Atmospheric Changes. 217 



understood ; indeed, they are impressed upon us by monthly experience. 

 But there are other atmospheric changes of vastly more importance to the 

 horticulturist, as influencing more directly the results of his labors. We 

 refer to the rapid and often excessive fluctuations in the amount of aqueous 

 vapor contained in the atmosphere ; that change from dampness to dryness, 

 or vice versa, so perceptible to the feelings, and accurately indicated by the 

 wet-bulb thermometer. These fluctuations frequently occur during mid- 

 summer, and, with their concomitant phenomena, exert an injurious influ- 

 ence on vegetation ; checking growth, and rendering plants liable to the 

 attacks of disease. How these results are accomplished, we shall endeavor 

 to explain farther on. 



There is in " The Report of the Department of Agriculture " for 1865, 

 quite recently published, a very instructive and important article headed 

 " Observations on Atmospheric Humidity," by J. S. Lippincott of Haddon- 

 field, N.J. Here we have these questions thoroughly discussed and scien- 

 tifically explained, and various means suggested for the partial protection 

 of growing plants from the results of sudden atmospheric changes. It is 

 a paper which should be read and pondered by all who feel an interest in 

 the fruits of the earth, whether they are the po-ssessors of extensive plan- 

 tations, or of only a few highly-prized trees and vines. It is princi- 

 pally in the hope of calling increased attention to this important subject, 

 and with the purpose of suggesting a careful perusal of Mr. Lippincott's 

 paper to the readers of this Magazine, that I have written these few notes. 



From the narratives of travellers, and from meteorological observations, 

 it has long been known that certain regions are subject to great depressions 

 of temperature after sunset ; and that this refrigeration arrives at its maxi- 

 mum a little before sunrise, when the cold is often excessive (sufficient even 

 for the formation of ice), though at noon of the same day the heat may be 

 intense. This peculiarity of climate is especially developed in parts of the 

 Sahara, and of what is called the "Great American Desert." It was also 

 known, that, in all regions so characterized, a veiy dry atmosphere prevailed. 

 Although generally accepted as a foct, that, the dryer the air of any coun- 

 try, the coldei" were its nights, this phenomenon remained unexplained 

 until the laws gpverning it were discovered by John Tyndall, F.R.S., and 

 demonstrated in the clearest manner in his well-known work " On Heat 



VOL. I. 28 



