GRAVITATIONAL METHODS 369 



forward, backward, and to the left for a considerable distance. Both the gravimeter 

 (G.M.) and the torsion balance (T.B.) are placed at the least favorable position for 

 each. The greatest effect on the meter occurs when it is directly over the bed and far 

 removed from the edge ; the greatest effect (maximum gradient) is experienced by the 

 T.B. when it is set up over the edge as shown. The calculated effects, which are 

 summarized in Fig. 217, show that the gradient effect is large, being about seven 

 times the probable error of the T.B. ; while the relative gravity effect is altogether 

 negligible. 



Case 2 deals with a boulder, which for simplicity has been taken as a sphere located 

 just below the surface. The calculated effects corresponding to the least favorable 

 positions of the two instruments are summarized in Fig. 217. They show again, that 

 in terms of the probable errors of the instruments, the effect on the balance is con- 

 siderably greater than on the meter. These calculations indicate that the gravimeter 

 should be capable of obtaining usable results in regions where shallow irregularities 

 of density would make torsion balance work practically worthless. 



The Gravimeter 



The gravimeter is an instrument with which relative vakies of the 

 force of gravity can be measured directly. It is similar to a field magneto- 

 meter in many respects. Gravity values obtained with the gravity meter are 

 relative only to some base station of the survey and, as in measurements 

 of the magnetic intensity, changes in value only need to be considered. 

 Latitude corrections and base checks for daily variation, which with the 

 gravity meter are purely instrumental matters, are handled in much the 

 same way as in magnetic surveying. 



Direct determinations of the relative gravity with a gravimeter or 

 gravity meter consist in "weighing" the same object with very great pre- 

 cision at several stations. The weight of an object at any location on the 

 earth's surface is equal to the force of attraction exerted by the earth on 

 the object. That is, the weight is equal to the product of the mass m of 

 the object, which remains the same at all locations, and the acceleration g 

 due to gravity. The acceleration g varies with the density of the materials 

 comprising the outer crust of the earth. Hence, the weight of a constant 

 mass VI at any station is affected by the natvn"e of the sul)surface materials. 

 For example, the value of g is greater at stations where the subsurface 

 material is relatively dense. The observed changes in weight are very small, 

 being of the order of one part in ten million of the total value of gravity. 



The Gravimeter as a Weighing Device, — Since the gravimeter is 

 essentially a very sensitive form of scales, its accuracy may be made 

 extremely high. To illustrate this degree of accuracy, it will be recalled 

 that 0.1 of a milligal is 1 part in ten million of the total force of gravity. 

 L. L. Nettletonf has put this matter into understandable terms along the 

 following line. In a spring-type gravimeter, a mass is suspended on a 

 spring. If the mass were such that it would stretch the spring to a length 

 of 30 centimeters, the accuracy of measurement necessary to detect a dif- 



t L. L. Nettleton, "Geophysical Prospecting for Oil," McGraw-Hill, 1930, p. 31. 



