m 
and the deposition of Dew. 
From this Table it appears, that when the difference between 
the temperatures of the two metrical cubes of air amount to 
two centesimal degrees, the quantity of vapour deposited by a 
metrical cube of the mixed air, amounts only to ^th of a grain, 
whereas the union of similar volumes at the temperatures of 4° 
and 20°, occasions a precipitation of 12 grains. The greater 
the difference, therefore, between the temperatures of the equal 
masses of air, the greater will be the density of the mist formed 
by their union. 
It does not follow, however, that nature, in the infinite di- 
versity of her operations, is confined, as in the particular case 
here considered, to the mingling together of equal volumes of 
perfectly saturated air, or of air containing a less proportion of 
moisture. Two or more parts of one particular temperature, 
and with any assignable degree of moisture, may be blended 
with other proportions of air, of a different temperature, and 
having other relations of moisture ; and from such will arise 
mists of every variety of density. 
Circumstances may be favourable to the deposition of dew 
on the borders of rivers, without contributing to the formation 
of mists. The atmosphere contains, at all times, some mois- 
ture ; and, therefore, when the relations of the temperature of 
the air and land are suitable, and a clear and tranquil sky pre- 
vails, a deposition of dew of some degree or other will take 
place. But, although the relations of temperature between the 
land and water may be favourable to the formation of mist, it 
by no means follows that the union of volumes of air, of un- 
equal temperatures, will produce a visible condensation *. In 
some examples which I have witnessed, the extreme tenuity 
of the mist has indicated, that the circumstances of tempera- 
ture and vapour were such as just to admit of its formation. 
The deposition of dew must always precede the formation of 
mists. This will appear evident, when we consider the prin- 
ciples to which each owes its origin. Suppose at some moment 
an equality of temperature to take place between the water, the 
* See Dr James Hutton’s paper on the Theory of Rain, Vol. I. p. 47. of the 
Transactions of the Royal Society of Edinburgh. 
VOL. IX. NO. 18. OCT. 1823. 
E 
