which extends along the plains from our northern to our southern boundary. The isohyetal or mean annual rainfall line of 20 inches, as indicated on the rain chart accompanying this report, begins on the southern boundary of the United States, about 60 miles west of Brownsville, on the Rio Grande del Norte, and intersects the northern boundary about 50 miles east of Pembina. Between these two points the line is very irregular, but in middle latitudes makes a general curve to the westward. On the southern portion of the line the rainfall is somewhat evenly distributed through the seasons, but along the northern portion the rainfall of spring and summer is greater than that of fall and winter, and hence the boundary of what has been called the Arid Region runs farther to the west. Again, there is another modifying condition, namely, that of temperature. Where the temperature is greater, more rainfall is needed; where the temperature is less, agriculture is successful with a smaller amount of precipitation. But geographically this temperature is dependent upon two conditions--altitude and latitude. Along the northern portion of the line latitude is an important factor, and the line of possible agriculture without irrigation is carried still farther westward. This conclusion, based upon the consideration of rainfall 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 southeastern 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 doubtless 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 exaggerated; but at 20 inches agriculture will not be uniformly successful from season to season. Many droughts will occur; many seasons in a long series will be fruitless; and it may be doubted whether, on the whole, agriculture will prove remunerative. On this point it is impossible to speak with certainty. 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.