Report on the lands of the arid region of the United States, with a more detailed account of the lands of Utah

by John Wesley Powell; Willis Drummond; Clarence E. Dutton; Grove Karl Gilbert; A. H. Thompson

METHODS OF DETERMINING THE EXTENT OF IRRIGABLE LAND UNLIMITED BY WATER SUPPLY.

In the few cases where the water supply is more than sufficient to serve the arable lands, the character of the problem is entirely changed, and it becomes necessary then to determine the area to which the waters can be carried. These problems are hypsometric; relative altitudes are the governing conditions. The hypsometric methods were barometric and angular; that is, from the barometric stations vertical angles were taken and recorded to all the principal points in the topography of the country; mercurial and aneroid barometers were used, chiefly the former; the latter to a limited extent, for subsidiary work. Angular measurements were made with gradientors to a slight extent, but chiefly with the orograph, an instrument by which a great multiplicity of angles are observed and recorded by mechanical methods. This instrument was devised by Professor Thompson for the use of the survey, and has been fully described in the reports on the geographical operations. To run hypsometric lines with spirit levels would have involved a great amount of labor and been exceedingly expensive, and such a method was entirely impracticable with the means at command, but the methods used give fairly approximate results, and perhaps all that is necessary for the purposes to be subserved.