Popular Lectures and Addresses. PROTECTION OF VEGETATION FROM COLD. [Paper read before the Royal Society of Edinburgh, THE effect of dew in protecting vegetation every clear still night of summer was long ago pointed out by Dr. Wells; the correctness and acuteness of whose views on this subject have been generally recognised. The hypothesis recently put forth by Dr. Tyndall, that absorption of radiant heat by aqueous vapour in the atmosphere is an effective defence against destructive degrees of cold, and the ready acceptance yielded to it by some of our highest authorities in the popular promulgation of the truths of science, seems to render it necessary VOL. II. B to recall attention to Dr. Wells's admirable work. In the first place, when Dr. Tyndall announces, as a result of his experiments on radiant heat, that "It is perfectly certain that more than ten per cent. of the terrestrial radiation from the soil of England is stopped within 10 feet of the surface of the soil," by the absorption it suffers from aqueous vapour; it must be remarked that this absorption cannot go on at the same rate through any great thickness of air. For at the same rate half the radiant heat would be absorbed in 70 feet; in 140 feet; in 210 feet, and so on, which is inconsistent with known facts; as, for instance, the influence of clouds on terrestrial radiation. Hence the quality of rays which passes through the lowest 10 feet of air suffers less than ten per cent. of absorption in the next 10 feet; and it is quite certain that after passing through several times 10 feet of air, the radiant heat must, by having been deprived of the part of it specially liable to absorption by aqueous vapour, be in a condition in which not one per cent. is absorbed from it in its passage through 10 feet of clear air. If true vapour of water really does exercise any influence in checking, by its absorption, the loss of heat by radiation from the earth's surface, it is, even in the most humid conditions of optically clear atmosphere, insufficient to prevent heavy dews by radiation into space of the latent heat of the vapour from which they are condensed. The quantity of heat thus radiated into space through the clear moist air close to the ground is so great that if instead of being taken from the vapour it were taken from the blades of grass, or other finer parts of plants, it would leave them destroyed by frost. In point of fact heat actually is radiated away into very high terrestrial atmosphere and distant interstellar air or æther, from the upper and finer parts of living plants, in so great amount every clear night of summer, that destruction by frost could not be delayed for many hours after sunset without a compensating supply of heat from some extraneous source. This source, on windy nights, is the thermal capacity of the air whirled about, up and down, and among the stems and leaves of the plants. On still |