WRIGHT: TRIGONOMETRIC COMPUTER 3 1 



problem: given two sides and the included angle of a spherical 

 triangle; find the third side. 



Description of instrument. — A photograph of the instrument is 

 reproduced in fig. i. It consists essentially of three concentric 

 graduated circles which can be inclined at different angles one 

 to the other. The inner circle I is horizontal and can be rotated 

 about a vertical axis. The outer circle III fits in the semicircu- 

 lar groove D and is free to turn in this bearing, the angle of turn- 

 ing being read off on the vernier at E, E; the circle in its bearing 

 can also be rotated about the horizontal axis F, F. The inter- 

 mediate circle II is not a complete circle and is attached to the 

 circle I by the horizontal axis G, G, and to the circle III by the 

 axis H. The angles of rotation and revolution of these circles 

 can be read off on appropriate verniers. The circles are equipped 

 with clamp screws and slow-motion devices for the accurate 

 setting of angles. 



The following steps are taken to solve the problem: given two 

 sides a, b, and the included angle C of a spherical triangle; de- 

 sired the third side c. 



Set the three circles at right angles to each other — all verniers 

 reading 90°. Turn circle III about the horizontal axis to the 

 position where it includes the angle C with the horizontal circle I 

 (angle read off on the vernier of circle II). Turn circle II in its 

 bearing until axis H includes an angle a with the axis F, F; 

 turn horizontal circle I about its axis until .axis G, G includes the 

 angle b with the axis H. The angle H-G is then the desired 

 angle c. The entire operation requires about one minute and 

 is simple and readily checked. 



The instrument is necessarily one of high precision, and first 

 class workmanship is required to produce a satisfactory com- 

 puter. In its construction care was taken to include adequate 

 adjustment facilities such that the instrument can be readily 

 adjusted in case of wear. The instrument is mounted on a 

 heavy cast-iron base. Credit is due to Mr. J. Jost of the me- 

 chanical staff of the Laboratory for the excellent mechanical 

 workmanship on this computer. 



