NEW FORCE ” OF M. J. THORE. 
463 
u 
Barometer — 767 ram. 
No. of 
experiment. 
Pressure. 
Direction. 
Maximum speed of 
one revolution. 
No. of 
revolutions. 
Exciting agent. 
54 
mm. 
400 
Negative 
47 seconds 
2-25 
Hot water 
55 
338 
50 „ 
2-15 
55 
56 
220 
52 „ 
1-75 
55 
57 
178 
61 „ 
1-50 
5 5 
58 
129 
130 „ 
1-00 
5* 
59 
100 
160 „ 
0-75 
55 
60 
80 
Slow 
0-75 
55 
61 
50 
0-25 
55 
62 
30 
55 
Very slow 
0-20 
55 
63 
20 
Still slower 
o-io 
55 
64 
14 
, # 
Gas flame 
65 
14 
55 
Just visible 
. , 
Hot spiral 
66 
14 
0 
No movement 
, , 
Hot water 
67 
8 
0 
0 
0 
'Gas flame 
Hot spiral 
68 
4 
0 
0 
0 
Hot water 
55 
69 
3 
0 
0 
0 
55 
70 
1-5 
Positive 
50 seconds 
2-0 
Gas flame 
71 
0-75 
20 „ 
3-00 
55 
72 
0-50 
700 
55 
73 
0-30 
55 
, . 
10-00 
Hot spiral 
74 
0129 
• • 
13-00 
75 
0-0495 
55 
• • 
13-00 
55 
It will be observed, on comparing these results with those obtained in 1875,* that 
the neutral point here is between 8 ra m, and 3 mm., whereas in the former case it 
was between 0'8 mm. and 0‘3 mm. I, however, attach little importance to this, as 
the older apparatus was much more sensitive than the one now used.t The important 
fact is that in each case the direction changes at a high exhaustion, and then the 
movement becomes five times as strong as it was originally. 
The motive force jraoducing these rotations is, at high exhaustions, the molecular 
impacts between adjacent surfaces of the suspended cylinders excited by the radiation 
falling on them from the hot water, hot spiral, or a candle (which is equally effective). 
* Loc. cit. 
t “ The barometric position of the neutral point dividing attraction from repulsion varies according 
to circumstances; among these may be mentioned the density of the substance on which radiation falls, 
the ratio of its mass to its surface, its radiating- and conducting-power for heat, the physical condition 
of its surface, the kind of gas filling the apparatus, the intensity of radiation, and the temperature of 
the surrounding atmosphere. When the surface exposed to radiation is pith, the neutral point is some¬ 
what low. I have had it vary between 50 millims. and 7 millims. below a vacuum. It is, however, 
impossible to ascertain exactly; for a point of rarefaction can be obtained at which the warm fingers 
repel and incandescent platinum attracts. With a heavy metal in the form of a sphere, so as to expose 
the smallest surface in proportion to the mass, I have not attained the neutral point until the exhaustion 
was within a very small fraction of a millimetre; whilst, if the metal is in the form of thin foil, the 
neutral point may easily be got lower than with pith.” “ On Repulsion resulting from Radiation. 
Part II.” (March, 1875.) ‘ Phil. Trans.,’ vol. 165, p. 540. 
