190 NINTH ANNUAL EEPORT OF 



from the south. This statement is amply supported b}'' Professor Cof- 

 fin's researches in the Smithsonian Contributions. 



The warm surface wind which sets in from the Gulf of Mexico over 

 the United States, is not only the great source of fertility, but is also the 

 great disturbing element of the atmosphere at all seasons of the year. 

 During the warm season, in this country, when the wind changes 

 to north or northwest, the sky becomes peculiarly transparent and blue in 

 color. I have frequently had occasion, in my tour through Canada and 

 the United States, to observe that the lower south and southwest wind 

 begins to blow, as in Great Britain, shortly after the sun heats the air at 

 the surface of the ground, and that the sky soon loses its peculiar trans- 

 parency. One point in regard to this surface wind from the south de- 

 serves special notice. I allude to the fact that it is often at rest or very 

 sluo[orish durino; the nitjht, and most active during the maximum heat of 

 the day. This vast surface wind which spreads over the region east of 

 the Rocky M(juntains, and over the Gulf of Mexico, is therefore daily put 

 put in motion hy the heat of the sun. The short time which I have been 

 able to devote to this subject leads me to believe that the breeze begins 

 to stir at an earlier hour in the day in the higher latitudes, and that it is 

 gradually propagated to the south. The sun rising earlier the farther 

 we advance northward is probably the cause of this phenomenon. 



Mr. Thom, in his work on the " Nature and Course ot" Storms," p. 255, 

 intc)rais us that south and southwest winds prevail during summer over 

 the projecting shoulder of South America, at Guiana; and I was in- 

 formed by the sugar-planters that in Cuba south winds are common 

 during the rainy season, namely, May, June, July, and August. 

 Mr. Phelps, also, in a recent communication to the meteorological 

 department of the Smithsonian Institution, mentions the fact that 

 at Fort Brown, on the Rio Grande, the "prevailing winds are from the 

 south, or probably a point or avo to the east of south. This, he says, 

 is more particularly the case during spring and the earlier part of 

 sunnner, when they are usually'' pretty constant, especially' during the 

 day time, blowing at the rate ot" fourteen miles an hour, or five degrees 

 of latitude per day. But Professor Coffin's report, already alluded to, 

 gives us the best view of this great aerial current, which fiowsover the 

 Mississippi valley, as well as along the Atlantic slope. 



The two great systems of atmospheric currents, viz : the lower and 

 warm surface wind from the south, and the cold and dry current flow- 

 ing constantly in the upper regions from the west, are intimately asso- 

 ciated with all the changes of the weather in the United States. 

 But belbre we attempt to trace the nature of these changes we must 

 direct attention to another element of meteorology, which we have as 

 yet almost left out ot" view, viz: the elastic and invisible vapor of 

 water contained in the air, and which plays so important a part in 

 almost all atmospheric changes. 



Science, as we have seen, was long perplexed with the problem of 

 the manner in which water existed in the air ; sometimes entirely in- 

 visible, at others obscuring the heavens with clouds, or falling as rain 

 or snow. For the solution of this question, we are also indebted to 

 John Dalton, who gave an explanatKjn ot" the matter, no less simple 

 than consistent and ingenious. He at the same time opened up a new 



