44 Prof. Marcet on the Variations of the Temperature 
ever considerable it may be, at the same time, near the sur- 
face of the ground. 
In winter, when the sky is obscured, the difference between 
the temperature of the successive strata of the atmosphere 
and those very close to the earth, is extremely small, even 
when the ground is covered with snow. And after we ascend 
above 100 feet, the temperature appears most frequently to 
be modified in a contrary sense to what takes place in clear 
weather—that is to say, it decreases in proportion as we as- 
cend. In eleven observations made both at sunrise and sun- 
set, and in the circumstances I have pointed out, the ther- 
mometer has been on two occasions lower by some centiémes 
of a degree at the height of 5 feet above the ground than at 
2 feet. The mean of the thermometrical difference between 
the height of 2 feet and that of 50 feet, has been only 0°.4 in 
favour of the highest elevation ; while the mean of the differ- 
ence between 52 feet and 105 feet, has, on the contrary, been 
0°23 in favour of the less elevated station. 
2dly, Winter is further distinguished from the other seasons 
of the year, by the excessively low temperature of the surface 
of the ground at certain periods of the day, compared with that 
of the stratum of air immediately adjoining it ; that is to say, 
situate at the height of about 2 feet. This difference ap- 
peared to me scarcely appreciable during the finest season of 
the year, even when the sky was perfectly clear and serene. 
A mean deduced from a great number of observations made 
during the summer and autumn of 1837, gave only 0°.54 for 
the times of sunrise and sunset, the periods of the day when it 
is at other times most considerable. In winter the difference 
in question becomes much more remarkable ; it amounted on 
one occasion to 6°, the ground being covered with snow. A 
mean calculated from twelve observations, in clear weather, 
both at the rising and setting of the sun, afforded me a dif- 
ference of 3° between the temperature of the surface of the 
snow and that of a thermometer placed 6 feet above the 
ground ; at the height of 2 feet this difference only amounted 
to 1° 5. 
I have remarked, oftener than once, in the course of the 
summer and autumn, that the surface of the ground has ap- 
