of Edinburgh, Session 1873-74. 
351 
Monday, 16 th March 1874. 
Sir WILLIAM THOMSON, President, in the Chair. 
The following Commnnications were read : — 
1. On the Resistance of the Air to the Motion of Fans. Ry 
James C. Fair weather. Esq. Communicated by George 
Forbes, Esq. (With two Plates.) 
The design of this paper is to describe the results of some experi- 
ments on “the Resistance of the Air,” which I made under the 
superintendence of Professor Forbes, in the physical laboratory of 
the Andersonian University, Glasgow. The apparatus employed 
resembles somewhat that used by M. le Chevalier de Borda, whose 
experiments on “the Resistance of Fluids” are recorded in the 
“Memoires de l’Academie Royale des Sciences.” It consists 
essentially of a wooden frame, which was firmly secured to the 
floor, and supporting a horizontal axis, at one extremity of which 
is fixed a wooden arm, on to which were bolted the vanes of different 
forms and sizes, the resistance of which it was desired to ascertain. 
A cylinder or barrel, about 3 inches in diameter, is fastened to the 
axis, whereon was wound a cord, which, being acted on by weights, 
gave a circular motion to the axis, and consequently to the arm, 
thereby carrying the vanes in the circumference of a circle of 3 
feet diameter, and causing them to impinge upon the air with 
velocities due to the force applied. The moving force consisted of 
weights of from J lb. to 20 lbs., suspended at the end of the cord, 
which passed over a guide-pulley, made fast in such a position that 
a drop of 35 feet was obtained. For each experiment the cord was 
wound on to the cylinder by hand, and afterwards abandoned to 
the action of the weight at a beat of a second’s pendulum ; and the 
vanes thereby allowed to make a definite number of revolutions, 
ascertained by a distinct mark on the cord ; the times of which 
were recorded for each observation. After a few trials the pendulum 
