Page:Notes on the State of Virginia (1802).djvu/122

112 rooms two or three times a week, in which they ſtay two hours at a time, the atmoſphere of which raiſes the mercury to 135° above nothing. Late experiments ſhow that the human body will exiſt in rooms heated to 140° of Reaumur, equal to 347° of Farenheit'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 autumn, and its receſs in 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 thermometer 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 38½°, which is 6½ degrees above the freezing point. That other circumſtances 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 the courſe of 40 years, during which it has been ſettled, there have been but two inſtances 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 combined with the