THE CONSTANTS OF THE CUP ANEMOMETER. 
785 
mentioned where it was whirled for several minutes without the anemometer, scarcely 
any motion of the balloons was observed though V = 52. Nor does the anemometer’s 
Y 
own rotation seem to have more effect, for — does not change with a given value of Y 
though v be reduced by brake friction to half its original amount ; it decreases, though 
slowly, with Y. Out of many comparisons, those given in the following table may 
suffice : — 
Table I. 
D = 75 
Brake. 
V 
V 
w 
D 
Mean V. 
Mean W. 
V 
w 
No. of 
Observations. 
0 
29-81 
12-79 
75 
29-53 
2-213 
13-34 
8 
6 
29-49 
12-66 
65 
27-15 
2-128 
12-75 
8 
12 
29-81 
12-20 
57 
25-40 
2-016 
12-60 
8 
18 
29-81 
12-71 
46 
21-66 
1-825 
11-87 
7 
24 
29-97 
1318 
39 
19-71 
1-689 
11-67 
7 
30 
29-17 
12-64 
28 
15-19 
1-370 
11-08 
7 
36 
29-25 
12-96 
23 
12-57 
1-296 
9-67 
5 
20 
10-40 
1-039 
10-01 
4 
18 
8-705 
0-923 
9-43 
4 
The three first columns belong to those single observations of the second set of 
No. I., in which the driving weight was 75, and allowing for the irregularities which I 
have described, may be considered to show that ^ is constant. The remaining columns 
relate to all in the first set of No. I., where the driving power is the same without 
regard to brake friction ; here the decrease of the ratio is obvious, though it might not 
have been evident from single observations. For instance, the first of the eight 
ratios belonging to D = 75 is 11*89, the fifth is 14'30. 
(16.) I obtained some additional information about the vortex by the air meter 
already mentioned. It is a small windmill of eight vanes (five would have been 
better), 2'8 inches diameter, connected by a stop and wheelwork with a series of 
indices registering its revolutions to a million. It is very sensitive, though less so 
than the balloons or vanes. A revolution is stated to be a foot ; but there was in 
its case a table of corrections which seemed to me so strange that I did not use them. 
Attaching it to the horizontal arm without the anemometer, and whirling for 8 m with 
V=17, I found a division = 0'867 foot. On placing the meter on the platform with 
the necessary precautions, with its axis of rotation in the plane of the arm and in the 
direction of W at 14 inches from the cross, I got in 2 ni 212 divisions with D = 35. 
This on the above scale is 1'53 foot per second, the balloons at the same time giving 
1'57 feet. It is, however, needless to turn the divisions into feet, as they serve for 
comparison. When the meter was placed to measure the radial current it gave 70 — a 
third of the other — but with this unexpected peculiarity : that during the two minutes 
it sometimes moved very rapidly and sometimes almost stopped. In another trial 
5 H 2 
