226 Royal Society :— 
If in either of these groups we separate into two classes the cases 
in which the elasticity was highest, from the cases in which it was 
lowest, and multiply the mean of each with the corresponding mean 
of the number of the revolutions of the anemometer, their product 
is nearly a constant, thus showing that the velocity of ascent of the 
atmospheric vapour is inversely as its elasticity ; and hence it follows 
that the velocity of the ascending current in the tube varies inversely 
as the density or elastic force of the vapour suspended in the atmo- 
sphere. ‘This was rendered evident by the aid of Tables appended 
to the paper. 
When the mean elastic force of vapour calculated from the dry 
and the wet bulbs is multiplied by the constant, 13°83, the result 
gives the whole amount of water in a vertical column of the atmo- 
sphere in inches; it follows therefore that when the difference of 
temperature between the external air and that in the tube, as shown 
by the differential thermometer, is constant, the velocity of the 
current in the tube varies inversely as the weight of the vapour 
suspended in the atmosphere. 
In an Appendix the author describes some additional experiments, 
made with the view of ascertaining whether the readings of the 
differential thermometer were mainly due to actual changes of tem- 
perature within the tube, or to extraneous causes acting on the 
external bulb. He found that when the external bulb was covered. 
with woollen cloth or protected by a zine tube of about 4 inches 
diameter and 6 inches long, the temperature of the bulb was 
increased about 2° on the scale of the instrument, and that when 
they were removed the prior reading was restored, while the number 
of revolutions of the anemometer per minute was not appreciably 
affected by the change. This explains why the readings of the dif- 
ferential thermometer varied from 33°:0 to 33°°5 as described in the 
paper, without producing a corresponding change in the velocity of 
the anemometer. 
For the purpose of obtaining a. more correct estimate of the 
influence of a given increase of heat within the tube, the author 
introduced into the tube at its lowest extremity, a phial containing 
eight ounces of water at the temperature of 100° Fahr., corked so 
that no vapour could escape. The result showed that in thirteen 
observations a quantity of heat equal to an increase of one-tenth of 
a degree on the scale of the differential thermometer, was equivalent 
to a mean velocity of the anemometer of 3°6 revolutions per minute, 
the greatest number being 3°8, the least 3°3 per minute. 
These observations render it still more evident, that if a higher 
temperature within the tube had been the main cause of the revolu- 
tions of the anemometer, the variations in their velocity would not 
have been in such exact relation to the elastic force of the atmo- 
spheric vapour, as has been shown to be the case. They also lead 
to the inference, that the apparent excess of heat within the tube 
alluded to by the author in his Paper read before the Society in 
1855 did not really exist, and to the conclusion that, if such excess 
had been present, the anemometer would not have been brought to a 
state of rest by depriving the air of the room of a portion of the 
moisture ordinarily suspended in it. 
