Dew and Hoar- Frost. 15 



in the air. Thus, on the 1st of September 1843, at 11 

 o'clock at night, the sky being overcast after a warm day, 

 when the grass was very wet with dew, a thermometer laid 

 on it rose to 60° ; and above it, in the air, fell to 58°. An 

 hour after, with the same state of sky, and the grass even 

 more loaded with moisture, the thermometer placed towards 

 the roots of the gi'ass rose to 62° ; and at its top, where co- 

 vered with dew, fell to 58°. A glass-tumbler, placed at the 

 same time on the grass-plot inverted, on its inside was co- 

 piously bedewed, whilst its outside remained quite dry. Does 

 not this shew, that the moisture precipitated rose from the 

 damp warm ground, through the medium of the air immedi- 

 ately in contact, warmed and saturated by it ? And, in this 

 instance, was not the dew produced after the manner that 

 fog is commonly produced \ I shall give another example. 

 On the 14th of the same month, at 8 A.M., when the grass 

 was loaded with dew, the air was 55° ; the wet surface of 

 grass, 58° ; the roots, 63°. At this time a mist prevailed, 

 and obscured the sun. I could give from my notes many 

 other instances of the like kind, in which the higher tempe- 

 rature of the grass on which dew was deposited, was accom- 

 panied by a hazy or foggy state of the atmosphere. They 

 occurred chiefly in the autumnal months, and in the early 

 winter, the season of fog, when the Rotha, and especially 

 lakes, and rivers running out of lakes, are commonly warmer 

 than the air at night, and calms are frequent. 



5. Hoar-frost, as it is well known, is the representation of 

 dew, commonly appearing when the radiating surface on which 

 it collects is below the freezing-point ; nevertheless, I have 

 sometimes found the temperatui'e of dew at or below the freez- 

 ing-point, without being frozen. This I witnessed on the 

 29th of March last year, and on the 4th and 7th of April at 

 night, when the thermometer on the grass was 29°, 32°, 30°. 

 In the first instance, it was observed at 12 at night, when the 

 air was very calm as well as clear ; observed an hour later, 

 the thermometer on the grass had fallen to 27, and the dew 

 was frozen. It may be conjectured, that the dew in this and 

 the like instances was precipitated at a temperature a little 

 above the freezing-point ; and that it remained unfrozen. 



