PHYSICAL CHARACTERISTICS OF THE ARID REGION. 3 
factor, and the line of possible agriculture without irrigation is carried still 
farther westward. This conclusion, based upon the consideration of rain- 
fall and latitude, accords with the experience of the farmers of the region, 
for it is a well known fact that agriculture without irrigation is successfully 
carried on in the valley of the Red River of the North, and also in the south- 
eastern portion of Dakota Territory. A much more extended series of rain- 
gauge records than we now have is necessary before this line constituting 
the eastern boundary of the Arid Region can be well defined. It is doubt- 
less more or less meandering in its course throughout its whole extent from 
south to north, being affected by local conditions of rainfall, as well as by 
the general conditions above mentioned; but in a general way it may be 
represented by the one hundredth meridian, in some places passing to the 
east, in others to the west, but in the main to the east. 
The limit of successful agriculture without irrigation has been set at 
20 inches, that the extent of the Arid Region should by no means be exag- 
gerated; but at 20 inches agriculture will not be uniformly successful from 
season to season. Many droughts will occur; many seasons ina long series 
will be fruitless; and it may be doubted whether, on the whole, agriculture 
will prove remunerative. On this point it 1s impossible to speak with cer- 
tainty. A larger experience than the history of agriculture in the western 
portion of the United States affords is necessary to a final determination of 
the question. : 
In fact, a broad belt separates the Arid Region of the west from the 
Humid Region of the east. Extending from the one hundredth meridian 
eastward to about the isohyetal line of 28 inches, the district of country 
thus embraced will be subject more or less to disastrous droughts, the fre- 
quency of which will diminish from west to east. For convenience let 
this be called the Sub-humid Region. Its western boundary is the line 
already defined as running irregularly along the one hundredth meridian. 
Its eastern boundary passes west of the isohyetal line of 28 inches of rain- 
fall in Minnesota, running approximately parallel to the western boundary 
line above described. Nearly one-tenth of the whole area of the United 
States, exclusive of Alaska, is embraced in this Sub-humid Region. In the 
western portion disastrous droughts will be frequent; in the eastern portion 
