•454 Dcx and Hoar-Frost. 



sarily fall in temperature, if situated so as not to receive heat 

 from other radiating bodies in their neighbourhood.* 



When bodies are thus exposed, some of them are found 

 to part with their heat faster than others : the former are 

 therefore called good radiaters, and may be observed covered 

 with dew ; while the latter, called bad radiaters, remain dry. 

 In fact, each body will cool down, and receive a deposition, in 

 proportion as it is a good radiater. 



A calm atmosphere is necessary. In order to the de- 

 position of dew, the air and vapour must remain under the 

 influence of the cold body until their temperature is brought 

 down to the dew point. Wind not only prevents this, but 

 rapidly reduces the objects exposed to it to a common tem- 

 perature, thus neutralising the effects of radiation. 



Fluids are bad conductors. Heat is slowly propagated 

 through them, when applied above or abstracted from below. 

 The cold, therefore, in the case we are considering, extends 

 to no great height ; indeed, it is generally confined to a few 

 feet from the surface of the ground. I often observe a ther- 

 mometer placed outside, at a height of about 7 ft., stand at 

 40° when the ground is white with hoar-frost ; and the Rev. 

 W. T. Bree (IV. 480.) has mentioned many striking instances 

 in point. The extent of the cold stratum is very obvious in 

 the neighbourhood of water ; the partially condensed steam 

 from it marking the limits very distinctly. (See, again, IV. 

 480.) 



I have said that dew will rarely be seen except in clear 

 weather f, for otherwise the surface of the ground will seldom 

 be found colder than the air; but it sometimes happens. 

 Last Saturday night (Nov. 18. 1833), the thermometer sunk 

 to 29°, and during Sunday did not rise to 33° in the shade; 

 at night it fell to 28°, and on the morning of the 18th it sud- 

 denly rose to 47 J°, the atmosphere becoming dull : a calm 

 prevailed all the time. The consequence was a copious de- 

 position of moisture, which, on the grass, indeed, could be 

 distinguished from the melted hoar-frost, but was very remark- 

 able on stones that had been previously dry. The rooms in 

 my house that had been without fires had partaken of the 

 severe cold, the temperature being 40°, and the windows of 

 them, as might have been expected, were covered with con- 

 densed vapour on the outside. This, though of the same 

 nature with dew, is not what, in common language, we would 

 call by that name. 



* Dew and hoar-frost are seldom observed in the close streets of towns, 

 or under shady trees, 

 f The common people here call a clear calm atmosphere " a frosty sky.'* 



