352 Proceedings of the Royal Physical Society. 



radius equal to '551 of the unit, describe a circle, and mark 

 it, anywhere, a ; and from the same centre, with radius '293 

 of the unit, describe another circle, and sign it c. These 

 are required, because ac, he will need to be viewed re- 

 spectively, in two different aspects. We may distinguish 

 the quadrants, as before, by roman numerals — v., VI., vii., 

 VIII. — taken counter-clockwise, starting with the front left 

 hand one. Where ox cuts the primitive between v. and 

 VI. and VII. and VIIL, mark a, and mark h in the corre- 

 sponding position at right angles to these. We may devote 

 quadrant VI. to the projection of the prism zone. Join the 

 point where the circle a cuts ox, with the right hand &, 

 and produce the line to cut the primitive in v. On this line 

 erect a perpendicular radial to 0, cutting the primitive at 

 a point to one marked m (110), and extended outward to 

 any convenient length M. Parallels to oh, oa are drawn to 

 meet M, and may be figured as 3 and 4, for convenience 

 of reference, if need be. Their use is, of course, to determine 

 the positions of the poles in the prism zone, and has been 

 sufficiently described already. The distances on the primi- 

 tive are to be marked off in corresponding positions in 

 each quadrant, as before. 



66. The poles on eh have next to be laid down. We 

 can do this by a direct method presently to be described ; 

 but at this stage it is better to project their relative posi- 

 tions on to oh, and afterwards carry them on to their correct 

 position by arcs of circles passing through those points 

 and a. Quadrant vii. is devoted to this, and in this we 

 are supposed first to view the sphere of projection from 

 the side at a. From h in vii. draw a line through the circle 

 c, where it cuts ox between vii. and viiL, and produce the 

 line to cut the primitive in viii. From o erect a perpen- 

 dicular to this line, and produce it beyond the primitive, 

 signing it E. A radial from the point where E cuts the 

 primitive through oh to the front a will cut oh in the 

 position from which e (Oil) can afterwards be readily drawn. 

 Also draw parallels to oa, oh, to meet E, and figure them 

 5, 6, for convenience of reference. They, of course, are to 

 be used for measuring the proportional lengths corresponding 



