196 NINTH ANNUAL REPORT OP 



is very transparent are rather deficient in moisture. The air is then 

 somewhat bracing, as the sun does not have the power of heating 

 it up to a high degree. The great changes in the weather which take 

 place at all seasons over the United States, and the very cold winters 

 in comparision to those of Europe, are intimately associated with the 

 hygromet;ic condition of the atmosphere, which we must now con- 

 sider. 



Why are the winters so cold at^^ashington in the SS*^ of latitude,' 

 the same as that of Lisbon, in Portil^l, where the orange-trees are now 

 in blossom? This is owing to the great prevalence of west and north- 

 west winds, which are very dry, and sweep over the vast territory 

 from Canada to Florida, and are converted into the northern of Cuba 

 and the coast along the Gulf of Mexico. President D wight, a most 

 accurate observer, in his Travels and History of New England describes 

 the character and effect of the winds so accuratel}^, that I will quote 

 two short passages. He says, " In 1787 the west wind began to blow 

 about the 20th November, and continued its progress with only four 

 short interruptions until the 20th of the following March— somewhat 

 more than 100 days. During the whole time the weather for the season 

 w^as very cold." Also, "in 1780 the wind blew from the west more 

 than six weeks without any intermission, and during the whole of this 

 time the w^eather was so cold that snow did not dissolve sufficiently to 

 give drops from the southern eaves of houses." So long as the westerly 

 wind continues to blow in winter there is no cessation of your cold, and 

 so lona,- as it continues to flow in a broad res^ular stream in summer 

 there is no end to 3'our drought. President Dwight maintains that the 

 west and northwest wind is merely the descent of the upper current 

 which flows so regularly right across the Rocky Mountains. Whoever 

 will take the trouble to examine the meteorological observations within 

 the Smithsonian Institution, I am inclined to think, will come to 

 the same conclusion. If the south wind was as common in winter as 

 in summer, and if there was no descent of this upper current, your 

 v/inters might be as mild as those of Portugal, whicli are tempered by 

 the moist wind from the southern Atlantic. 



The question naturally arises, why is the west wind so extremely cold ? 

 The answer is, simply, because in crossing the Rocky Mountains it is 

 robbed of its moisture ; and becoming so dry that the sun pours down 

 his rays of heat ; in vain neidier the air by day nor the ground by 

 night can retain them ; they fiy off into space. The principle on which 

 air is dried in passing over high mountains is a very simp.e one, if we 

 merely bear in mind that air expands under diminished pressure and 

 becomes colder by the heat being diffused over a greater space. Pro- 

 fessor Espy has very ably investigated this subject on theoretical 

 grounds. But, as our space is exceedingly limited, I shall merely 

 adduce some examples by way of illustration. Mr. Walsh made an 

 ascent in a balloon on the 26th August, 1852, from Kew Gardens, in 

 England. The barometer stood at 30 inches, and the dew point was 

 61°; or, in other words, the air contained 6,06 grains of water in every 

 cubic foot. At the height of 18,370 feet the barometer stood at 35 

 inches, temperature of air 70, dew point 2°8, or equivalent to 0.8 grain 

 of water in a cubic loot. But at this elevation the air being expanded 



