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have acquired more in the day they lose at night 

 by radiation, the force of which depends on the 

 state of the surface of the radiating body, the 

 interior arrangement of it's particles, and above 

 all on the clearness of the sky,, that is on the 

 transparency of the atmosphere and the absence 

 of clouds. When the declination of the sun 

 varies very little, this luminary adds daily the 

 same quantities nearly of heat, and the rocks are 

 not hotter at the end than in the middle of sum- 

 mer. There is a certain maximum, which they 

 cannot pass, because they do not change the 

 state of their surface, their density, or their ca- 

 pacity for caloric. On the shores of the Oroo- 

 noko, when you get out of your hammock during 

 the night, and touch with your bare feet the 

 rocky surface of the ground, you are singularly 

 struck by the sensation of heat, which you ex- 

 perience. I observed pretty constantly, in putting 

 the bulb of the thermometer in contact with the 

 ledges of bare rocks, that the laxas negras are 

 hotter during the day than the reddish-white 

 granites at a distance from the river ; but the 

 latter cool during the night less rapidly than 

 the former. It may be easily conceived, that 

 the emission and loss of caloric is more rapid 

 in masses with black crusts, than in those which 

 abound in laminae of silvery mica. When you 

 walk, between the hours of 1 and 3 in the after- 

 noon, at Carichana, Atures, or Maypures, among 



