58 



Cumana, in the centre of vast plains, of small 

 height above the level of the ocean, the thermo- 

 meter is generally toward sunrise four or five 

 degrees lower than at two in the afternoon. In 

 the Atlantic ocean, on the contrary, between 

 eleven and seventeen degrees of latitude, the 

 greatest variations of heat rarely exceed 1*5 or 

 two degrees ; and I have often observed, that 

 from ten in the morning to five in the evening 

 the thermometer did not vary 0 8 of a degree. 

 In looking over fourteen hundred thermometri- 

 cal observations made hourly during the voyage 

 of Mr. Krusenstern, in the equatorial region of 

 the South Sea, we see, that the temperature of 

 the air changed from day to night only one or 

 1*3 centesimal degree*. 



I have often endeavoured to measure the 

 -power of the Sun by two thermometers of mer- 

 cury perfectly equal-}-, one of which remained 

 exposed to the Sun, while the other was placed 

 in the shade. The difference resulting from the 

 absorption of the rays in the ball of the instru- 



* I constantly observed the thermometer on the deck, to 

 windward, and in the shade. Perhaps the thermometer and 

 barometer of Mr. Krusenstern were in a more sheltered 

 place, for instance in the great cabin. 



t This instrument had a ball of three lines diameter, which 

 was not blackened. The scales were contained in tubes of 

 glass very distant from the ball. Travellers prefer at present, 

 and with reason, Mr. Leslie's photometers. Nicholson's 

 Journal, 4to edition, vol. iii, p. 467. 



