ILLUMINATION. 35 



on a level with the top of the glass plate, and in 

 a direct line with NS. 



A wooden pillar F, the top being cut obliquely is 

 placed as shown ; on it is fastened a thick brass 

 plate, by a binding screw, B, passing through the 

 slot at one extremity, the other extremity being per- 

 forated to receive a knitting needle, H. 



This Knitting Needle must be perfectly straight, 

 and work very smoothly in E and G. On it between 

 E and G a boxwood wheel, I, four inches in circum- 

 ference is fixed, and above G a small mirror K. 



A motion four times as slow as that of the hour 

 hand of the clock is imparted to the needle and thus 

 to the mirror, by a silk cord passing round the box- 

 wood wheels M and I. 



An elastic band and a small hook is the best 

 means of keeping the cord tight, it should always 

 be detached when the instrument is not in use. 



Another and larger mirror, L, mounted to allow of 

 its rotation in all directions, and elevation to any 

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

 generally on its left hand-side. 



A stout card R 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 pole PF the axis, fig. 18. 



d2 



