154 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 



parallel with the clock-shaft. When the beam of light from the 

 mirror extends out parallel with these lines, as indicated by the 

 streak of light, the mirror will be in the correct position. 



245. Arranging the second mirror. For getting the light in a 

 desired direction, a second mirror is used in the path of the beam 

 extending directly northward, from the first mirror, and so arranged 

 that the light is reflected as desired (fig. 81 M 2 ). 



246. Heliostat with the mirror parallel with the clock-shaft. 



With the other heliostats described in this chapter, the clock-work 

 rotates the shaft once in 24 hours, but with this form, the rotation 

 is once in 48 hours, i. e., half the rate of rotation of the earth. The 

 clock-shaft is somewhat extended and the mirror is fixed directly to 

 the shaft and parallel with it. The mirror is therefore in a plane 

 which if extended would cut the celestial north pole (fig. 82). 



Light reflected from this mirror may be made to take any direc- 

 tion in a circle. 



247. Setting up the heliostat with the mirror parallel with the 

 clock-shaft. The heliostat is placed in a proper position for 

 receiving the sunlight. The support is made level, and the instru- 

 ment set north and south. The clock-shaft is then elevated from 

 the horizontal until it is at an angle equal to the latitude of the 

 place where it is to be used. As the mirror in this form may be set 

 to reflect the light anywhere in a circle, it is best to loosen the clamp 

 of the clock-shaft and rotate the mirror until it receives the full 

 light of the sun and reflects it in a convenient direction. Then 

 clamp the shaft to the clock-work and the mirror will follow the 

 sun. 



248. Arranging the second mirror. The second mirror is now 

 placed so that it will receive the beam from the movable mirror, 

 and then it is turned, raised, or lowered on its stand, until the light 

 extends in the desired direction. It should continue to hold the 

 light in one place so long as the sun shines on the movable mirror 

 (fig. 82). One must make sure that the position of the second 

 mirror is such that it will not shade the heliostat mirror as the 

 sun moves toward the west. 



