Atmosphere, and on I he Densily of Sea-iuater. 29 



That the specific gravity of the water of the ocean, in all its 

 parts, however remote, should be nearly the same, is easily ex- 

 plained; it is indeed what might be expected from theory. It 

 is more difficult, it appears to me, to account for the slight va- 

 riations; I may remark, they appeared to me greatest when the 

 sea was rough and agitated ; and once the specific gravity of 

 the water seemed diminished by a heavy fall of rain, viz. in lat. 

 4' north, and in long. 18^ IS" west, where we experienced a 

 quick succession of tropical squalls. 



Whether there is a specific gravity peculiar 'to the water of 

 each Zone, as a modern traveller of high authority endeavours to 

 prove, I am greatly in doubt. From my own experiments, in 

 which I cannot but put some reliance, 1 feel much inclined to 

 infer the contrary, and especially from those made on land, which 

 I know to be perfectly accurate. Several of these agree in 

 giving the same specific gravity to specimens of water taken 

 from parts of the ocean very remote from each other : for in- 

 stance, the water from lat. 0" 12" south, and 22" 36" south, 

 and that from 34" 25" south, and the water that washes the 

 shores of Colombo. 



For ascertaining the temperature of the air and of the water 

 of the ocean, I used delicate pocket-thermometers, the bulbs of 

 which projected about an inch from the ivory scale. In the ex- 

 periments on the temperature of the ocean, the water was tried 

 the instant it was drawn, before it was affected by the air. To 

 find the temperature of the air, I always chose the coolest part 

 of the ship on deck, and always put the instrument in the shade, 

 and exposed it to the wind, taking care not to bring it near any 

 surface that had the power of radiating much heat; circumstances, 

 I need not remark, of importance to be attended to, and, in 

 conse(]uence 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 tbe mates of the ship, 

 Messrs. Sleight and Powell, intelligent and obliging 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 op- 

 portunity of making, between and bordering on the tropics, it 

 appeared to me perfectly regular at a great distance from land, 

 when the weather was fine, and the wind steady. In these cir- 

 cumstances, I found the air at its maxinuun temperature pre- 

 cisely at noon, and at its mininmm towards sunrise. I shall give 

 in illustration of the fact two instances from my note book. 



April 



