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Show 123 The mean daily evaporation at Fort Yuma from March 19 to April 2,1876, was 0.5 inch, and the discharge of the river per second, as found above, was equal to 7,659 cubic feet. Heuce the evaporation from a lake- surface of 570 square miles would be equal to the quantity of water which the river could supply. Since the river was at its lowest stage, this gives minimum amount supplied by the Colorado aud Gila combiued. A few observations were made to discover the extent of the influence of small bodies of water, as the surface of a river, upon the increase of humidity in the air. Observations made on the river- bank, with the psychrometer, show not only a decrease of temperature, but also an increase in the relarive hamidity. A distance of a few hundred feet from the river this influence is hardly perceptible, as a great bulk of air is mingling continuously with the stratum of cooler and moister air, which rests on the surface of the river, i Where the river- bank is lined with trees, the cooler air is longer retained and does not mix so quickly with the neighboring strata; hence iu such localities the difference in temperature and humidity is considerable. Thus it was repeatedly observed at Stone's Ferry that the air under the trees on the river- bank had a tempeiature of 96° to 98°, when that upon the mesa, but a few yards dist- aut, showed a temperature of 105° to 10b° F. In the following table a few observations are given, which show the extent of the influence of the river on the air : Date. Time. 3 2 ,2 S Remarks. July 24 Sunrise. July 24 1.... do . .. August 31 1 do . .. August 31 - do August 31 2p. m... August31 1 . . . d o . . . 90 92 72.3 74.4 92 6 95.0 73.2 71.5 61.0 60.2 69.1 66.1 0.399 , On river- bank. 0.339 I 400 yards from river. 0.510 River- bank. 0.428 ' On mesa, 400 feet from river. 0.280 . River- bank. 0.209 On mesa, near river. TOPOGRAPHICAL WORK. This was carried on by my efficient topographer, Mr. Gilbert Thompson, assisted by Mr. George H. Birnie, on the first trip, and by topographer P. A. Clark and G. H. Bir-nie on the second. The route throughout was meandered with the Caselia theodolite, and distances measured with the odometer. Bearings were taken to prominent peaks and the topography of the country adjacent was noted. Whenever time and opportunity permitted peaks were ascended, and topographical and triangulation stations established. Owing to the excessive heat experienced while in the valley of the Colorado, during the summer- months, it was impossible to make as many ascents as might have been made in a cooler climate. The mountains being destitute of water and grass, rendered it necessary to carry these supplies np the mountains, where an ascent was made, and this it was not often practicable to do. In order to obtain the profile of the route the aneroid barometer was read at each meander- station, and at camps near the river the altitude of the O OI> t u e barometer above the level of the water was determined by leveling. At* Stone's Ferry, Camp Mohave, aud Fort Yuma a daily record was kept of the fall aud rise of the water in the river, and at Camp Mohave a permanent bench- mark was placed, to which the height of the river cau be referred in future. This bench- mark is a stout iron pin driven down into the ground at the east end of the hospital. Its head, which is flush with the ground, is at au altitude of 755.2 feet above sea- level, as determined by our barometer observations. Au arrangement for observing r. he rise aud fall of the river was placed a short distance from the bank. * ' A piece of iron water- pipe was taken and cross- wires placed at each end. The pipe was then firmly fasteued to two stout posts in such a position that the line joiniug the intersections of the cross- wires was horizontal. A loug graduated rod is placed upright at the edge of the water, when the observer looks through the pipe and takes the reading. The axis of the pip*- is 42.52 feet below the bench- mark, and the surface of the water on September 2 was 50.24 feet below, or " 704.96 feet above sea- level. At Fort Yuma a bench- mark was also established by driving an iron pin into the ground uear the southeast comer of thu platform which surrounds the flag- staff. The altitude of the top of this piu, as obtained from Southern Pacific Railroad levels, is 204.56 feet above sea- level, referred to the Fort Point tide- gauge. The altitude of pin above surface of water in the river near engine- house, March 18, 1876, is 84.41 feet. A record of these altitudes was placed in a cavity in the upper end of the pin. The distance of surface below high- water mark was measured wherever the point |