ILLUMINATION. 37 



drawn pointing to the pole will be parallel to PF, as 

 the pole is infinitely distant. 



If OC be now drawn and TT' at right angles to it 

 and tangent to the place, the angle AOT, i.e. the 

 altitude of the pole = the angle OCF, i.e. the latitude 

 of the place. 



Now since the sun completes a revolution in 24 

 hours it is evident that a mirror fixed at an angle 

 with the horizon equal to the latitude of the place, 

 will point to the pole, and if .nade to revolve by 

 clockwork once in 24 hours will always face the sun. 

 If instead of facing the sun it is required to throw its 

 reflection on a given point, we must then employ 

 another mirror to receive the reflection from the one 

 looking towards the sun, and give the latter a mo- 

 tion twice as slow to enable it to keep the reflected 

 ray in position on the second mirror. It is on this 

 account the boxwood wheel on the needle is four 

 times the circumference of that on the hour axle of 

 the clock. 



When adjusting the heliostat it is best to fix it 

 permanently in position. The window of some room 

 looking towards the south and devoted entirely to 

 this purpose, should be chosen. 



The stand is placed on a firm support at the win- 

 dow, (which should be open), so that the line NS 

 points north and south, this can easily be done by 

 means of a compass laid on the glass plate D over 

 NS. The levelling of D being carried on at, the 

 same time, by means of the screws + + + and a 

 small spirit level. 



