XXViii INTRODUCTION. 



Table 21 gives the temperature correction 



Zx 0.002039 (6 — 50°). 



The side argument is the mean temperature of the air column (6) given 

 for intervals of 1° from 0° to 100° F. The top argument is the approximate 

 difference of altitude Z obtained from Table 20. 



For temperatures above 50° F. , the correction is to be added, and for 

 temperatures below 50° F., the correction is to be subtracted. It will be 

 obser\^ed that the correction is a linear function of Z, and hence, for example, 

 the value for Z= 1740 is the sum of the corrections in the columns headed 

 1000, 700, and 40. 



In general, accurate altitudes can not be obtained unless the temperature 

 used is freed from diurnal variation. 



Table 22 gives the correction for latitude, and for the variation of gravity 

 with altitude in its effect on the weight of the mercury. When altitudes 

 are determined with aneroid barometers the second factor does not enter the 

 formula. In this case the effect of the latitude factor can be obtained 

 by taking the difference between the tabular value for the given latitude and 

 the tabular value for latitude 45° The side argument is the latitude of the 

 station given for intervals of 2° The top argument is the approximate 

 difference of height Z. 



Table 23 gives the correction for the average humidity of the air at 

 different temperatures; the values of the factor (i + ^3) adopted by Prof. 

 Ferrel and given on page xxiii have been used. This correction could have 

 been incorporated with the temperature factor in Table 21, but it is given 

 separately in order that the magnitude of the correction may be apparent, 

 and in order that, when the actual humidity is observed, the correction may 

 be computed if desired, by the expression 



Z 



(0.378 f) 



where e is the mean pressure of vapor in the air column, and d the mean 

 barometric pressure. 



The side argument is the mean temperature of the air column, varying 

 by intervals of 2° from — 20° F. to 96° F., except near the extremities of the 

 table where the interval is 4° The top argument is the approximate 

 difference of altitude Z. 



Table 24 gives the correction for the variation of gravity with altitude 

 in its effect on the weight of the air. The side argument is the approximate 

 difference of altitude Z, and the top argument is the elevation of the lower 

 station h^. 



The corrections given by Tables 22, 23 and 24 are all additive. 



