Page:Notes on the State of Virginia (1853).djvu/103

Rh 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, that 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