: "Theoretically, the greatest height a pump can be fixed above the water level depends on certain conditions: the atmospheric temperature, and the altitude the pump is to be fixed above the sea-water level. The higher the temperature, and the greater the altitude, the less distance the height of the pump can be above the water. The height to which water can be drawn from the source to the top of the bucket, or under side of a piston or plunger, when at the top of the stroke, or what is termed the 'height of suction,' cannot reach more than about 33 feet when the pump is at the sea level. If a tube about 34 feet long is immersed in a well, and the air is extracted by means of an air pump at the upper so that a vacuum is formed, the water will not rise in the tube until the air is expelled, when it will not rise more than 33 feet, even though there is a complete vacuum formed in the upper end of the tube. The reason why the water will not rise in the tube higher than this, is that the height of the water counterbalances the pressure of the atmosphere. This height is the theoretically greatest height that water will rise in a suction pipe. For the pump to discharge water, it is necessary for the water to be in motion, and to set and keep it in motion a portion of the water will rise, due to the atmospheric pressure. The shorter the suction pipe, the more certain the pump is of being completely filled at every stroke of the pump handle.

"The action of the pump is as follows: The bucket on moving upward attracts the air, so that the atmospheric pressure on the surface of the water in the well causes the water to follow the bucket up the suction pipe, through the suction valve, into the working barrel. On the return stroke, the suction valve will close, the valve in the bucket will open, and the water which before was under the bucket will pass through it to the top side. When the bucket is again raised, the water will be lifted through the delivery valve into the delivery pipe. There is practically no limit to the height of lift, which may be any height consistent with the strength of the pump and the available power. The ordinary pump used for raising water to the level of the top of the bucket, is termed a lift pump; for raising water above this, a force pump or a plunger pump must be used, when the water is displaced by a solid plunger on its downward stroke, when the quantity of water raised will be equal to the volume of the plunger. This system may be repeated when water is to be lifted more than ordinary heights."

X

WALL MAKING AND PLUMBING

"This will never do," said Mr. Gregg. "We must protect the bank at this point, or the water will soon undermine and demolish our pier, for you see it is only near the landing where the bank shows signs of injury, and it is as badly damaged on one side as the other. This is caused by projection of the pier into the river, which prevents the water from flowing in its regular course, and causes it to rush into the angle formed by the junction of the pier with the bank, thus cutting away the latter."

"Perhaps it will be best to build a sort of retaining wall against the bank for ten or twelve feet each side of the pier to prevent this rush of water from cutting away the earth. If we had field stones enough on the ground, it would be cheaper to use them, though they would not make as good a 'job' as either cut stones or concrete; since we haven't the stones, we'll build it of concrete, as you have some knowledge of that material, and I will engage Nick to help you."

The next day Mr. Gregg ordered Portland cement and all the other materials required to build the wall, and engaged Nick, who promised to come the following morning. In the evening, Mr. Gregg had the boys in his den, and explained to them how to go about constructing the wall. He decided to have it built of concrete blocks about 12 × 24 × 12 inches, to be faced with good, strong, cement mortar on the face and ends, which would give the exposed wall a nice, smooth appearance. Mr. Gregg explained that there must be a foundation of stone under the concrete, formed by large bowlders or "fielders," laid as closely together as possible, the joints filled in with smaller stones and, when possible, cement mortar, to bind the whole into a solid mass—as shown by dotted lines in the illustration which he made on the blackboard. The blocks for the work were to be cast in wooden moulds or forms, which Fred and George could easily make out of boards taken from the dismantled barn. At the points where the wall was wanted, the bank was about 8 feet high from the bottom of the river, and it was determined to make the wall 8 feet high, 2 feet wide at the top and 3 feet at the bottom, with the batter on the water side, the weight of the wall being 140 pounds per cubic foot. It is always best to have the inclined surface on the side of the wall where the water will be. The water at high tide rises to a level of 6 feet above the base C D.

"In designing such a retaining wall," said the father, "for water one side, and earth the other, or determining its stability, the principles generally followed may easily be worked out by Fred, or even by George.

