84 



will exist in rooms licated to 140° of Reaumur, equal 

 to 347" of Farcnheit's, and 135" above boiling water. 

 The hottest point of the 24 hours is about four o'clock, 

 P. M. and the dawn of day the coldest. 



The access of frost in autufnn, and its recess the 

 spring, do not seem to depend merely on the degree 

 of cold ; much less on the air's being at the freezing 

 point. White frosts are frequent when the thermo- 

 meter is at 47°, have killed young plants of Indian corn 

 at 48°, and have been known at 54°. Black frost, and 

 even ice, have been produced at 38 i°, which is 6h de- 

 grees above the freezing point. That other circum- 

 stances must be combined with the cold to produce 

 frost, is evident from this also, on the higher parts of 

 mountains, where it is absolutely colder than in the 

 plains on which they stand, frosts do not appear so 

 early by a considerable space of time in autumn, and 

 go off sooner in the spring, than in the plains. I have 

 known frosts so severe as to kill the hiccory trees round 

 about Monticello, and yet not injure the tender fruit 

 blossoms then in bloom on the top and higher parts of 

 the mountain: and in tlie course of 40 years, during 

 which it had been settled, there have been but two in- 

 stances of a general loss of fruit on it : while, in the 

 circumjacent country, the fruit has escaped but twice 

 in the last seven years. The plants of tobacco, which 

 grow from the roots of those which have been cut off 

 in the summer, are frequently green here at Christmas. 

 This privilege against the frost is undoubtedly combin- 

 ed with the want of dew on the mountains. That the 

 dew is very rare on their higher parts, I may say with 

 certainty, from 12 years observations, having scarcely 

 ever, during that time, seen an unequivocal proof of its 

 existence on them at all during summer. Severe frosts 

 in the depth of winter prove that the region of dews 

 extends higher in that season than the tops of the 

 mountains: but certainly, in the summer season, the 

 vapours, by the time they attain that heighth, are be- 

 come so attenuated as not to subside, and form a dew 

 when the sun retires. 



