168 On the Influence of the Humidity of the Atmosphere 
sphere, at least where the surface of the earth affords a sufficient 
supply of moisture ; and that the latter, in its turn, re-acts upon 
the temperature, and prevents it from being in any case greatly 
depressed below the point of deposition. In cases where this 
check operates in a feeble manner, which must always happen 
when the atmosphere is reduced to great dryness by the trans- 
lation of air from a higher latitude, or across elevated table-land, 
the lowest temperature of the night must always be considerable. 
In the months of April and May, for example, the air over our 
island is generally extremely dry, on account of the cold and 
parching north-east winds which prevail at that season, and the 
minimum temperature of the night is accordingly often reduced 
below the freezing point. Last season afforded a striking illus- 
tration of this. The spring was throughout extremely dry, and 
the minimum temperature descended almost regularly every 
night below the freezing point, the only exceptions having oc- 
curred when a partial change took place in the humidity of the 
air. Even on the day of the summer-solstice, though the 
maximum temperature rose at Perth to 70°, yet the moisture in 
the atmosphere being so scanty that the point of deposition was 
so low as 28°, the temperature of the ensuing night sunk to 80° ; 
nor did the self-registering thermometer cease to approach the 
freezing point, until the absolute humidity of the atmosphere 
became permanently and considerably augmented. 
From the remarks which I have made on this important sub- 
ject, a method of determining the average hygrometric state of 
the air, by means of the minimum temperature, readily sug- 
gests itself, it being evident, that places, where there is little dif- 
ference between the greatest heat of the day and the extreme 
cold of the night, must be exposed to a moist atmosphere, while 
those situations which possess a wider range, with respect to the 
diurnal returns of temperature, must enjoy a corresponding de- 
gree of dryness, and probably, too, of salubrity. But it is to 
the influence of moisture, as the means of checking the diminu- 
tion of temperature during the absence of the sun, the great 
source of light and heat to our globe, that I wish chiefly to 
draw the attention of meteorologists. After the sun has sunk 
below the horizon, the temperature of the air, which had been 
previously declining with his altitude, undergoes a more rapid 