"Taking the earth side first, as shown in diagram Fig. 72a, W C X, angle of repose of earth to be retained—30 degrees; G C, the line of rupture; G C A, the wedge of earth at 112 pounds per cubic foot to be accounted for, the weight of which equals—

(GA × AC)/2 × 112 lb. = (4' 7" × 8')/2 × 112 lb. = 2,053 lb.

"This will act at a point one-third the height of the wall H. From H erect a perpendicular H I equal to 2,053 lb. Set out the angle H I J equal to angle of repose, 30 degrees. From H erect a perpendicular to A C, cutting I J in J. Then J H equals the direction and magnitude of the weight of the earth acting on the wall.

"Produce J H through the wall toward the water side. Find centre of gravity of wall in K and the weight of the wall, which in this illustration equals—

(AB + CD)/2 × AC × 140 lb. = (2 + 3)/2 × 8/1 × 140 lb. = 2,800 lb.

"From where J H produced meets a vertical line drawn through the centre of gravity, K, in L set of L N equal to 2,800 lb.; make L M equal to J H; complete parallelogram L M O N, when L O equals resultant of earth and wall.

"The magnitude and direction of P R can be found as in the first part of this article. Produce R P through the wall, and from where it cuts the resultant L O in S make S T equal R P. Let the diagonal L O now be produced so as to make S V equal to L O. Complete the parallelogram S T U V, when the resultant S U equals the combined resultant of earth, water, and wall, and as it passes within the middle third it can be considered safe.

"Now, boys," said Mr. Gregg, "I have not only told you how to build a retaining wall, I have also told you how to make all the necessary calculations for designing it, as the same figuring and diagraming, on this principle, will answer for any sea wall requiring like conditions.

"I know you both understand figures and geometry enough to make such calculations, if you are ever called upon to do so."

The next morning, before the boys had finished their breakfast, Nick was on hand ready to go to work, equipped with a pair of hip rubber boots which would enable him to wade in water two feet deep and remain dry.

Fred and George were soon ready and Mr. Gregg went out to tell them the proper way to commence. The foundation was the first consideration, so an examination of the site and was made, the length of the proposed walls measured off. While waiting for the tide to ebb to its lowest point, Nick and the boys busied themselves gathering up stones for the foundation and wheeling them to the point nearest where they were to be used.

After gathering all the stones thought necessary, the question of making the moulds for the concrete blocks was considered, and, as the greatest bulk of the blocks would be simply blocks with square ends and square faces, the moulds for these would be a box having inside dimensions of 12 inches deep, 12 inches wide, and 24 inches long. These dimensions would then allow of blocks being made in the moulds that will contain exactly 2 cubic feet. The mixed concrete was dropped gently in the mould and lightly tamped so as to make it solid. The mixture consisted of not less than 3 of cement, 5 of sand, and 7 of very fine gravel or broken stone, no piece being larger than a white bean. It was mixed in the same manner and in accordance with the rules given for making concrete for the sidewalk in Chapter I.

The mould should rest on a smooth block of stone, wood, or other suitable material, while being filled and tamped, and when full the surplus should be levelled off, by a straight-edge—wood or iron—drawn over the top of the mould, until all the surplus is removed. The mould is then allowed to stand a little while until the concrete "sets" fairly hard, when the mould may be removed. To make it easy to take the block out of the mould, the inside should be well sprinkled with neat cement before the concrete is put in, and the box itself might be made slightly tapering to permit the block to move out easy. This method, however, is not to be recommended, as the blocks do not fit so well in a wall as when left perfectly square. There are a number of devices for making moulds so that delivery of blocks may be easy. One of the best is to hinge one corner of the mould with heavy hinges, while the opposite diagonal corner is left loose but held in place by a strong hasp and staple. When the box or mould is full and the block ready to remove, the hasp is loosened, the mould opens across at the two corners and frees the block. Should there be any holes or defects on the face of the blocks, they can be filled with cement mortar made with 2 of cement and 3 of clean sand. Blocks of this size should season not less than 4 or 5 days, to set hard before being used.

