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. THE SELECTION OF IRRIGABLE LANDS. From the fact that the area of arable lands greatly exceeds the irrigable, or the amount which the waters of the streams will serve, a wide choice in the selection of the latter is permitted. The considerations affecting the choice are diverse, but fall readily into two classes, viz: physical conditions and artificial conditions. The mountains and high plateaus are the great aqueous condensers; the mountains and high plateaus are also the reservoirs that hold the water fed to the streams in the irrigating season, for the fountains from which the rivers flow are the snow fields of the highlands. After the streams leave the highlands they steadily diminish in volume, the loss being due in part to direct evaporation, and in part to percolation in the sands from which the waters are eventually evaporated. In like manner irrigating canals starting near the mountains and running far out into the valleys and plains rapidly diminish in the volume of flowing water. Looking to the conservation of water, it is best to select lands as high along the streams as possible. But this consideration is directly opposed by considerations relating to temperature; the higher the land the colder the climate. Where the great majority of streams have their sources, agriculture is impossible on account of prevailing summer frosts; the lower the altitude the more genial the temperature; the lower the land the greater the variety of crops which can be cultivated; and to the extent that the variety of crops is multiplied the irrigating season is lengthened, until the maximum is reached in low altitudes and low latitudes where two crops