THE CONSTANTS OF THE CUP ANEMOMETER. 
787 
Table III. 
No. 
N. 
A. 
b. 
V. 
V. 
w. 
I. 
28 
45 
78-8 
18-15 
6-43 
1-421 
II. 
28 
46 
18-15 
6-57 
III. 
2817 
45-5 
71-4 
1815 
6-50 
1-566 
IV. 
2817 
45-25 
66-4 
18-26 
6-46 
1 689 
V. 
28 
45 
18-15 
6-43 
VI. 
28 
45 
66-4 
18-15 
6-43 
1-689 
VII. 
28 
44-5 
18-15 
6'36 
VIII. 
28 
45 
68-2 
18-15 
6-43 
1-643 
IX. 
27-5 
44-8 
66-4 
17-82 
6-40 
1-689 
X. 
27-5 
44 
17-82 
6-28 
W becomes sensibly constant after four minutes, and this time was allowed in 
subsequent experiments. 
(18.) On the whole, the estimation of W is a weak, perhaps the weakest, part of 
this investigation ; and what I have observed respecting this air vortex makes me 
doubt whether the conclusions deduced from such experiments can be fully applicable 
to real wind. Still, it may be hoped that they will give useful information. 
(19.) Friction is an important agent in anemometric motion, and one in which 
I found more difficulty in the measuring than I had anticipated. It was fourfold : 
(1°) that due to the instrument’s weight ; (2°) that caused by the application of the 
brake ; (3°) that caused by the lateral pressure of the axle on the upper bearing, 
caused partly by the action of the wind, partly by the force which, as in the gyro- 
scope, is required to change the plane of the anemometer’s motion ; and (4°) that 
due to centrifugal force. There are three degrees of this resistance ; the highest is 
that which occurs when a machine is started from rest ; the intermediate is that 
which acts during continuous motion, which is the one to be considered here, and the 
least which prevails when the parts that rub are put into a state of vibration by 
tapping or jarring, which may be sometimes only half the first. Friction occasionally 
varies from the effect of temperature or chemical change on the oil used to lubricate 
the bearings, or else of dust finding its way into them (as sometimes happened). 
(20.) Nos. 1 and 2 were measured thus. A cast-iron disc, weighing 103T7 oz., 
was fixed on the shaft in place of the anemometer ; on its circumference a groove was 
cut whose bottom was TO foot diameter, in which a fine silk thread was wound; to 
this, weights were appended till when the disc was barely moved they continued the 