A portion of these blocks must have a bevel face on them to form the batter on the front of the wall. There must also be a proper proportion of them having their ends bevelled to the batter of the wall, to use as "headers." A header in brick, stone, or concrete, is a unit, or piece, that is laid in the wall with its ends showing through on the face, while a "stretcher" shows its whole length on the face of the wall. Other portions of brick or stone, when built in a wall, are called "closers."

The batter on the blocks is formed by making one side of the mould lower than the other. In this case, the difference in the width of the sides of the mould would be 1¹⁄₂ inches; because the height of the wall being 8 feet, the blocks 1 foot thick, and the batter 1 foot, there would be a falling off on each block of 1¹⁄₂ inches in order to have the top front of the wall 12 inches back from the bottom front. The ends of the header blocks may be battered by placing in the ends of the mould a piece of wood 12 inches wide, and the lower edge 1¹⁄₂ inches thick, and the top edge planed to a thin wire edge. The end or section of the plank will then have the appearance of a wedge 12 inches long, 1¹⁄₂ inches thick on one end, and tapered to nothing at the other end. When the block is taken from the mould, and the wedge piece removed, the block will show the same batter on its end as the stretchers do on their face, and they can be built in together without showing any difference in the slope, if the work is carefully done.

Nick, who had had some experience in this kind of work, found no difficulty in understanding the whole process.

At low tide he set to work to make a solid bed for the foundation, while the boys handed him the stone and the prepared mortar as he required it, so that before the tide rose one side of the stone foundation was ready to receive the concrete blocks. During the interim between tides, Nick and the boys made the moulds, prepared for mixing the concrete, and got old timbers and lumber for a temporary scaffolding. After the moulds were made and some concrete mixed, Nick began on the blocks. It was not long before he had a sample, which seemed all right, and before he stopped quite a number of them were ranged on boards "setting."

On the sixth day after it had been commenced, the job was entirely finished. The joints in the wall had been nicely "pointed" up with cement mortar by aid of a fine-pointed trowel. The back, or ground side of the wall was filled in with earth, and danger to the pier was entirely removed.

That night Mr. Gregg told the boys and Jessie—who had watched closely the growth of the wall—quite a lot about Portland cement and concrete, which interested them very much. Portland cement as we have it now was unknown a hundred years ago, but an Englishman invented the method of making it and properly proportioning the various materials used. Fifty years ago there was scarcely any made in this country, the little that was used being imported from England, and later from Belgium; but now more of it is made and used in the United States than anywhere else in the world. He pointed out that the building of the Panama Canal was made much easier and less costly because of cement, and that the largest dam ever built had just been suggested, to dam the Mississippi near Keokuk, Iowa. This would be over 5,800 feet long and nearly 40 feet high and from 25 to 35 feet thick. He told of the various big storage dams being built and contemplated by the United States, in Montana, Arkansas, Nebraska, Wyoming, New Mexico, Dakota, Texas, and many other places, at a cost of hundreds of millions of dollars—which never would have been attempted if concrete had not been available. He also made mention of the great wall that now protects Galveston from the ravages of the sea. It is not many years since Galveston was almost destroyed by tidal waves that caused an enormous loss of life, and destruction of property amounting to over $17,000,000. The wall was built to prevent a recurrence of similar disasters. It is 17,503 feet long, 17 feet high, and 16 feet thick at the base. Another recent work is the enormous dam built by English engineers across the river Nile at Assiout, about 250 miles above Cairo in Egypt, which increases the area of good land some 300,000 acres. Ancient Babylon is again to blossom and become a beautiful country to live in, for British engineers are laying out plans for building storage dams and irrigating canals in these now sandy and barren lands. All, or nearly all, of these works and proposed works would never have been thought of, if Portland cement had not been in existence.

Mr. Gregg, after finishing his talk on concrete, noticed that George had two fingers on his right hand tied up, and on inquiry was told that George had his fingers hurt by a concrete block falling on them just as the retaining wall was being finished. The father insisted on seeing the bruised fingers and found they were not badly hurt, though the skin in one place was broken. George explained that his mother had washed his hand, dressed the wound, and applied an antiseptic to it, so that it was all right now and did not pain him.

