Daniell on Meteorological Observations. 107 



and at different periods of the year, we should obtain, by the 

 substitution, a much more useful result. The evaporation from 

 the land must be much more irregular than that from the 

 ocean and other deep accumulations of water : it must vary 

 with almost every moment of the day, and every variety of 

 situation. The fallow-field, and the field of corn, yield their 

 moisture with different facility ; and sun-shine and shade must 

 produce still greater modifications. 



I have found, from experiment, after a shower of rain, while 

 the temperature of the air has been 66, and that of the vapour 

 60, that the temperature of a running stream has been 62 ; of 

 wet grass, when the sun had been shining upon it, 69 ; of wet 

 sand, 76 ; of stagnant water, 67 ; of damp garden-mould, in the 

 sun, 84. So that at the very same moment of time these situa- 

 tions were respectively yielding at the rate of 0.18 grains, 0.89 

 grains, 1.82 grains, 0.67 grains, and 3.16 grains, per minute, 

 from a surface of 6 inches diameter. Not that vapour of these 

 various degrees of force existed at one time ; for it is evident 

 that those which were above the temperature of the air must 

 have been instantly precipitated, and as instantly redissolved at 

 the general temperature. 



The facility and rapidity with which vapour of such different 

 degrees of force in its nascent state is resolved into one general 

 mass of equal density, is very worthy of observation. I have 

 often endeavoured to detect it before its change, with the hy- 

 grometer, but with equivocal success. For example, when the 

 temperature of the air was 56, after sun-set, and the hygrometer 

 denoted the dew-point to be 47, 1 placed myself in the middle of 

 a stream whose temperature was 63 ; the precipitation took 

 place at 50, only 3° higher than before. As the water was here 

 yielding vapour of 16° higher temperature than the general 

 average, I expected to have found it more nearly approaching 

 the point of saturation ; especially as there was very little air 

 stirring. From hence we must conclude that it diffuses itself 

 with immense velocity, which, perhaps, we shall have the less 

 difficulty in conceiving, when we call to mind that the rising 

 vapour possessed a force of 0.578 inch, which was only opposed 



