462 



mometer. While the differential thermometer indicated the same 

 relative differences hetween the heat of the atmosphere within and 

 without the tube, the velocity of the revolutions was found to vary 

 considerably. This variation was discovered to be chiefly, if not 

 wholly, dependent on the elasticity of vapour, due to the hygro- 

 metrical state of the atmosphere, as estimated from the dry- and the 

 wet-bulb thermometers, and calculated from the tables of Regnault. 



240 observations were recorded and afterwards separated into 

 groups, each group comprising those in which the differential ther- 

 mometer gave the same indication. 



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 zinc 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 



