176 



diiFerent temperatures, is - -. The results, for tempe- 

 ratures from 0° to 35° cent, are exhibited in the last column 

 of the table. For the temperatures 0°, 5°, and 10°, they 

 agree very well with the height in which Mr, Welsh found a 

 lowering of temperature of 1° cent; and we may conclude 

 that at the times and places of his observations the lowering 

 of temperature upwards was nearly the same as that which 

 air saturated with moisture would experience in ascending. 



It is to be remarked that, except when the air is saturated, 

 and when, therefore, an ascending current will always keep 

 forming cloud, the effect of vapour of water, however near 

 saturation, will be scarcely sensible on the cooling effect of 

 expansion. Hence the law of convective equilibrium of 

 temperature in upward or downward currents of cloudless air 

 must agree very closely with that investigated above, and 

 must give a variation of 1° cent in not much more or less 

 than 330 feet. 



It appears, therefore, that the explanation suggested by 

 Dr. Joule is correct ; and that the condensation of vapour in 

 ascending air is the chief cause of the cooling effect being so 

 much less than that which would be experienced by dry air. 



The following extract of a Letter from Professor W. 

 Thomson, LL.D., &c., to the President, was also read : — 



"About two years ago I wrote to you that a metal bar, 

 insulated so as to be moveable about an axis perpendicular to 

 the plane of a metal ring made up half of copper and half of 

 zinc, the two halves being soldered together, turns from the 

 zinc towards the copper when vitreously electrified, and from 

 the copper towards the zinc when resinously electrified. 



" If the copper half and the zinc half of the ring are insu- 

 lated from one another, and if they are connected by means 

 of wires with two pieces of one metal maintained at any stated 

 difference of potential by proper apparatus for dividing the 



