466 
MR. W. CROOKES OK THE SUPPOSED 
No. of 
Pressure. 
Direction of rotation 
Xu inter of revolutions per minute. 
experiment. 
of vanes. 
millims. 
76 
760 
/ 
Positive 4 
13 
/ 
77 
200 
Positive 
7 
00 
1^ 
100 
/ 
Positive <■ 
2 
79 
/ 
30 
Positive 
Very slight when tapped 
80 
20 
0 
No movement 
81 
10 
0 
No movement 
82 
1 
0 
No movement 
/ 
83 
700 M* 
Negative -> 
40 
/ 
84 
450 
Negative 
/ 
30 
85 
300 
/ 
N egative 
20 
86 
200 
0 
No movement 
87 
138 
0 
No movement 
/ 
88 
110 
Positive <$- 
/ 
Positive <$- 
18 
89 
98 
33 
90 
54 
Positive 
60 
91 
17 
/ 
Positive -4r 
150 
/ 
92 
10 
Positive 
600 
93 
5 
/ 
Positive -4r 
1000 
/ 
94 
2-5 
Positive -4? 
/ 
Positive 
/ 
Positive 
Increasing, but too quick to count. 
95 
0-5 
J? J 5 
96 
0-1 
Some points in this series of experiments are noteworthy. The vanes were 
arranged at such a slope in relation to the heated ring that the effect of the rising 
current of hot air should be a maximum. Owing to this, the first action of heat is to 
drive the vanes round in the positive direction, in opposition to the tendency to 
negative rotation which is almost always observed with air of moderate density. 
This antagonism lasts until a pressure of about 25 mm. is reached, when the two 
opposing forces balance and no movement is observed. After this point is reached 
the negative rotation continues till a pressure of about 250 M is reached, when it 
dies out, to be succeeded at an exhaustion of about 100 M by the positive rotation 
which all experiments show to be the natural direction for high vacua. The neutral 
point arrived at in Experiments 86 and 87 is, I believe, the analogue of the neutral 
* M = one-milliontli of an atmosphere, or 0'00015 mm. At low exhaustions I speak of millimetres of 
pressure, but at high exhaustions I prefer to count in millionths of an atmosphere. The inconvenience 
of using two units of measure is less than that of employing one system for both ends of the scale. 
