580 
Proceedings of Societies. 
[August, 
a plank of wood drawn through the water of a canal. It is 
desirable to have a set of laboratory experiments, however, as 
the conditions can be varied more than can be done by such 
methods, and for this purpose the author had designed a special 
apparatus. In Mr. Froude’s experiments there was a practically 
unlimited mass of water and a definitely limited extent of solid 
surface ; and his results are not free from certain anomalies. 
The author thought it might be instructive to try the other case 
of a limited mass of water, and a virtually unlimited surface ; a disk 
in rotation gives such a surface. In some respeCts a cylinder 
would (as suggested by Prof. Ayrton) be the simplest to treat 
theoretically, but there are experimental difficulties in its way. 
The apparatus of the author consists of a metal disk rotated on 
a vertical axis in a vessel of water ; and the problem is to 
determine its resistance to rotation, since this will be equivalent 
to the water friction upon it. Within the outer vessel is placed 
a thin copper chamber, the diameter of which is unalterable, but 
the depth is variable at pleasure. The disk is placed concentri- 
cally inside the chamber, where there are two cheese-shaped 
masses of water, one above and one below the disk, which are 
dragged into rotation next the disk and retarded next the sides 
of the pan. The couple required to rotate the disk is equal to 
the couple exerted by the disk or the fluid when the motion is 
uniform. Hence the tendency of the chamber to rotate is 
measured by suspending the latter from three wires in a manner 
similar to the bifilar suspension of magnets. An index marks 
whether it rotates or not on a graduated scale, and a weight 
suspended by a cord measures the force required to keep the 
index at zero. Let M be the moment of the fractional resistance 
of the disk ; N the number of revolutions per second. Then 
M = CN*, where C and x are constants. The author has ob- 
tained a number of results, which are, however, not yet ready 
for publication. He mentioned, however, that a rough cast-iron 
disk has a frictional resistance almost exaCtly as the square of 
the velocity ; whereas a turned brass disk gave a value of x 
decidedly less than 2. The resistance is a little greater when 
the mass of water is larger. These results were calculated for 
a speed of 10 feet per second. The author hopes to try the 
effedt of temperature, &c., on fluid friction, and viscous as well 
as thin fluids. 
Prof. Unwin also exhibited a piece of apparatus with which 
he hopes to study the stress of rivetted plates under shear by 
means of elastic substances, such as caoutchouc. He purposes 
to stretch the rivetted caoutchouc and photograph the appearance 
of stress-lines upon it. 
Lieut. G. S. Clarke, R.E., explained the process invented by 
Prof. McLeod and himself for determining the absolute pitch of 
tuning-forks (see “ Monthly Journal of Science,” 3rd series, 
vol. I., p. 257). 
