98 Canadian Record of Science. 
give an extraordinary quantity of milk, while the dry- 
ness of the air and ground is especially favorable to sheep 
as well as cattle. 
How the Canadian plains, in spite of their interior and 
northerly situation, come to have so warm and dry a 
climate is worthy a moment’s consideration, though the 
instruction which this audience has already received from 
Professor McCleod, makes any remarks from me hardly 
needful. It is to be remembered that south of western 
Canada lies the vast plains-country of the United States, an 
arid space thousands of square miles in extent, towards 
which blow steadily the warm currents of air from the 
Gulf of Mexico, attracted by the heated air issuing from 
these ample spaces of treeless land. The ground becomes 
baked, and the air, heated by contact with it, rises rarified 
in enormous volumes, sucking in the northward-bound 
currents to take its place, and at the same time buoying 
them up and preventing the condensation or precipitation 
of moisture. This overfiow of heated air continually drifts 
polewards, or northward, where, it must not be forgotten, 
the land is far lower; and as it goes it is joined by similar 
currents from the Nevada and Idaho deserts, and from the 
coast of California and Oregon. Combined, this current 
pours steadily northward, attracted by the rarified air now 
rising from the Canadian plains, and still bearing a large 
part of its original moisture. 
But over the Saskatchewan valley it meets the cooler air 
flowing from the north, also attracted by the heated prairies, 
and in contact with this cooling current the moisture of 
the south and west winds is condensed into clouds and falls 
as rain. A secondary characteristic of this movement is 
the diversion of the northward-blowing wind eastward, al- 
though, as the earlier lecturers in this Course have shown 
us, the natural tendency of these antitrades is toward the 
west. : 
But as winter approaches the conditions are altered. The 
cooling of the plains diminishes their attractive power, and 
the warm southerly winds tend away from the east, toward 
