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. Care

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.

inch (12 millihalf-round piece, This piece must inches (14 centi

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 101 inches (26.7 centimetres) long, 61 inches (16 centimetres) wide, and metres) thick. Then make a flat, shaped like the figure marked G. be inch (7 millimetres) thick, 5 metres) 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

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 0. 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.