374 BAROMETRICAL MEASUREMENT OF HEIGHTS. 



Table V. furnishes the small correction -^.^vz-r for the decrease of sravitv on 



10443430 ^ -^ 



a vertical acting on the density of the air ; the height of the barometer h at the 



lower station representing its approximate altitude. Like the preceding correction, it 



is always additive. 



Use of the Tables. 



In Table I. find first the numbers corresponding to the observed heights of the 

 barometer h and h'. Suppose, for instance, h = 29.345 in.; find in the first column on 

 the left the number 29.3; on the same horizontal line, in the column headed .04, is 

 given the number corresponding to 29.34 = 28121.7 ; in the last column but one on 

 the right, we find for .005 = 4.5, or for 29.345 := 28126.2. Take likewise the 

 value of h', and find the difference. 



If the barometrical heights have not been previously reduced to the same tem- 

 perature, or to the freezing point, apply to the difference the correction found in 

 Table II. opposite the number representing t — t' ; we thus obtain the appro.ximate 

 difference of level, D. 



For computing the correction due to the expansion of the air according to its 



temperature, or D x ( —^7.^; )■> make the sum of the temperatures, subtract from 



that sum 64 ; multiply the rest into the approximate difference D, and divide the 

 product by 900. This correction is of the same sign as (t -\- t' — 64). By apply- 

 ing it, we obtain a second approximate difference of level, D'. 



In Table III., with D' and the mean latitude of the stations, find the correction for 

 variation of gravity in latitude, and add it to D\ paying due attention to the sign. 



In Table IV. with D', and in Table V. with D' and the height of the barometer at 

 the lower station, take the corrections for the decrease of gravity on a vertical, and 

 add them to the approximate difference of level. 



The sum thus found is the true difference of level between the two stations, or Z ; 

 by adding the elevation of the lower station above the level of the sea, when known, 

 we obtain the altitude of the upper station. 



The use of the small table, VI., by means of which approximate differences of 

 level can be obtained by a single multiplication, is explained below, page 90. 



Example 1. 



Measurement of Mount Washington, New Hampshire, by A. Guyot, August 8th, 

 1851, 4 p. M. ; the barometer at the lower station being at 825 English feet above 

 the mean level of the sea; at the upper station at one foot below the summit. 



The observation gave, 



Barometer. Attached Thermometer. Temperature of Air. 



Gorham, h = 29.272 in. r = 70°.70 F. t = 72°.05 F. 



Mount Washington, h' = 24.030 " t' = 54°.52 F. t' = 50°.54 F. 



t — t'=: 16°.38 F. 122°.59 F. 



— 64° 



t-\-t' — 6A= 58°.59 F. 

 D 34 



