Bowie — Recent Gravity Work in the United States. 101 



Akt. X. — Recent Gravity Work in the United States /* by 

 "William Bowie. 



This paper gives a summary of the gravity work done by the 

 Coast and Geodetic Survey during the past few years. This 

 work comes under two general heads. First, the instruments 

 and the field work and methods ; and, second, the reduction 

 and discussion of the observations. 



In 1891 the Survey began the use of two sets of half -second 

 pendulums. f They proved to be economical in their operation 

 in the field and gave accurate results in the relative determi- 

 nation of the intensity of gravitation. The pendulum is 

 essentially a flat stem, supporting a bob, with devices at the 

 other end of the stem (called the pendulum head) to support 

 the whole upon an agate knife edge fastened to the pendulum 

 case. In the head is fastened rigidly an agate plane which 

 rests upon the knife edge. As originally designed, the knife 

 edge was in the head and the plane attached to the case. By 

 having the plane in the head a more nearly invariable length of 

 pendulum was secured, for any wear on the planes would have 

 negligible effect on the period of oscillation. 



Before beginning a swing of a pendulum the air in the case 

 is practically exhausted (to about 60 millimeters of mercury). 

 With a low air pressure a long period of swinging may be had 

 with a small arc of oscillation. By means of a break-circuit 

 chronometer, a flash apparatus and two mirrors, one on the 

 case and the other fastened to the pendulum head, the obser- 

 vations are made by noting the time of a coincidence between 

 the beat of the pendulum and that of the chronometer. If 

 two such coincidences are observed, one can then obtain the 

 period of the pendulum with reference to the chronometer 

 time. The pendulum is so made that its period is slightly 

 greater or less than a half second and a coincidence between 

 the beat of the pendulum' and that of the chronometer occurs 

 every five or six minutes. The rate of the chronometer is 

 obtained by observations with an astronomic transit on stars as 

 they cross the meridian. Another piece of apparatus now in 

 use is the interferometer. This is used instead of the static 

 method formerly employed to get the flexure or movement of 

 the pendulum case due to the horizontal force applied by the 

 swinging pendulum. The case moves with the pendulum and 

 tends to lengthen the period of oscillation. 



The use of the interferometer to determine flexure was pro- 

 posed by Mr. John F. Hayford while he was Inspector of 



* Read before the Philosophical Society of Washington, May 6, 1911. 

 f See Appendix 15, C. and G. S. Report for 1891, and Appendix 1 Report 

 for 1894. 



Am. Jour. Sci.— Fourth Series, Vol. XXXII, No. 188.— August, 1911. 



