42 
MR, A. MALLOCK ON EXPERIMENTS ON FLUID VISCOSITY. 
the cylinder E is another, G, and the space between E and G, and the interior of A, 
are kept filled with water in which thermometers are placed, and the temperature of 
the fluid in the annulus between E and A is taken as the mean of the temperatures 
in A and the outer annulus. 
The cylinder A has its floor about half-an-inch up from the lower edge, and during 
the experiments the space under this floor is filled with air. Thus the fluid in the 
annulus only touches the cylindrical surface of A. The axis F of the outer cylinder 
is hollow, and a rod, H, passing through a watertight joint at its upper end, carries a 
short piece of cylinder K, of the same diameter as A. The arm L at the lower end 
of H, where it projects beyond F, is held fixed, thus keeping the short cylinder K 
stationary whilst E revolves. The space between E and K is filled with mercury. 
The object of this arrangement is to cause the lower surface of the water in the 
annulus to be in contact with a surface whose velocity at any point is nearly the same 
as its own. Of course, in the mercury, in consequence of its being in contact with 
the floor of the cylinder E, there is a certain amount of circulation set up, which causes 
an outward radial drift in the mercury in contact with the floor and a return current 
flowing inwards in the mercury surface in contact with the water, but the radial velocity 
is slow compared with the circumferential, and the water in the annulus between E and 
A is very nearly in the same condition as it would be if E and A were infinitely long. 
Figs. (2) and (3) show the arrangement used in the second and third series of 
experiments respectively. 
Attached to reading telescope T was a small camera-lucida, by means of which the 
image of the divided circle could be seen on the surface of a cylinder, driven by clock¬ 
work and carrying continuous paper, P. On this paper an electric clock recorded 
seconds, and another pen and electromagnet in connection with a contact-maker on the 
axis F marked every revolution of the cylinder E. A third pen, worked by a hand 
lever, was used to follow the motion of the image of the divided circle seen on the paper 
through the camera-lucida. 
In making an experiment the usual course was to bring the wire of the telescope 
to the zero of the divided circle when everything was at rest, then to start the paper 
cylinder and mark the position of the zero on the paper. The experimental cylinder 
was then set in motion, and when the suspended cylinder had reached its approxi¬ 
mately stationary position (about four minutes after the revolving cylinder was 
started) the mark on the divided circle, which was nearest to the centre of the paper, 
was followed by the third pen for a minute or more, after which the zero mark was 
again made on the paper, and the number of the division (on the circle) followed by 
the pen was written on. In some experiments, however, the motions of the 
suspended cylinder were followed from the moment the revolving cylinder was 
started. The diagrams so made gave a permanent record of the result of each 
experiment, which could be analyzed at leisure. 
Table I. (p. 47) gives the dimensions and constants of the apparatus. 
