20 ROYAL SOCIETY OF CANADA 
Then by projections in OA 
a.OC cos 4, = a.OB cos 8, + a BC cos 6,. 
Also O5'C? being the projec- 
tion of the triangle OBC on a plane 
perpendicular to OA 
OC. = 0C sme, 
OBY = OB amos 
BC = BC sini, 
and 
OO 6, OT. ec 2B: mee: 

A 
are the sides of the vector triangle 
OB''C"', which is OB C' when turned in its plane through 90°. 
P & 
