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Show REPORT OF THE CHIEF OF ENGINEERS. 343 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 secoud, as found above, was equal to 7,659 cubic feet. Hence 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 and Gila combined. 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 relative humidity. 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 aud moister air, which rests on the surface of the river. Where the river-hank is lined with trees, the cooler air is longer retained and does not mix so quickly with the neighboring strata: hence in 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 bad a temperature of 96° to Ur*J, when that upon the mesa, but a few yards distant, showed a temperature of 105- to 10-- F. In the following table a few observations are given, which show the extent of the influence of the river on the air: I>at«\ Time. Remark*. i , | •Jnly-24................ Sunrise................. HO 73.9 0.399 ' On river-bank. July24................___ilo................... !»2 '71.5 0.339 400 yardu from river. August 31.................do................... 7£ 3 61.0 0.510 liiver l.ank. Auburn 31.............i___do................... 74.4 00.2 0.44s On mesa, 400 feet from river. August 31............. 2p.m................... »6 6». 1 0. 't*0 Kiver bank. Aui;uftt31.................do................... 'J5.0 C6.1 0.20!* Ou iiu-aa, near river. TOPOGRAPHICAL WORK. This was carried.on by ray efficient topographer, Mr. Gilbert Thompson, assisted by Mr. George H. liirnie, on the tirst trip, aud by topographer F. A. Clark and G. H. Hir-nie on the second. The route throughout was meandered with the Casella theodolite, and distances measured with the odometer. Hearings were taken to prominent peaks and the topography of the country adjacent was noted. Whenever time and opportunity permitted peaks were ascended, aud 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 up 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-rrtation, and at camps near the river the altitude of the O ot the barometer above the level of the water was determined by leveling. At Stone's Ferry, Camp Mo-have, and Fort Yuma a daily record was kept of the fall and rise of the water in the river, and at Camp Mohave a permanent bench-mark was placed, to which the height of the river can be referred in future. This bench-mark is a stout iron pin driven down iuto the ground at the east end of the hospital. Its head, which is flush with the ground, is at an altitude of 755/2 feet above sea-level, as determined by our barometer observations. An arrangement for observing the rise and 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 fastened to two stout jnwts iu such a position that the line joiniug the intersections of the cross-wires was horizontal. A long 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 pipe is 4*2.5:! feet below the Wmli-mark, and the surface of the water on September 2 was 50/24 feet below, or 7«»4.iN» feet above solevel. At Fort Yuma a bench-mark was also established by driving an iron pin into the ground near the southeast corner of the platform which surrounds the flag-star)'. The altitude of the top of this pin. 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 •bove surface of water in the river near engiue-house, March lrt, l^G, is 84.41 feet. A record of these altitude* was placed in a cavity in the upper end of the pin. The distance of surface below Jiigh-water mark was measured wherever the point Dry bulb. Wet bulb. Relative liu uiitlity. |