ILLUMINATION. 37 



volve 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 

 motion four times 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 XXX and a small spirit level. 



The plumb-line is attached to the top of the needle 

 H, and the brass plate G moved till the lead of the 

 plumb-line hangs directly over NS. At the same 

 time the card E (which should be very accurately 

 made), is placed with its upper extremity at E, the 

 lower side being parallel with NS, and the needle H 

 made parallel with the line corresponding to the 

 latitude of the place on the card. H now points 

 to the pole. 



During these adjustments, the level of the appar- 

 atus, and the north and south positions of the line 



