upon the Minimum Temperature of the Night . 165 
most intense cold of winter, when the air was loaded with va- 
pour, not less than when it was almost entirely bereft of humi- 
dity ; insomuch, that, by ascertaining the hygrometric state of 
the atmosphere in the evening, I have no difficulty in determin- 
ing the minimum temperature of the ensuing night, the devia- 
tion seldom being beyond the limits within which the nicest 
thermometrical observations are made. 
The agreement, however, which I have endeavoured to trace 
between the minimum temperature and the point of deposition, 
is, for obvious reasons, most remarkable from the end of J uly to 
the end of December, the temperature of the year being then 
on the decline, and rendering the relative humidity greater than 
during the other half of the year ; but at no season is the devia- 
tion so great as not to indicate a mutual connection between 
them. It may also be proper to remark, that, after the tempe- 
rature of the night reaches the point of deposition, which it sel- 
dom fails to do, the check to its farther diminution operates 
most effectually at the higher temperatures ; because the quan- 
tity of vapour which passes into the liquid state being greater, 
it exerts a corresponding influence over the thermal state of the 
air, and thus prevents the lowest temperature of the night from 
ever being greatly depressed below the point of deposition. In 
the month of July, for example, when the mean point of depo- 
sition may be taken in this latitude at 45°, the quantity of mois- 
ture in the air is 0 2099 grains in 100 cubic inches ; whereas, in 
the month of December, when the mean point of deposition is 
15° lower, the quantity of moisture in the same volume of air 
is only O’ 1278 grains. So that a depression of 1° of temperature 
in the former case, would cause nearly double the quantity of 
vapour to pass to the liquid state, and evolve a proportional 
quantity of caloric, to counteract any tendency (whether from 
radiation or any other cause,) which the air might have to a 
farther reduction of temperature, I shall not occupy the pages 
of this Journal with a tedious comparison of numerical results 
in support of the general positions which I have laid down, but 
confine myself to a few interesting details of some observations 
which I made, under circumstances peculiarly well fitted to sub- 
ject the theory to a severe trial. These I shall briefly describe : 
1 