"You were wise to go to your mother and have your bruise attended to immediately, otherwise you might have had something serious happen to you, as lockjaw frequently comes from wounds of that kind, if deep enough and not attended to immediately. It is often said that lockjaw or tetanus is caused by a wound made by a rusty nail. It is certainly bad to be wounded with a rusty nail—or any other rusty iron—and tetanus may follow; but it does not follow because the nail is rusty, but because the tetanus microbe that may be on the nail, or on the skin when the wound is made, is carried into a favourable place for development.

"This tetanus microbe, which has a long name, is very plentiful and is scattered broadcast by every gust of wind. It is a microbe of dirt, and the ground and street abound with it. Its first home and breeding place is in the intestines of horses and other domestic animals, but its greatest danger to the human family is when it gets into the blood by way of a wound. Cleanliness, in this as in many other cases, is both a preventive and a cure."

"Father," said Jessie, "I saw a very funny thing to-day while watching Nick and the boys finish the wall. The train across the river came to a standstill for some reason or other, and, as I was watching it, I saw three puffs of steam go out of its boiler, and a short time after I heard three loud whistles. This seemed to me quite curious, but while I was thinking over it, there were three more jets of steam, followed by three more 'toots.' How was it that I saw the toots before I heard them?"

"This is a question, my dear, that will require some little time and thought to answer properly. In the first place, you must understand that light travels very much faster than sound and that sounds do not reach you until some time has elapsed, if you are a little distance away. You see a flash of lightning, and a little while after you hear the thunder; and if you count 1, 2, 3, in the ordinary way, between seeing the flash and hearing the thunder, you may be fairly satisfied the source of the thunder is well on to three miles away. This, of course, is not exactly correct, but approximately so. Every time you count one, it stands for a mile. According to science, light travels 186,000 miles a second, while sound only travels at the rate of 1,090 feet per second at a temperature of 32 degrees Fahrenheit, or freezing, its velocity being increased at the rate of one and one tenth feet per second for every degree above this temperature. So you see light travels nearly a million times faster than sound, and this accounts for your seeing the puffs quite a little while before you heard the 'toots', as you call them. There are many curious and interesting things about light and sound which I'd like to describe to you sometime.

"Sound travels in dry air at 32 degrees, 1,090 feet per second, or about 170 miles per hour; in water, 4,900 feet per second; in iron, 17,500 feet; in copper, 10,378 feet; and in wood, from 12,000 to 16,000 feet per second. In water, a bell heard at 45,000 feet, could be heard in the air out of the water but 656 feet. In a balloon, the barking of dogs can be heard on the ground at an elevation of four miles. Divers on the wreck of the Hussar frigate, 100 feet under the water, at Hell Gate, near New York, heard the paddle wheel of distant steamers hours before they hove in sight. The report of a rifle on a still day may be heard at 5,300 yards; a military band at 5,200 yards. The fire of the English, on landing in Egypt, was distinctly heard 130 miles. Dr. Jamieson says he heard, during calm weather, every word of a sermon at a distance of two miles. The length of the sound waves in the air is sometimes many feet, while the length of the longest light wave is not more than .0000266 of an inch; it is no longer a mystery why we can hear, but cannot see, around a corner."

The children were greatly interested by these familiar marvels and made their father promise that he would resume the talk some other evening and tell them about thermometers and barometers.

The late afternoon next day was taken up with an excursion on the Caroline down the river to Newark, where Fred induced his father to purchase a full soldering outfit, as the boys wanted to try some plumbing and soldering work. There had been a plumber at the Gregg home nearly all that day doing repair work of various kinds, and Fred, who had watched the workman, concluded he could have made the repairs himself if he had had the proper tools.

An hour or two in the city, then a pleasant sail home, proved a fine ending for a day's labour.

