Dew

Abstract

HAVING read with interest the abstract in NATURE of January 14 (p. 256) of Mr. Aitken's observations on dew, I noted attentively during a walk this morning the behaviour of the hoar-frost as deposited on different objects. The morning was fine and frosty after a clear cold night. There was a copious deposit of hoar-frost upon the grass, upon the upper side of wooden rails, and upon the topmost twigs of the bushes in the tall hedges (6 to 8 feet high), but the lower twigs in the hedges had little or none. On stones in the road, as Mr. Aitken observes, there was little hoar-frost on the upper surface, only lines of ice crystals along the salient angles, but their under surfaces were thickly covered. With the loose heaps of broken stones by the road-sides the case was different: here the uppermost stones were thickly coated with frost on their upper surface, but had little on their lower surface; the stones underneath the uppermost layer, on the contrary, were coated with hoarfrost on their under, but not on their upper, surfaces. The hollow “cat's ice” on the road-side puddles, where previously unbroken, was copiously coated with ice crystals above, but only scantily underneath; but where the ice had been broken so that the cavity beneath it had communicated freely with the open air, the crop of ice crystals was equally copious on both sides. A large hollow iron roller, 24 inches in diameter, had a copious crop of ice crystals on the upper surface, but little on the sides. Underneath, however, it had a coating of ice which extended about an inch from the point at which it rested on the earth, and then ended abruptly.