in experimenting we will use either the sun or a lamp, as happens to be most easy and convenient. THE HELIOSTAT. In looking out-of-doors in the daytime we find that the sunlight fills all the air, and extends as far as we can see. It shines in at the window and fills the room. Even on a cloudy day, and in rooms where the sunshine cannot enter, the light fills everything, and is all about us on every side. Now, in studying light we do not wish a great quantity. We want only a slender beam, and we must bring it into a dark room, where we can see it and walk about it and examine it on every side, bend it, split it up into several beams, make it pass through glass or water, and do anything else that will illustrate the laws that govern it. Choose a bright, sunny day, and go into a room having windows through which the sun shines. Close the shutters, curtains, and blinds, at all the windows save one. At this window draw the curtain down till it nearly closes the window, and then cover this open space with a strip of thick wrapping-paper. Cut a hole in this paper about the size of a five-cent piece, and at once you will have a slender beam of sunlight entering the hole in the paper and falling on the floor. Close the upper part of the window with a thick shawl or blanket, and, when the room is perfectly dark, our slender beam of light will stand out clear, sharp, and bright. As soon as we begin to study this beam of light, we find two little matters that may give us trouble. The sun does not stand still in the sky, and our beam of light keeps moving. Besides this, the beam is not level, and it is not in a convenient place. We want a horizontal beam of light, and some means of keeping it in one place all day. An instrument that will enable us to do this, and that can be adjusted to the position of the sun in the sky at all seasons of the year and every hour of the day, may be readily made, and will cost only a small sum of money. On the next page are several drawings, giving different views of such an instrument and some of its separate parts. It is called a heliostat, and we shall find it of the utmost value in our experimenting in light, heat, sound, electricity, and other branches of physical science. The first drawing represents a front-view of the heliostat. The second drawing gives an end-view, and we can now make one by simply following these few directions: The part marked A in the two drawings is a piece of pine board, 23 inches (58.4 centi metres) wide and two or more feet long, or as long as the window where it is to be used is wide. Any boy who can use plane and saw can make this piece of work out of common inch-board, and, if you have no pieces so wide as that, it can be made of two or more pieces fastened together with cleats; but, in this case, all the cracks must be close and tight. In the middle of this board, cut a round hole 5 inches (12.7 centimetres) in diameter, with its centre 8 inches from the bottom of the board. In the first drawing this hole can be seen at B, and in the second drawing it is shown by dotted lines at B. On one side of the board screw two iron brackets, using brackets measuring 14 inches (35.5 centimetres) by 12 inches (30.5 centimetres). These brackets are placed one on each side of the hole in the board, and are placed 14 inches (35.5 centimetres) apart, and with the short arm of the bracket against the board. In the first drawing the two brackets are shown, and in the second drawing one is shown in profile, and they are marked C in both drawings. On the end of the brackets is placed a flat piece of board, 6 inches (16.5 centimetres) wide and 14 inches (35.5 centimetres) long, or long enough to reach from one bracket to the other. This board may be screwed up to the brackets, and thus make a shelf. Gare must be taken in fastening this shelf to the brackets to place it so that the outside edge of the shelf will be 16 inches (40.6 centimetres) from the large board. On the outside edge of this shelf another board, 7 inches (17.8 centimetres) wide, is placed upright, and secured with screws and small strips of wood at the ends, as in the drawing. This shelf, with the wooden back, is marked D in the drawings. These things make the fixed parts of the heliostat, and we have next to make the movable parts, or the machinery whereby it can be adjusted to the movement of the sun in the heavens. First, get out a flat piece of board 10 inches (26.7 centimetres) long, 61 inches (16 centimetres) wide, and 1 inch (12 millimetres) thick. Then make a flat, half-round piece, shaped like the figure marked G. This piece must be inch (7 millimetres) thick, 5 inches (14 centimetres) along the straight side, and with the circular part with a radius of 3 inches (7.6 centimetres). A hole, inch (12 millimetres) in diameter, is made in this, as represented in the drawing, and then the halfround piece must be screwed to the flat piece of wood we just cut out. In the figure marked N you will see these two pieces fastened together. Fig. I is the most difficult piece of all. It should be made of ash or some hard wood. One end is square, and has a |