36 



ILLUMINATION. 



required height, is placed in front of the mirror K and 

 generally on its left hand -side. 



A stout card E- whose sides are the same length 

 as the needle, is divided into degrees, minutes, and 

 seconds. 



A small plumb line, P, a spirit level and a compass 

 complete the apparatus. 



Before describing the adjustment of the heliostat, 

 it is necessary to call to mind the following facts. 



The sun moves apparently in a circle of which the 

 earth is the centre and the line joining the poles PP' 

 the axis, fig. 18. 



FIQ. 18. 



At any place on the earth's surface a line AO 



