on the Formula for measuring Heights by the Barometer . 227 
be obtained, by taking the temperature at which f is the maxi- 
mum tension of vapour. This, by Dalton’s table, is 42° when 
f — *284. Hence in that particular case, 
.904, .qqzL 
* = 1+ -002086 (60 - 48) = V0375 = ' 274 > which C ° rre ‘ 
sponds with the maximum tension for 41°. If greater preci- 
sion be sought, the calculation may be repeated with r = 41, when 
q> would be found '278. Now, by the experiments of Gay 
Lussac, the weight of vapour is to that of air, having the same 
elasticity, as 5 to 8 ; and since the weight of 100 cubic inches of 
air is, by the experiments of Biot and Arago, equal to 82.9 
grains English, at the freezing point, and with a pressure of 
80 inches of mercury, the weight of the volume of vapour at the 
temperature t, and under the pressure <p, is, in grains, 
| x 82 9 <P -6854 <p 
30 (1 + -002086. t — .32), °*' 1 + 002086 (r —32) 
When <p = .278 and r = 41, we obtain from this expression 
•18867 grains for the weight of the moisture in 100 cubic inches 
of air, completely saturated with humidity, at the temperature 
of 41°. Lastly, since the actual temperature of the air was 60°, 
the weight of the moisture in 100 cubic inches of air, in the 
stateof dilatation it undergoes, bypassing from the temperature of 
• 1 8867 
41° to 60°, will be Y + . 002086. 60=41 ° r 17671 grainS ' 
The temperature r is the point of deposition , and is some- 
times denominated the dew point. It is scarcely necessary to re- 
mark, that this point is essentially different from the temperature ac- 
quired by the thermometer, with the moistened bulb ; nor should 
I have thought of noticing the distinction, had I not seen them 
very erroneously confounded by Mr Colebrooke, in a paper con- 
taining many judicious meteorological observations, which he pu- 
blished in the 27th Number of the Journal of Science of the 
Royal Institution. More mature attention to the subject, will, 
I am satisfied, convince a person of Mr Colebrooke’s discernment 
and philosophical attainments, that the temperature of the 
moistened thermometer can never reach, by a very considerable 
range, especially in dry air, the point of deposition. In the above 
example, which I have so amply illustrated, the point of deposi- 
