1818.] Philosophical Transactions for 1817, Part II. 57 



barometer ; the 12th, of the winds ; and the last, the account of the 

 weather generally. The observations were continued, without 

 much interruption, from the middle of February to the middle of 

 August, when the author arrived at Ceylon, commencing in the 

 49th degree of north latitude, and 6^ degrees west longitude, 

 and proceeding round by the Cape of Good Hope and the Isle 

 of France. The experiments on the density of the sea water 

 were made on portions of water drawn from the surface of the 

 ocean, its temperature being reduced by calculation to 80°, a 

 number which was fixed upon because it is nearly the mean 

 annual temperature of Ceylon, and of the sea generally in the 

 intertropical regions. The results of these experiments show 

 that the ocean resembles the atmosphere with respect to the 

 general uniformity of its composition, the specific gravity of the 

 water being very nearly the same in all the different trials. The 

 number of observations recorded is 36 ; the highest specific 

 gravity is 10277, and the lowest 12051. These variations seem 

 to have no connexion with the temperature, or at least not to 

 bear any regular proportion to it. The differences seemed rather 

 to depend upon what may be regarded as incidental circum- 

 stances, as the roughness of the surface, a heavy fall of rain, 

 and a succession of tropical squalls. Dr. Davy's observations 

 controvert an opinion which has been adopted, that the different 

 zones of the sea have each their peculiar specific gravity. 



With respect to the temperature of the air and water, the 

 obseivations were made every two hours with delicate thermo- 

 meters. Dr. Davy conceives that the temperature of the atmos- 

 phere in hot climates has been frequently overrated from the 

 thermometer not being sufficiently protected from the radiation 

 of caloric by neighbouring bodies. The highest temperature 

 that is noted is 82° ; this occurred at 2° 10' north latitude, about 

 five days before they arrived at Ceylon ; the uniformity of the 

 temperature in these regions is very remarkable, the maximum 

 and minimum not differing more than 3° or 4° in ordinary cases, 

 and seldom more than 5° or 6°. The author has made an obser- 

 vation on the diurnal variation of the temperature of the atmos- 

 phere at sea which had not been before noticed, which, when 

 the weather is fine, and the wind steady, appears to have few 

 exceptions : the air was " at its maximum temperature precisely 

 at noon, and at its minimum towards sun-rise." But many cir- 

 cumstances were found to disturb the regular progression : on 

 the one hand, in a perfect calm the accumulation of heat, not 

 only in the ship, but in the water itself, cause the greatest heat 

 to occur some time after the hour of noon, and by showery and 

 Unsettled weather the regular variation was still more disturbed. 



Contrary to what is commonly asserted, the diurnal change of 

 temperature in the sea is veiy nearly as great as in the atmos- 

 phere. When there were the fewest disturbing causes, the 

 weather fine, the surface smooth, and no land near, the 



