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Show 151 • In Colorado and New Mexico the cisterns of the United States Signal Service barometers at Santa F6*, N. Mex., and Colorado Springs, C. T., were taken as distance- points, and the temporary camps of the parties, as in California, taken as reference stations for all observations made in their vicinity during occupatiou. Horary tables were secnred for Los Anpeles, Cal., and Fort Mohave, Ariz. The horary tables published by Colonel Williamson in his paper on the " use of the barometer, & c," for stations in California, and that for Camp Independence, Cal., secured by your expedition of 1871, were also used as far as they were deemed applicable. In Colorado and New Mexico no additional horary corrections were secured, but those determined in 1873 and 1874 being applicable, were used. The observations taken in the field were copied by the observers on formB adapted to their complete preparation for final reduction, avoiding errors in copying and transcribing again and again upon special computation sheets. When filled, these forms were forwarded, when practicable, to the office in Washington, D. C, for revision and reduction, the certificate of the chief of party that they contain true copies of the original observations being required. In following out the system of observation and record required by the printed instructions published by the survey, a great number of observations at various points were secured and have been reduced. Upon the receipt of the observations from the field, the records of the various instruments were carefully examined, and their relative indices of error determined at every comparison, and therefrom their absolute errors upon the standard of the United States Signal Service in Washington. The hourly observations were then corrected for instrumental errors, reduced to 32° F. and to level, and after correcting erroneous or erratic observations revealed by plotting. Horary tables were formed for the reduction of isolated cistern barometer observations and for use in reducing thfc aneroid profiles. In forming the tables of horary corrections I have had the observations reduced to second level plotted, and have selected those days only when well- marked diurnal oscillations are exhibited. The relative humidities were not computed for each observation separately, but the meanB of the temperatures and wet- bulb indications were taken at each hour for the entire series, and the resulting computed humidities for the differences of these means taken for use in the barometric formula. Single results are liable to be so erratic and unreliable, that it is supposed that this course would give a sufficiently near approximation with less labor of computation for use in the barometric formula. In computing the altitudes of camps, & c, from cistern barometer observations, the full formula of Plantamour containing a term for humidity, and as represented by the tables in Williamson, has been used. In the reduction of aneroid observations the smaller terms of this formula have been omitted, since the aneroid differences of altitude are generally small, and from the construction of the instrument some of these terms necessarily do not apply. The observations have been referred by divers means to corresponding observations at the base station. When a long series of observations have been taken, the observations have been referred by a mean of the daily means, or of the observations at 7 a. m., 2, and 9 p. m. Isolated observations have been corrected for hoiary oscillations when suitable tables were known, and referred to the corresponding daily mean at the reference station ; otherwise simultaneous barometric observations have been obtained by interpolation and an approximate mean daily temperature used. Observations taken upon peaks have been referred, ( save in a few instances where they have been compared direct with the initial reference stations) to temporary stations at their bases, the approximate daily mean temperature being used in the temperature term, this temperature being observed at the base station, and found approximately as follows for the mountain station : The camps of the ascending party ( as a rule) were established at the limit of tree growth, where observations of the thermometer were taken at 7 a. m. and 9 p. m. on the day of the ascent before and after the occupation of the station. These observed temperatures have been reduced 3° F. for every 1,000 feet difference in elevation between this camp and summit, and taken in connection with the 2 p. m. observation on the peak for an approximate daily mean. Where a peak is high above the reference station, and for lack of fuel, cold will prevent observation at night upon its summit, some such device must be inaugurated for securing approximate daily mean temperatures. The temperature will necessarily always be observed near the hottest part of the day, and will give results the more erroneous the higher the peak is above the reference station. The changes in temperature must take place principally near the heating body, i. a., the earthy surface; and especially in the excessively dry atmosphere of the West, where a conservator of heat in the shape of aqueous vapor is wanting, the air directly in contact with the earth undergoes great fluctuations in temperature, during the day giving ranges of climate temperature not at all commensurate in effects with the less horary oscillations of the barometer, whereas, if there be quite a thick and wide stratum of air between the summit of the peak and the reference |