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158 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI
§ 250. If one were looking at the north pole of the earth from a position along the earth's axis, the direction of the earth's rotation would appear in a direction opposite to the hands of a clock or watch. To compensate for this, a mirror to hold a spot of sunlight in one position would need to be rotated around an axis parallel with that of the earth, but in an opposite direction to the earth's rotation, that is in the clockwise direction.
§ 251. At the equator, the clock-shaft must be horizontal in order to be parallel with the earth's axis. The clock-shaft must be turned from east to west. This can be accomplished either by a clock-work located at the southern end of the shaft turning in the clockwise direction as in fig. 77-79, or by a clock-work located at the northern end of the shaft turning in a counter-clockwise direction.
§ 252. At the north pole of the earth, the axis of rotation of the shaft would be vertical and the direction of rotation as seen from above, would be clockwise.
At the south pole the axis would also be vertical and the direction of rotation would be clockwise as seen from below — i. e., from the north — or counter-clockwise as seen from above.
§ 253. A heliostat constructed for the southern hemisphere would be exactly similar to one for the northern hemisphere except that the clock-shaft must rotate in the counter-clockwise direction, that is, from right to left.
§ 254. Setting up a heliostat in the southern hemisphere. — If
a heliostat is properly constructed for the southern hemisphere it is set up at any given south latitude by arranging the instrument due north and south with the free end of the clock-shaft pointing south. Then the clock-shaft would be elevated above the horizon a number of degrees corresponding with the south latitude. This would make the clock-shaft parallel with the earth's axis and it would point toward the celestial south pole (fig. 85). Indeed, the entire procedure for getting the light in the desired direction, the use of the condenser and projection objective, etc., is exactly as for the northern hemisphere.