OCR Text |
Show 152 station not in contact with the earth, the actual mean temperature of the entire stratum varies but slightly, if at all, during the 24 hours, unless disturbed by abnormal warm or cold currents; so that while the observed temperatures near the surface would when plotted, during say a month or a year, show well- marked daily maximum and minimum, the mean temperature of the thick stratum would show nearly uniform changes of temperature from season to season, but slightly affected by diurnal oscillations. This would seem but a natural consequence of the fact that in a dry climate, where but little heat is made sensible by the condensation of aqueous vapor in the formation of clouds and rain, and still less is absorbed by the dry air, the entire diurnal oscillation in temperature is due to the actual heating of the particles of air by contact with the heated earth, which heat, taken by the air, may disappear by radiation or be expended in work in expanding the particle and overcoming the resistance to its upward motion, leaving but comparatively little sensible heat, carried by connection, to be distributed throughout the great body of this stratum. If this surmise be correct, not only is it necessary to obtain the daily mean temperatures for use in the barometric formula, but the means for a longer time, if practicable. It would be interesting to find by experiment the limit of time for which the mean observed temperature, taken in connection with the observed height of the barometer corrected for norary oscillation, would give the best results. Corrections for difference in phase of abnormal waves or for abnormal oscillations have not been directly attempted. As far as practicable, however, observations at points where altitudes are required, especially at semi- permanent stations, have been referred to two or more reference stations and the results weighed in accordance with the known general directions of atmospheric waves and the distance of the point whose altitude is sought from the reference stations estimated in the direction of this wave motion. This supposes that the position of the wave between the two referenoe stations is an inclined plane, which is true only within small limits compared with the area covered by an entire wave at any given instant, but this seems the only means at our disposal by which we may readily and approximately eliminate the effects of abnormal disturbances from the resulting altitudes, when, as must necessarily be the case in the thinly-settled west, observations must be referred to distant points in other phases of disturbance, and when the observations are not at points sufficiently numerous to determine the barometric gradients. Since the middle of December the following work has been performed: Cistern- barometer stations computed 930 Aneroid- barometer stations computed '. 5,013 Hourly observations recomputed and plotted, days 270 Horary tables deduced 14 Cistern- barometer altitudes copied into permanent record- books, and indexed.. 2,424 Aneroid- Barometer altitudes copied into permanent record- books, and indexed.. 8,696 Total number of altitudes computed from barometric readings since 1871 11,125 All of the barometric work has been re- examined and such results as are considered most useful have been arranged for publication with Vol. II. The computation shave been made by Assistants F. M. Lee and George M. Dunn, and privates William Looram and John F. Kirkpatrick, Battalion of Eugineers, except Lieutenant Bergland's observations, which were computed under his own supervision at Los Angeles, Cal. Respectfully submitted. W. L. MAR8HALL, First Lieutenant of Engineers. Lieut. GEO. M. WHEELER, Corps of Engineers, in charge. APPENDIX G 2. ON THE METEOROLOGICAL CONDITIONS OF THE MOHAVE DESERT. UNITED STATES ENGINEER OFFICE, GEOGRAPHICAL SURVEYS WEST OF THE 100TH MERIDIAN, Washington D. C, February 10,1876. DEAR SIR : I have the honor to submit herewith a report upon the meteorological conditions of the Mohave Desert. During the past five years observations cpon the daily temperatures were made at the two military posts of Fort Yuma and Fort Mohave, both m the Colorado Valley; and to these military authorities thanks are due for information furnished. I hope |