The next day, after school, George and Jessie assisted their mother "making garden," planting flowers, trimming bushes, and destroying weeds, while Fred gave the Caroline another coat of varnish, and finished painting his little workshop, which now looked very snug and tidy. He soldered up all the leaks in every kitchen utensil he found defective, much to the delight of his mother and the maid. Fred found many things about the house wanting more or less attention, so he determined to try to put them in order. He discovered that to make a good job of soldering, he must first make the metal to be fastened together, perfectly clean and free from rust, dirt, or grease, the parts around the leak being scraped bright and smooth. He found some little difficulty in getting the solder to the exact place he wanted. In the outfit his father bought him, was not only a soldering iron,—which is not iron but copper—but a scraper, a lump of solder, a box of rosin, a piece of chamois leather, a bottle of muriatic acid, and a piece of sal-ammoniac, to be crushed fine and dusted over any surface that is to be finished bright. Fred had no trouble in soldering holes of small size in teakettles, tins, or such things as he could handle easily, for the impaired portions could be placed in a horizontal position before him and the solder applied readily. A leak in an upright water pipe in the shed, however, gave him a hard time, for he could not get the solder either to run up hill or to stay on the place where it was put. He got over this difficulty, however, by making a clay dam, a "tinker's dam"—mixing clay until it was soft, then winding a strip of it around the pipe just below the leak and applying the solder until the hole or crack was entirely covered, when a good solid job resulted. Of course, before applying any solder, all the water was drained from the pipe, and the defective part was thoroughly scraped. When the work was done, there was an edge of solder left projecting from the pipe, which Fred rasped away with a course rasp, leaving just enough solder to cover the leak properly. He then sandpapered the work and it looked almost as "good as new."

It is easy enough to solder across the work when level, even if the article being soldered is round, because the metal can be worked across the top and down the sides; but on the under side, it may be necessary to make use of a clay dam. A plumber's work covers a lot of things, among which may be mentioned metal roofing, wall flashings, water-pipes of all kinds, drain connections, hot water and steam fittings, hot-air and ventilation fittings, stove and range settings, and many other things connected in some way or another with the foregoing. Many times an offensive odour is noticeable in the cellar, or near the line of drainage, and it is often difficult to locate the source, so that expensive excavations are made before the trouble is remedied. Plumbers and drainage men often use what is termed "the peppermint test," to find where the leakage exists, and this is particularly suitable for the examination of existing soil pipes and drainage fittings. This test consists in pouring a small quantity of oil of peppermint or other substance possessing a pungent, penetrating, and distinctive odour, into the pipe or drain. The defective pipe or joint is then located by the escaping odour.

It is very important that defects of this kind should be located and repaired immediately, for odours emanating from drains or soil pipes carry with them germs of the kind most dangerous to human health and life.

Some taps in the bath room and over the kitchen sink were not working freely, and others were "dropping" a little. Fred, after cutting off the water from the main, unscrewed these and put new rubber washers in some, wound cotton twine around the plugs of others, and made the tight ones work easy by removing worn out washers and cut strings. He also fixed the hydrants on the lawn in the same manner, and made all the taps in and about the house work tightly and smoothly.

When Mr. Gregg arrived home, Fred told him all he had done, showing the tin pans and the leaky pipe he had soldered, and he straightened up with pride at being told that he was already "quite a plumber."

After tea, the family went down to the river's bank and chatted awhile on home matters; then shortly after the sun went down, they adjourned to "the lion's den."

"Now," said George, "father will tell us about barometers and thermometers, as he promised."

"Well," said Mr. Gregg, "I'm pleased to know you are so ready to listen to my talks, and I hope you'll remember some of the facts I've been telling you.

"There are many kinds of barometers, but all are constructed about on the same principle, and on the old theory that 'nature abhors a vacuum'. There may have been some kind of an instrument that did service as a barometer in the early ages, but we have no knowledge of it. The instrument as we now know it had its beginning with Galileo, Torricelli, and Pascal, but was not perfected until about 1650. Good barometers require the greatest possible care in their construction, and there ought to be two or more standing together as checks on one another in order to obtain correct results. The mercury used must be pure and good, free from all other substances and from air bubbles or films of air on the sides of the bulb. Simple barometers, suitable for ordinary purposes, can be easily made. I will describe one, and make a sketch of it on the blackboard.