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 rota- 

 tion 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 direc- 

 tion. 



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. 



