tion of the manner of making and using that instrument : The sun, in his daily apparent path through the sky, moves as though he were fixed to the surface of a vast globe, which makes a daily revolution around an axis. This axis is found by drawing a line from a point near the pole-star to the centre of the earth, and then continuing this line beyond the earth till it meets the heavens in a spot which can be pointed out only by those who live south of the earth's equator. This line is the axis around which the sphere of the heavens appears to revolve once in a day. The knowledge of this motion of the sun enables us to construct an instrument with a movable mirror which will reflect his beams in one direction from sunrise to sunset. The Figs. 1 and 2 are drawings of the heliostat. scale at the bottom of Fig. 1 gives in inches the size of the parts of the lower drawing. H is a round wooden rod, which we call the polar axis of the heliostat, because it points toward the pole-star when the instrument is in the proper position for use. This axis turns freely in a hole in the board A B, and in the block K. A wooden washer M, which is slid over the axis and is fastened to it, rests on the block K, and thus keeps the axis from slipping down. The end H of the axis has a slot cut in it, and a semicircle of wood G, which is screwed to the back of a board carrying the mirror N, turns in this slot around a carriage-bolt, as shown in the figure. This movable mirror is fastened to the board either by strings or by elastic bands, which go around the ends of the board and mirror. The mirror should be of silvered glass, not of common looking-glass. It is, as stated in Fig. 2, 9 inches long and 6 inches wide. Since the sun in his daily course through the heavens appears to move as though attached to the surface of a sphere, which revolves on an axis parallel to the polar axis of the heliostat, it follows that, if we tilt the mirror N so that the sunbeam which strikes it is reflected downward in the direction of the polar axis H, then, by simply turning this axis with the sun as he moves in the sky, we can keep his rays constantly reflected in that direction. The dotted line and arrow going from the mirror N to 0 show the direction of the reflected rays. But this is not a convenient direction in which to have the sunbeam, so we fix at O another mirror, 6 inches high and 5 inches wide, which reflects the beam from 0 to B, through a hole of 5 inches diameter cut in the board A B. Brackets, 14 x 12 inches, with their 12-inch sides screwed to the board A B, support a shelf D, which holds the mirror 0. Each morning in the year the sun appears on the horizon at a different point on the celestial sphere, so that on different days we have to give the mirror N a different tilt toward the sun. At the equinoxes, that is, on the 20th day of March and September, the rays fall at right angles to the axis H, as shown in Figs. 1 and 2, and the mirror in Fig. 1 is placed at the proper tilt for those days. In Fig. 2 the tilt of the mirror is also given for the days of the summer and winter solstices. As we go north, say to Boston, the north star rises to a greater height above the horizon, so the axis of our heliostat at Boston must stand more upright than at New York, and have the position marked "42° 22′, Boston." Going south, say to New Orleans, we shall see the pole-star shining above the horizon, at a height which is one-fourth less than the height it appears at in New York; therefore, at New Orleans, the polar axis of the heliostat is lowered into the position marked "29° 58', New Orleans." So we see that in different latitudes the axis of our heliostat has to be placed at different angles with the horizontal line. In order that the instrument may work correctly, the angle which it should make with the horizon is the same as the latitude of the place. These are the angles written before the places named in Fig. 2. These changes in the slant of the polar axis for different latitudes need like changes in the shape of the block K; but if one first draws the correct line in which the axis goes through the board A B, the block K can be formed without trouble. EXPERIMENT 1.-To place the heliostat in position for use, we raise the sash of a southern window, and secure the board A B between its jambs, with the mirrors outside and the polar axis inside the room. With a shawl or blanket closely cover that part of the window above the board A B, so as to keep out all light except what comes into the room through the hole B. The movable mirror is now turned toward the sun, and tilted so that the beam from it is reflected by the fixed mirror 0 into a horizontal direction, and at right angles to the board A B. If the window faces the south the heliostat will work with entire success. If the window does not truly face the south, then the board AB should be tilted sideways till it does face that direction, and any opening thus made between the board A B and the window-sash may be closed with a strip of wood. THE WATER-LANTERN. Fig. 3 represents a wooden box containing a mirror placed inside at an angle of 45°, and supported by wood slats fastened to the sides of the box. The side of the box opposite the mirror is open. In the top of the box is a round hole 5 inches (12.7 centimetres) in diameter. In this hole rests a hemispherical glass dish, 51⁄2 inches (14 centimetres) in diameter, made by cutting off the round top of a glass shade. At the back of the box is a wooden slide carrying a horizontal shelf on its top. This slide has a long slot cut in it, and, by means of a bolt and nut fastened to the back of the box, it can be made fast at any required height. This slide is 16 inches (40.6 centimetres) long, 5 inches (12.7 centimetres) wide, and inch (19 millimetres) thick. The shelf is 7 inches (17.8 centimetres) long and 5 inches (12.7 centimetres) wide, and has a hole 34 inches (8.3 centimetres) in diameter cut in its centre. A block of wood is fastened to the back of the box in the |