272 PROCEEDINGS OF THE AMERICAN ACADEMY 



current, which is probably of neai'ly equal magnitude, flows from north- 

 Avest to southeast, or nearly at right angles to the other, and the 

 resultant of the two is a current almost directly from the west. The 

 reaction of these two currents appears to be the principal cause of the 

 sudden changes of weather in our latitude. They give definite direc- 

 tion to our storms, accordingly as the latter are more influenced by 

 the motion of the one or the other of these great aerial streams. The 

 principal American storms may, from our present knowledge, be di- 

 vided into two classes ; namely, those which have their origin in the 

 Caribbean Sea, and those which enter our territory from the north, at 

 the eastern base of the Rocky Mountains. Those of the first class, 

 which have been studied with much success by the lamented liedfield 

 and others, follow the genex'al direction of the Gulf-Stream, and, over- 

 lapping the eastern portion of the United States, give rise to those 

 violent commotions of the atmosphere which are in many instances so 

 destructive to life and property along our eastern coast. These storms 

 from the south are frequently two or three days in traversing the dis- 

 tance from Key West to Cape Race, and their approach and progress 

 might generally be announced by telegraph in time to guard against 

 their disastrous effects. Though the general direction of these storms 

 appears to be made out with considerable certainty, much remains to 

 be done in settling the theory of their character and formation. 



The materials which have been collected at the Smithsonian Insti- 

 tution during the last seven years, relative to the other class of storms, 

 have enabled us to establish general facts of much value, not only in a 

 scientific point of view, but also in their application to the prediction of 

 the weather. [This statement was verified by a series of maps, exhib- 

 ited to the Academy by Professor Henry, on which were indicated the 

 beginning and pi'ogress of some remarkable changes of weather.] From 

 these maps it appears that the great disturbances of the atmosphere 

 which spread over the surface of the United States enter our territory 

 frotn the possessions of the Northwest Company, about the sources of 

 the Saskatchawan, at the base of the Rocky Mountains, and are thence 

 propagated south and east, until, in many instances, they spread over 

 the whole of the United States, and probably a large portion of the 

 British possessions. 



For example, the great depression of temperature which occurred in 

 January of the present year, and which will be remembered by every 



