176 
12 • 38 
different 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 l°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 
