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Show 52 plot, which are selected because they show both under and over compensation, and because the observations and comparisons having been made at the same temperatures, they can be plotted together. In no case have I found the aneroids affected by an amount exceeding one- half the reduction to 32° Fahrenheit of a mercurial barometer, at a mean height of 22 inches, for a range of temperature of from 35° to 40° Fahrenheit, and since this, even iu extreme value, is less than the smallest subdivision of the aneroid's graduated face, aud not perhaps far exceeding the ordinary errors in reading the instrument, I have not directly corrected for it, especially since I do not feel justified in making tables of corrections for temperature to be applied to aneroid readings from the defective data at my command. It would take much longer series of careful comparisons of each aneroid, through a wider range of temperature, to properly eliminate errors in the individual comparisons, and give us the true movement of the instrument, which is only indicated approximately by the tables I have formed and represented on the plate; but it seems to me that they indicate enough to make it clear that no very material errors in the results, computed as they have been, can be due to this cause. The aneroids to be used in future upon this survey have been made to order by Casella, of London, are 3 inches in diameter, with attached thermometers, and a movable scale of feet. The graduation upon the scale of feet is made by one of the usual barometric formulas, where there is no special term for humidity, leaving out the correction for the variation in the force of gravity upon the mercurial column, the zero or reference plane being assumed where the mercury stands at 30 inches and the mean temperature at 50° Fahrenheit. When, therefore, these conditions are fulfilled, the indications of the aneroid in feet in latitudes where this datum- plane coincides with sea- level will give directly the approximate altitude of the station above the sea, provided the aneroid- errors be corrected for. To enable tbe observer to correct for index- error, and to avoid the necessity for his adding or subtracting a constant quantity to the indications of his instrument, is the objeot of the movable scale. The aneroid, before the day's work begins, is compared with the mercurial barometer at 32° Fahrenheit, and its index- error determined. The zero of the scale of feet is then set by turning the movable ring upon which the scale is engraved to the right or left of 30". 0 by a quantity, expressed in inches- of the scale, equal to the index- error of the instrument; the effect is to keep the zero of the feet- scale at the constant datum-plane where the cistern- barometer at 32° Fahrenheit would stand at 30". 0. Altitudes, then, read off the face of the aneroid, are affected by the erroneous assumed position of the sea- level; by all periodic and non- periodic fluctuations in the barometric height: by the difference in the temperature- correction between that due 50° Fahrenheit and the actual mean temperature of the two stations; and by the undetermined instrumental errors. Profiles, therefore, given by the direct use of the aneroid scale of feet, are but rough approximations, the principal source of error in which, in short intervals of time, is the temperature- correction above or below that due 50° Fahrenheit, which often amonnts Jto many feet in the computed difference of altitudes between two stations very near each other horizontally. I give below the profile of a single day's march, as derived directly from the face- readings of the aneroid, and from computation, which shows pretty fairly the differences which we may expect in the gradients derived from the two methods. Camp or station. Camp 10 , , T r . , . * - x T , - r . . r . 9 3 4 5 1 6 7 8 9 10 11 12 Camp 11 Altitude from aneroid face. 8720 8875 9025 9160 9200 9355 9530 8960 9245 8675 8790 9090 9470 8520 Altitude computed. 8961.9 9118.9 9273.4 9403.2 9436.0 9602.1 9771.9 9180.9 9454.7 8821.6 8949.9 9207.0 9666.1 8655.8 Difference, Feet + 241.2 243.9 948.4 943.9 930.0 946.1 941.9 990.9 909.7 146.6 159.9 187.0 196.1 135.8 Distance betw'n stations. Miles. 0.000 0.903 9.930 .730 .753 9.393 .839 1.633 3.431 3.195 .663 .816 1.920 9.00 Grade per mile, from feet- scale. 761.0 67.9 185.0 53.1 64.8 908.3 349.0 83.1 147.3 174.9 980.5 368.8 475.0 Grade per mile, computed. 769.0 69.3 177.7 43.6 69.4 901.3 361.9 80.0 198.2 194.4 313.5 376.3 505.1 © P. r + 8.0 + 9.1 - 7.3 - 9.5 + 4.6 - 7.0 + 19.9 - 3.1 + 50.9 + 90.9 + 33.0 + 9.5 + 30.1 |