36 Prof. Marcet on the Variations of the Temperature 
the air at 4 or 5 feet above the ground, is notably warmer than 
the ground itself ;* 2d/y, During calm and clear nights, and rec- 
koning from the height of 5 or 6 feet, the air becomes warmer 
as we ascend, according to some unknown law, and to an ele- 
vation the limit of which has not yet been determined. 
Before proceeding to detail the experiments which I under- 
took with the design of throwing some light on such parts of 
this subject as are still obscure, let us return to the observa- 
tions of Six, the only ones which have been made with some 
degree of continuity, and at considerable differences of eleva- 
tion, and let us determine whether they took place in such cir- 
cumstances as were likely to lead to correct results. 
The two following considerations induce me to believe 
that the results of Six’s experiments must very often deviate 
from the truth. 1s¢, These experiments were made in the 
centre of a populous town, in which the temperature of the 
surrounding atmosphere must necessarily have been affected 
by the vicinity of a constantly renewed source of heat. The 
influence of this artificial heat must have been particularly felt 
during the period of nocturnal radiation, by preventing the 
cooling of the atmospheric strata nearest the earth. In con- 
sequence of this, the relative increase of heat, in proportion 
as we ascend, must have often appeared to Mr Six not so 
great as it really is. 2d/y, The heat acquired by the walls of 
the Cathedral, exposed during the day to the rays of the sun, 
must have often exercised an influence on the results obtained 
by the English philosopher. No one is now ignorant to what 
a degree stone and brick buildings become heated, when ex- 
posed to the direct rays of the sun, particularly during the 
warm season ; it is so considerable that when a person is pas- 
sing along by the side of a wall in the evening, he very often 
feels the heat proceeding from it. Now, the strata of air next 
the Tower of Canterbury Cathedral must necessarily have been 
affected, particularly during the earlier part of the night, by 
the neighbourhood of a mass so heated, the more especially as 
* Often, according to Wells, from 4 to 5 degrees. Wilson has seen this 
difference amount to 8 degrees, the surface of the ground being covered with 
snow. 
* 
