1874] lOi [Blodget. 



or movement from Oregon or Washington Territories eastward to tlie 

 country of the Upper Missouri, or to the line of Red River of the north. 

 There is no progressive march of a i-efrigerated area, or of a barometric 

 depression, along that line from west to east across the mountains. And 

 the line of separation is far east of the mountains themselves, apparently 

 as far as the Yellowstone, nearly , though of course, there is some partial 

 correspondence of phenomena west of this line, and some general relation 

 of the principal conditions. And here I anticipate the more precise re- 

 sults I hope to obtain, in explanation and corroboration of these positions, 

 in order to put forth a view of the causes of these phenomena which ap- 

 pear to me new, and which I hope others will examine also. It is that 

 in the system of atmospheric circulation before referred to, thei-e must 

 be a general descent of atmospheric volumes to the surface at or near 

 the northern border of the belt ; — that this descent may be of masses 

 sometimes large, and depleted of both heat and moisture before they de- 

 scend ; — that descending volumes may come also from the adjacent at- 

 mosphere on the north, not containing heat or moisture brought from the 

 tropics ; — and that, as a general fact these cold, dry masses of air, sink- 

 ing quietly, or poured down with force and violence, to spread over the 

 surface as cold and violent winds, do cause many otherwise inexplicable 

 extremes of cold in the winter and spring particularly. 



The descent of masses of heavy, cold air, must often be induced simply 

 to fill the void caused by contraction of the volume of air from which rain 

 and snow fall. All along the belt of westerly winds this contraction is 

 going on, and this very rapidly during all the colder months. Moving 

 with a constant motion toward the earth, as well as along the surface, it 

 is only a natural vicissitude of this condition, that the descending mass 

 should, at intervals, be poured, like a mass of cold water, over the border 

 of the humid belt, producing the extremes that so often appear to strike 

 down from above. 



I do not remember seeing much reference, hitherto, to descending vol- 

 umes of air on the northern border of this belt of circulation, yet as the 

 trade "winds steadily withdraw the air beneath, toward the tropics, it 

 must necessarily return above ; and it must descend as it returns. If all 

 these movements were perfectly regular, we should see no spasms of se- 

 verity, but as, in fact, there are many days of steadily expanding heat in 

 spring, the days on which the contraction occurs are only the more vio - 

 lent. Hence those heavy pouring winds, that bring such severity of cold 

 during the spiring months ; winds which are neither winds of propulsion, 

 nor of aspiration, but merely the forcing down of cold masses of air from 

 the upper atmosphere, to spread along the surface to some extent, but to 

 be perpetually recruited and renewed from above. An easy experiment 

 will illustrate the condition, by dropping the upper sashes of high win- 

 dows in a heated room on a cold day, — the downward movement will 

 prove unexpectedly tangible and heavy, and as conspicuously marked, 

 almost, as if water were poured through the windows. 



