73 



Suppose, also, that the mean annual temperature at the station 

 is 56°. 



Table V gives 0.103 for 1,500 feet and temperature 56°. To reduce, 



therefore, from the " actual elevation " of 1,516.4 feet to the " station 



elevation " of 1,482.7 feet, the following correction is necessary, viz, 



33 7 



-yTjTT X 0.103 = 0.035, which is the "removal correction" required on 



Form No. 1059-Metl. The correction in this case must be used with 

 the plus sign. 



TABLES VI AND VII. ^DETEKMINATION OF HEIGHTS BY BAROMETER 



199. The use of these tables requires that at least two observations 

 of the temperature and the pressure of the air be made, simulta- 

 neously if possible, at two stations. The elevation of one of the 

 stations must be known. 



From Table VI we find the first approximate difference of eleva- 

 tion of the two stations. Table VII gives the allowance that must 

 be made on account of the temperature of the air. 



To be strictly accurate, allowance should also be made for three 

 other effects: (a) For the amount of moisture in the air; (&) for 

 the effect of gravity on the weight of the air; (c) for the effect of 

 gravity on the weight of the mercury (not required when the aneroid 

 is used or when the readings of a mercurial barometer are separately 

 corrected for gravity). By neglecting these effects in computing a 

 high elevation, say 10,000 feet, other conditions being average, an 

 error of fully 100 feet may be made. Greater inaccuracies than this, 

 however, are likely to result if the computations are based upon only 

 a few barometric and temperature readings, and especially if the 

 readings are not strictly simultaneous. Moreover, the air tempera- 

 ture required is the mean temperature of the whole column between 

 the two stations, but this is only approximately given by the mean 

 of the observed temperatures. 



The two tables following are, therefore, sufficiently accurate for 

 use of tourists who desire a knowledge of the approximate altitude 

 corresponding to the more or less limited and incomplete observa- 

 tions they may make. Example : 



Barometer reading at base station inches— 27.58 



Barometer reading at upper station do 21. 47 



Air temperature at base station degrees — 68 



Air temperature at upper station do 42 



Estimated mean temperature of air column is — 

 68+42 110 _ 



Feet 



Table III, at 21.47, gives 9, 002 



at 27.58, gives 2,195 



First approximate difference at elevation 6,807 



Table IV, for T = 55° and 6,800 feet gives: 



Feet 

 Allowance for temperature of air +69 



Hence difference of elevation is 6, 876 



Suppose the elevation of the base station is 1, 851 



Then the elevation of the upper station is 8, 727 



