CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 149 



Sometimes the co-latitude and the vertical line are used instead 

 of the horizontal line and the latitude. This brings about the same 

 result, for if the clock-shaft is vertical to start with, it must be 

 tipped over toward the north from the vertical an amount equal to 

 the co-latitude. That is, in Ithaca, it must be inclined 47.5 degrees 

 from the vertical. 



In general the clock-shaft must be inclined upward from the 

 horizontal, a number of degrees corresponding with the latitude at 

 the place of observation or it must be inclined downward toward 

 the north from the vertical position a number of degrees correspond- 

 ing with the co-latitude. The sum of the latitude and the co-lati- 

 tude in every case equals 90 degrees. (See fig. 85 and its explana- 

 tion) . 



INSTALLATION OF A SINGLE-MIRROR HELIOSTAT 

 241. Setting up the heliostat. 



1. In the first place the instrument must be leveled and 



arranged accurately in a north and south direction. 



2 . The clock-shaft must next be elevated to an angle correspond- 



ing with the latitude of the place where it is to be used 

 ( 240) so that it will point toward the north star. It will 

 then be parallel with the earth's axis (fig. 85). 



3. To give the proper angle to the mirror, depending on the 



declination of the sun, and to get also the correct local 

 time, loosen the clamp holding the clock-arm (fig. 79 c) 

 and turn the clock-arm toward the sun until the light 

 shines through both sights along the line q-p. Then 

 clamp the set screw at c. 



4. To get the spot of light in the desired place, loosen the clamp- 



screws in the position arm F-B and below H in the rotating 

 collar and then raise or lower the shaft o, fig. 79 and rotate 

 the position arm around the column A till the light is 

 reflected where it is wanted, then tighten the clamping 

 screws and the clock-work should cause the mirror to move 

 so that it will reflect the beam of light in the same place so 

 long as the sun shines on the mirror. 



