242 
Canadian Agriculture. 
Simpson, 770 miles due north of Fort McLeod, is warmer. To- 
understand thoroughly the climate of the North-West we must 
pass beyond the artificial boundary at the south of the Dominion. 
On the other side of this line (lat. 49°) there is, in the United 
States territory, a vast region of arid country, covering an area 
of about half a million square miles, and occupying a general 
elevation of about six thousand feet. Over this, the Great 
American Desert, as it is called, the moisture-laden winds flow- 
ing northward from the Gulf of Mexico, have to pass on their 
way to the Canadian prairies — " south-west winds come across 
the dry, hot plains west of the Mississippi." * Any cause which 
tends to reduce the temperature of the atmosphere thereby 
lessens its capacity for moisture, so that when an atmospheric 
current is sufficiently cooled, its vapour is usually precipitated in 
the form of rain. But, in summer time, the beat arising from 
the burning plains of the American Desert is so great that the 
northward flowing air-currents retain their temperature, and 
therefore their moisture, and it is not till the winds encounter 
the cooling influence of the grass-covered plains of the prairie 
that their temperature becomes sufficiently reduced to allow of 
the precipitation of their moisture in the beneficent showers of 
rain which constitute so marked and so essential a feature in- 
the meteorology of the North-West. The American desert is- 
thus the indirect cause of the summer showers of the prairie, aSy 
were it not for this desert, the air currents from the south would 
probably be deprived of their moisture long before they reached 
the North-West. But this same desert is in winter the cause of 
that dryness of the atmosphere which renders the winter climate of 
the prairie so much less trying than it otherwise might be, for at 
this season the " suspension of those desert effects which gave the 
summer rains," in other words, the cooling of the desert, induces 
the main air-currents from the Gulf of Mexico to trend to the 
east. This change of direction takes them over the region of 
the great lakes, where they deposit an abundant rainfall.t 
" Bad Lands." — The area within which is comprised what I 
have referred to under the name of the Great American Desert 
is not described as " desert " by the American geographers. 
The region in question extends over considerable portions of the 
States of Nebraska, Colorado, Wyoming and Utah, between the 
latitude of Santa Fe (36° N.) and that of Cheyenne (41 J° N.),. 
and between the meridians of 99° and 111° W. Much of it is 
occupied with what are known as " bad lands," and it is these, 
with their arid climate and scant vegetation, which impart the 
desert character. They also extend around the Uinta Mountains, 
* Ansted, 'Phys. Geogr.,' Fifth Ed., p. 295. 
t ' Manitoba aad the Great North-West,' p. 130. 
