ON DEW, &c. 157 



while I was absent 45 minutes, the temperature 

 of the grass rose 15, while that of the neigh- 

 bouring air increased 3j. During a fourth 

 night, the temperature of the grass at half-past 

 9 o'clock was 32. In 20 minutes afterwards, 

 it was found to be 39, the sky having in the 

 mean time become cloudy. At the end of 20 

 minutes more, the sky being clear, the tempera- 

 ture of the grass was again 32. These were 

 the most remarkable of my observations on this 

 subject ; but I may add to them, that I have 

 frequently seen, during nights that were gene- 

 rally clear, a thermometer lying on the grass- 

 plat rise several degrees, upon the zenith being 

 occupied only a few minutes by a cloud. On 

 the other hand, upon two nights I observed a 

 very great degree of cold to occur on the 

 ground, in addition to that of the atmosphere, 

 during short intervals of clearness of sky, be- 

 tween very cloudy states of it. 



I did not speak in the preceding section of 

 another obscure state of the atmosphere, that 

 occasioned by fog, or mist, as the moisture de- 

 posited in it attaches to all bodies, indiscri- 

 minately ; on which account, I was unable to 

 determine, whether or not dew forms during its 

 continuance. But, with respect to the con- 

 nexion of this condition of the atmosphere with 

 cold, I have to remark, that I have several 



