

Bowie — Recent Gravity Work in the United States. 103 



The General Instructions for Gravity Determinations state, 

 in part : 



(1) The determination of the intensity of gravitation shall 

 commence with standard determination of time, and shall end 

 with a second determination of time, made not less than 42 

 hours later than the first. Between these time determinations 

 the pendulums shall be kept swinging as nearly continuously 

 as is feasible. 



(2) A standard determination of the intensity of gravitation 

 shall consist of not less than six swings of which the probable 

 error of the mean result is not less than ± '004 dyne. 



(3) Each pendulum shall be swung in the direct position 

 only, and two successive swings are ordinarily to be made with 

 each pendulum. The standard conditions for each swing shall 

 be a pressure (reduced to zero degrees centigrade) of 60 milli- 

 meters, and total arc at beginning 5 millimeters. These stan- 

 dard conditions are to be approached as closely as is feasible 

 without the expenditure of extra time for that purpose. 



(4) The latitude of each gravity station must be determined 

 within 1'. 



(5) The elevation of each gravity station must be determined 

 within 50 feet, unless to secure this degree of accuracy would 

 require the expenditure of much time and money. 



Such computations were made in the field as were necessary 

 to ascertain whether the required degree of accuracy was 

 obtained. 



If it is assumed that there is no error in the value 980 , 111 

 dynes for the intensity of gravitation at the base station, then 

 the average actual error in the values for the other stations is 

 about *003 dyne, with a maximum error, in rare cases, of about 

 •010 dyne. The probable error of the absolute determination 

 of gravity at Potsdam is ± *003 dyne. The probable error of 

 the determination of the value for the Coast and Geodetic 

 Survey station, relative to Potsdam, is about ± "002. Hence, 

 the probable error of the base station is about ± "004 and the 

 average probable error of the other stations in the United 

 States is ± "005 dyne. It is interesting to compare this 

 accuracy with that of the stations determined at sea by Dr. 

 Hecker. It is probable that the Tincertainty in the value of 

 the intensity of gravitation at an ocean station is, in some cases, 

 as much as "10 dyne. 



Hayf ord applied Pratt's hypothesis of isostasy in his investi • 

 gations of the figure of the earth. It is a well-known fact that 

 the plumb line is attracted by the known excesses of mass 

 above the theoretical sea surface, and is repelled by the defect 

 of mass in the oceans. Hayf ord showed by solution B, in his 

 " Supplemental Investigation in 1909 of the Figure of the 



