100 A MANUAL OF TOPOGKAPHIC METHODS. 



with the value of the additional thousandths, in a separate column. These 

 values have been diminished by a constant, which does not alter the differ- 

 ence required. 



Table II gives the correction 2.343 feet X ( r — T> ) f° r the difference of 

 the temperature of the barometers at the two stations, or r — r'. As the 

 temperature at the upper station is generally lower, r — r' is usually posi- 

 tive and the correction negative. It becomes positive when the temperature 

 of the upper barometer is higher and r — r' negative. When the heights 

 of the barometers have been reduced to the same temperatures, or to the 

 freezing point, this table will not be used. 



z _i_5<>959 

 Table IV shows the correction D' — J~' ' to be applied to the 



approximate altitude for the decrease of gravity on a vertical acting on 

 the density of the mercurial column. It is always additive. 



Table V furnishes the small correction „ ^ , ,„ ,„~ for the decrease of 



10443430 



gravity on a vertical acting on the density of the air ; the height of the 



barometer /* 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 zz 28121.7; 

 in the last column but one on the right, we find for .005 zz 4.5, or for 

 29.345 =z 28126.2. Take likewise the value of h', and find the difference. 



If the barometrical heights have not been previously reduced to the ' 

 same temperature or to the freezing point, apply to the difference the cor- 

 rection found in Table II opposite the number representing r — t' ; we thus 

 obtain the approximate difference of level,' D. 



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



to its temperature, or D X ( c ~ ) make the sum of the tempera- 

 tures, subtract from that sum 64; multiply the rest into the approximate 



