86 LANDS OF THE ARID REGION OF THE UNITED STATES. 
period. This has been done by very simple methods. Usually in each 
case a section of the stream has been selected having the least possible 
variation of outline and flow. <A cross-section of the stream has been 
measured, and the velocity of flow determined. With these factors the 
capacity of the streams has been obtained. In some cases single measure- 
ments have been made; in others several at different seasons, rarely in 
different years. The determination of the available volume of the several 
streams by such methods is necessarily uncertain, especially from the fact 
that it has not always been possible to gauge the streams exactly at the 
critical period; and, again, the flow in one season may differ materially from 
that in another. But as the capacity of a stream should never be rated by 
its volume in seasons of abundant flow, we have endeavored as far as pos- 
sible to determine the capacity of the streams in low water years. Alto- 
gether the amount of water in the several streams has been determined 
crudely, and at best the data given must be considered tolerable approxi- 
mations. In considering the several streams experience may hereafter 
discover many errors, but as the number of determinations is great, the 
average may be considered good. 
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 goy- 
erning 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 gra- 
dientors to a slight extent, but chiefly with the orograph, an instrument by 
which a great multiplicity of angles are observed and recorded by mechan- 
ical methods. his instrument was devised by Professor Thompson for the 
use of the survey, and has been fully described in the reports on the 
