? 
Double Altitude Problem. 47 
(which is to find the volume of saturated air at 32°, that of dry 
air at zero being unity), he has, 30: 30°216:: 1: 1:0072, which, 
added to the expansion = ‘07802, gives 1:08522. 
The process ought to be 30: 30°216:: 107802: 1:08578. 
In my own comparison I assumed that the air was saturated 
at zero; and though the formula gives all the numbers a little 
in excess, they are nearer than those resulting from Mr. 
Daniell’s calculations. 
If these principles of the mixture of vapour with air be cor- 
rect, a cubic foot of dry air, of the temperature 7, will be sa- 
turated by a grains of vapour of the same temperature. 
Hence, if 2 be the temperature of the point of deposition, and 
¢ the temperature of the evaporating surface, we shall have 
£ FRE IN pF, 2 
abe ( 450+ ¢ pope) Pay 
& St = ‘ 
5600 a ( Feoeca as a) = E, or the evapo- 
tion from a surface one foot square in grains per minute. 
As ¢ is only the temperature of the evaporating surface, the 
E 
general temperature will be T=¢ + —. 
The dynamical question respecting the velocity with which 
vapour will rise from the evaporating surface remains to be 
considered, and will most likely give employment to some of 
yous eaders. Tuomas TREDGOLD. 
P.S. My thanks are due to Canpour for his references 
to the preceding corrections of Dr. Ure’s results: I had over- 
looked them in the one Journal, and the other I do not regu- 
larly see. 
VI. Reply to the Remarks of Mr. Rippxe on the Double Al- 
titude Problem. By James Burns, Esq.* 
To the Editor of the Philosophical Magazine and Journal. 
Sir, 
R. Riddle in his concluding remarks on my solutions of 
the problem of double altitudes, takes it for granted that 
“ we are perfectly agreed,” though there is not a single sylla- 
ble in my communication (nor has he furnished a single proof) 
* [We had hoped that this controversy would have been concluded in our 
preceding volume, and shall be well pleased if our correspondents will now 
allow it to terminate.—-Ep1r.] ba 
a 
