Chap. 7] GRAVITATIONAL METHODS 123 



Swick.^" For the Potsdam apparatus, observations with the one-pendulum 

 and twin-pendulum methods are found in Schmehl's publication; Vening 

 Meinesz has illustrated the application of his method by photographs of 

 records and calculation examples.'* 



E. Gravimeters 



The pendulum methods discussed in the preceding section are sometimes 

 referred to as "dynamic" gravity procedures since they involve the meas- 

 urement of time. Other possibilities in the same category are: (1) the 

 determination of the time and distance characteristics of free fall in vacuum ; 

 (2) a comparison of gravity with the centrifugal force of a rotating body by 

 measuring the slope of a surface of mercury subjected to rapid rotation in 

 a vessel. None of these methods has been perfected to the same degree of 

 accuracy as that found in pendulum or gravimeter methods. 



The term "gravimeter," or "gravity meter," is customarily applied to an 

 instrument involving a static method of comparing gravity with an elastic 

 force and a measurement of the deflection or position of certain "indica- 

 tors" when gravity and comparison force are in equilibrium. In the 

 barometric method, atmospheric pressure as measured with an aneroid (or 

 boiling-point thermometer) is compared with the reading of a mercury 

 barometer; in the volumetric method (Haalck gravimeter), both sides of a 

 mercury barometer are connected to two vessels with the air under different 

 pressure. In all remaining static gravity methods, the elasticity of springs 

 is used for comparison with gravity. 



The mechanical gravimeters fall into two groups : nonastatic and astatic. 

 Gravimeters that are modifications of horizontal seismometers may be 

 called horizontal seismo-gravimeters, while those resembling vertical 

 seismographs may be designated vertical seismo-gravimeters. 



1. Barometric method. If atmospheric pressure is measured at different 

 localities with an aneroid and a mercur}^ barometer, discrepancies result 

 because the mercury barometer is affected by variations in gravity. Since 

 the aneroid does not give sufficient accuracy, the boiling point of water is 

 measured instead to give atmospheric pressure. The barometric method 

 was the first to make possible determinations of gravity on board ship and 

 was perfected principally by Hecker. To obtain a mean error of ±40 

 milligals it is necessary to read the boiling point with an accuracy of 

 1/1000° C. and the barometer with an accuracy of 0.01 mm. How much 



^2 Loc. cit. 



'^ Schmehl, Zeit Geophys., loc, cit. 



'^ Loc. cit. 



^^ See page 580, Fig. 9-94. 



