332 
PEOFESSOE ^Y. THOMSON A2sT» DE. J. P. JOULE 0^'' THE 
of air among themselves, which Professor Stokes has proved to exist, by his researches 
on the motion of pendulums. We may easily apprehend that in such experiments as 
our last, the entire face of the disc is covered with a film of air, which revolves along 
with it at very slow velocities. As the velocity increases there will still be a film of air 
adhering to the disc, but with the difference that it will be constantly replaced by fresh 
stopped air, the thermal efiect of which will ultimately be the only recognizable pheno- 
menon. 
A very interesting and important branch of our subject was to inquire into the 
thermal phenomena which take place at the surface of a sphere passing rapidly through 
air. Some of our experiments on this subject have been made by blowing air from a 
large bellows against a ball ; others by whirhng a ball or sphere in the air by means of 
the apparatus already described. We shall commence by describing the latter, in some 
of which a thermo-electric junction was employed, and in others an ether thermometer. 
Seeies XV. — Wooden ball 2 inches in diameter, with a thermo-electric jimction 
of fine copper and iron wires made even with the surface. 
Position of the junction in respect 
to the direction of motion. 
yelocities in alternate 
experiments. 
Difference of 
thermal effect. 
r 
73-6 and 23-1 
0-269 
In front, or anterior < 
118-4 and 23-1 
0-517 
L 
141-5 and 39-5 
0-745 
r 
74 and 28-5 
-0-146 
At the side, or equatorial. ..< 
115 and 26-3 
0-283 
1 
120 and 40 
0-020 
r 
71-5 and 25 
0-093 
In the rear, or posterior ...>^ 
112-4 and 19'3 
0-414 
L 
113-7 and 42 
0-280 
In the above experiments differential results for the several pair's of velocities are 
alone given, so that, although one of the quantities has a negative sign, there is no proof 
of actual cooling effect. In the next experiments we whirled a thin glass globe, 
3'58 inches in diameter, placed at a distance of 38 inches from the axle of the appai'atus. 
The small bulb of an ether thermometer was kept in contact rrdth the glass. 
