40 



CONSTRUCTION OF ISOBARIC CHARTS 



The dimension or "dimensional equation" of the quantity V is obtained from 

 formula (1), and in fact this quantity is expressed in terms of —-. 2^, that is to say 



the dimension for work done upon a unit mass. In Table 3 the unit for V is 



mile*^ 

 chosen as one ^ -^ in order that the velocities resulting from the solenoids may be 



mile 

 expressed in r . In order to obtain from Table 3 the value of V at any given ele- 

 vation, e. g., 3 487 feet, above the level of the station barometer at Omaha, we pro- 

 ceed as follows. First in the principal Table 3 we seek the value of V corresponding 

 to z = 3 480 feet, viz., 70 570 ; then by the aid of the small auxiliary table of pro- 

 portional parts we find for z = l feet the additional portion of F= 100 and thus the 



mile'^ 

 complete V= 70 670 for z = 3 487 feet. Consequently work amounting to 70 670 , 



must be performed in order to raise the unit mass from sealevel to the altitude of 

 3 487 feet above the station barometer at Omaha, or we may say that there are 70 670 

 level surfaces of gravity between sealevel and the point standing 3 487 feet above the 

 Omaha station barometer. 



This method for the calculation of V can be applied at all stations where g„ has 

 been previously determined by pendulum observations. At points where no such 

 measurements of r/o have been made the following well-known formula for the calcula- 

 tion of gravity at the earth's surface must be employed, 



g^ = 32.1726(1 - 0.002 59 cos 2X) (1 - 0.000 000 059 7z^). 



Table 4. 

 The Accklkeation of Gravity at Sealevel. 



(6) 



