deep slot cut in it; the rest is round, and may be 11 inch (32 millimetres) in diameter. The square part must be large enough to slip over the half-circular piece, G, as is shown at H. A hole, inch (12 millimetres) in diameter, is cut in the two ends, as marked by dotted lines at J, and through these holes an iron bolt and nut are fitted, so as to hold the circular piece, G, and yet allow it to turn freely in every direction. A hole, 1 inch (32 millimetres) in diameter, is cut through the triangular piece of wood K, as shown by the dotted lines, and then this block is securely fastened to the back of the large board, as shown in the second drawing. An opening of the same diameter, and having the same direction, is also cut through the board, and the movable piece, marked I, is put through this hole, as in the drawing. Finally, we want a wooden washer, 3 inches (8.7 centimetres) wide, as represented at M. This we slip over the long wooden handle, as shown in the second drawing, and this washer rests on the block K, the top of which is 3 inches square. This makes all the movable parts of the heliostat, and, when we have put in the mirrors, the instrument is finished and ready for use. We must have two mirrors, one 6 inches (15.2 centimetres) square and one 10 inches (25.4 centimetres) long and 6 inches (15.2 centimetres)

wide. These may be made of common looking-glass; but plate-glass with silvered back is far better, and costs only a little more.

Any carpenter can make this instrument, and the cost will be about as follows: Wood, 50 cents; labor, $1.75; glass, $1; iron nut, 5 cents; brackets, 50 cents -total, $3.80. When finished, the instrument should have a coat of shellac-varnish, and, when this is done, the mirrors may be put in place, and fastened on with very heavy bands of rubber. This will enable us to take the glasses off when the instrument is not in use, and, if the elastic bands or rings are very strong, they will answer perfectly. The long mirror is to go on the movable piece at N, and the small mirror stands on the shelf, facing the opening in the board, at O. This mirror stands at the angle shown in the next drawing (Fig. 2), and the other mirror is adjusted to the sun at its various positions in the sky at different seasons of the year.

Here is a diagram showing the position of the handle of the heliostat, and the mirror for different seasons and in different parts of the country. The handle must be placed on a line parallel with the axis of the earth, and the four dotted lines give its position when the heliostat is to be used in Boston, New York, Washington, and New Orleans. This

also causes the block of wood marked K to have a slightly different shape, so that the hole through it will be in the middle. The dotted line marked "At Equinox" shows the path of the light from the sun, and

the three dotted lines show the paths of the reflected light as it passes from one mirror to the other. The position of the movable mirror is also shown in the positions it has at summer and winter solstices.

FIRST EXPERIMENT WITH THE HELIOSTAT.

Choose a bright sunny day, and take the heliostat into a room having a window facing the south. Raise the sash and place the instrument in the window, and fasten it there so that it will be firm and steady. Before closing the window down upon it, move the larger mirror on its axis till it reflects a beam of light into the small mirror. Then turn the handle to the right or left, and a round, horizontal beam of light will enter the room. When this is done, close all the windows, so as to make the room as dark as possible. To do this, shawls or blankets or enameled cloth will be found useful inside the curtains and shutters. Then get a piece of cardboard, about 6 inches (15.2 centimetres) square, and lay a five-cent piece in the centre, and, with a knife, cut a hole in the card just the size of the coin. Then fasten this, with pins or tacks, over the opening in the heliostat.

We have now a slender beam of light in a dark room. Walk about and study it from different sides. See how straight this slender bar of light is; it bends to neither the left nor right, but extends across the room in an absolutely straight line. As the sun moves, turn the handle of the heliostat to keep the light in place.

Here is a picture of a dark room, in the window

of which is the heliostat. In the centre of the piece of cardboard is the small hole where the light enters the room. A boy is holding one end of a long piece of linen thread just at the bottom of the hole in the

card, and another boy has drawn the thread out straight and tight, so that it just touches the beam of light throughout its length.

Were you to try this experiment, you would see

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