ON DEW, &c. 155 



observed by me this night on grass, will exceed 

 the greatest ever observed by Mr. Six by 1 

 degree. 



According to a few observations made by me, 

 the greater coldness of grass, than that of the 

 air, begins to appear, in clear and calm weather, 

 in places, sheltered in the afternoon from the 

 sun, but still open to a considerable portion of 

 the sky, soon after the heat of the atmosphere 

 has declined. A similar coldness continues 

 upon grass in still and serene mornings, for 

 some time after the rising of the sun, in places 

 shaded from its direct light, but otherwise open 

 to the sky. My experiments on this point have 

 also not been many, and none of them were 

 made in winter f which, I presume, are the rea- 

 sons, that I never observed a cold, from this 

 cause, later in the morning, than an hour after 

 sunrise. The surface of snow, however, was 

 once, in the depth of winter, observed by Mr. 

 Wilson of Glasgow to be considerably colder 

 than the air, till a little after midday*. 



In cloudy nights, particularly if there was 

 wind, the grass was never much colder than 

 the air. On such nights, the temperatures of 

 both were sometimes the same ; at other times 

 that of the grass was the higher of the two, 



* Paper in Phil. Trans. 1781. 



