temperature of the ocean and atmosphere , &c. 281 
strument in the shade, and exposed it to the wind, taking care 
not to bring it near any surface that had the power of radi- 
ating much heat, circumstances, I need not remark, of im- 
portance to be attended to, and, in consequence of the neglect 
of which, the temperature at sea, in the intertropical regions, 
has by most observers been overrated. 
During the greater part of the voyage, observations were 
made every two hours, on the temperature both of the air and 
of the water; and with the kind assistance of the mates of 
the ship, Messrs. Sleight and Powell, intelligent and oblig- 
ing men, they were carried on during the night as well as 
the day. 
I am not aware that the law of the diurnal variation of the 
temperature of the atmosphere at sea, has been described by 
any writer. From the numerous observations, which I had 
an opportunity of making, between and bordering on the tro- 
pics, it appeared to me perfectly regular at a great distance 
from land, when the weather was fine, and the wind steady. 
In these circumstances, I found the air at its maximum tem- 
perature precisely at noon, and at its minimum towards sun- 
rise. I shall give in illustration of the fact two instances from 
my note book. 
April 2d. S. lat. 21 0 3". W. long. 27° 27". Wind E. by S. 
Hour. 
6 A. M. 
8 
10 
12 
Temperature, 
o // 
78 
79 
~ 79,5 
80 
2 P. M. - - 79,25 
4 - - 79 
O o 2 
