Cn. VI] HELIOSTATS FOR THE MAGIC LANTERN 145 



zontal must the mirror be. As the sun gets higher and higher the 

 mirror must be raised more and more on its hinges ; and as the sun 

 seems to move toward the south as well as upward, the mirror must 

 be rotated on its axis toward the south. 



(C) For a western exposure If a western exposure is used, 

 the entire instrument should be placed pointing east and west if 

 possible. The mirror will be raised on its hinges and turned south- 

 ward early in the afternoon. As the sun sinks toward the west the 

 mirror will be made more and more nearly horizontal, and as the 

 sun seems to move toward the north as well as toward the west, 

 the mirror will finally be nearly horizontal on its hinges and rotated 

 somewhat northward. 



These movements of the mirror become intelligible if one 

 observes the position of the sun in the different periods of the day. 

 By consulting fig 86, 87, it is also clear that the mirror must have 

 different positions owing to the declination or position of the sun 

 with reference to the horizon at different times of the year. 



HELIOSTATS DRIVEN BY CLOCK-WORK 



239. Types of clock-driven heliostats. A fundamental 

 character of all heliostats is that the clock-work rotates a shaft 

 corresponding with the post carrying the hour hand of an ordinary 

 clock, and that it is this shaft which directly or indirectly gives 

 motion to the mirror. 



This shaft must be made parallel with the earths axis wherever 

 the instrument is used. 



(A) Single-mirror type. This is so constructed that the clock- 

 work gives a double motion to the mirror something as one can 

 give a double motion to a mirror held in the hands, i. e., an up and 

 down motion and a motion of rotation on the axis (fig. 77-79). 



(B) Double-mirror type. In this type one mirror is fixed at 

 the end, or the side of the clock-shaft. The second mirror is not 

 moved by the clock-work, but is set by hand at the beginning of 

 each experiment (fig. 81-84). 



As one might conclude, the second or two-mirror type is of 

 simpler construction and therefore correspondingly inexpensive. 



