2i8 Atmospheric Changes. 



considered as a Mode of Motion." From long-continued and delicate 

 experiments, Prof. Tyndall arrives at the conclusion, that aqueous vapor is 

 opaque to the rays of heat of low intensity ; in other words, that the pres- 

 ence of a considerable amount of moisture in the air prevents the radiation 

 into space of heat from the soil and plants of that locality, and conse- 

 quently a low night-temperature : but if, by the action of drying winds or 

 otherwise, the proportion of aqueous vapor in the air be much reduced, the 

 barriers to radiation are removed, and considerable nocturnal refrigeration 

 follows. " The removal, for a single summer night, of the aqueous vapor 

 from the atmosphere that covers England, would be attended by the de- 

 struction of every plant which a freezing temperature would kill." In the 

 paper cited, Mr. Lippincott gives at considerable length the results of 

 personal observations made in Camden County, N.J., during 1864 and 1865, 

 in which there are many notable instances of a low degree of atmospheric 

 humidity followed by excessive cold, in accordance with the theory of Prof 

 Tyndall. " On the 22d of July, at two, p.m., the force of vapor, or pressure, 

 in inches on the barometer, was but .188; which is lower than we have ever 

 observed it during summer and autumn, and lower than is sometimes no- 

 ticed even at the freezing-point." On the morning of July 23, the tem- 

 perature was but " 46° at six feet above the soil ; a narrow escape from 

 frost." 



It may now be asked in what manner these fluctuations in the humidity 

 of the atmosphere concern the horticulturist, and whether it is in his 

 power to protect his plants from the evils following in their train. These 

 questions are discussed at length in Mr. Lippincott's paper. He mentions 

 that the first appearance of mildew and rot in vines almost immediately fol- 

 lowed the low morning temperatures consequent on the diminution of the 

 quantity of aqueous vapor in the atmosphere of the locality.* 



Why these sudden changes should cause mildew in the vine (and per- 

 haps many other plant-diseases), may, I think, be thus explained. In a 



* This agrees entirely with my own observations, although my experience with mildew has been limited 

 to a few spots on odd leaves ; the close vicinity of sea-water in nearly all directions preventing those sud- 

 den changes so injurious to the vine. In the present season, 1866, my vines were entirely healthy during 

 the ver>' hot and rainy weather of July and the early part of August : but with the first low morning tem- 

 peratures, about Aug. 9, a few spots of mildew appeared ; and, before Aug. 17, it had largely increased, 

 and rot was discovered in a few clusters. Some of the worst cases were sulphured, and the disease 

 checked. The last three weeks of August were cold, damp, and unfavorable. 



