42 Prof. Marcet on the Variations of the Temperature 
that is to say, beyond the latter elevation the increase of 
temperature ascending was not generally very sensible, how- 
ever clear the weather might be. I could adduce a great 
number of observations in support of this assertion ; among 
others, those of the Ist and 2d August, of the 8th and 18th 
September, of the 3d and 4th October, &c., in which the li- 
mit of the increase of temperature was always found below 
100 feet. We shall afterwards see that in winter, especially 
when the weather is not very clear, the increase of tempera- 
ture at the moment of the rising and setting of the sun, is 
most frequently not observable except from the surface of the 
earth to the height of 5 or 6 feet ; from that elevation up to 
a height of 100 feet, the temperature remains uniform, and 
sometimes even goes on diminishing in proportion as we ascend. 
4TH QuEsTIon.—Does the inercase of temperature which takes place 
as we ascend at certain periods of the day, vary according to 
the different seasons of the year ? 
The answer is in the affirmative ; for although I have not 
found conspicuous differences between the summer and au- 
tumn, such is far from being the case with the winter season, 
which has presented to me some remarkable results in this re- 
spect, especially when the surface of the earth is covered with 
snow. 
Viewed in relation to the increase of temperature as we 
ascend, winter differs from the other seasons of the year in 
the two following respects :— 
1st, The difference of temperature between the strata of 
air adjoining the earth is much more considerable than at 
any other period of the year; such is the result of the series 
of observations made this year during the months of De- 
cember and January. The maximum of difference observed 
was on the 20th January, at half-past eight in the even- 
ing, the weather being perfectly calm and clear, and the 
earth covered with snow to the depth of about a foot. This 
difference amounted to nearly 8° for a change in an ele- 
vation of 50 feet; the thermometer, at the height of 2 feet, 
indicating—16°.2, and at the height of 52 feet—8°.4; at the - 
