space about m 2 a vector representing the acceleration which would be experienced 

 by a mass particle m 2 . This vector is called the attraction of m 2 and is given by 



_ JUL-L. . (4) 



m 2 



The unit of attraction in the c.g.s. system of units has been named the gal 

 after Galileo. The unit commonly used in geophysical prospecting is the 

 milligal or 10 — 3 gals. Sometimes a unit called a "gravity unit," equal to 

 10"~ * gals, is used. 



Suppose a mass particle m to be situated at a point Q whose cartesian co- 

 ordinates are (u,v,w,) and that we wish to write down an expression for the 

 attraction of m at the point P whose coordinates are (x,y,z) . The point occupied 

 by the mass is called the mass point or source point. The point P is called the 

 field point. The field point is the point where the attraction is observed or 

 computed. It would be the point occupied by the gravity meter. Starting with 

 equation (4) and writing r in terms of its components, the expression for the 

 attraction would become 



]• 



(x - u) (y - v) (z - w) 

 A= -km i + j + k — (5) 



yo v3 yo 



where 



r = [ (x - u) 2 + (y - v) 2 + (z - w) 2 ] 112 



From this we see that the components of the attraction are given by 



A. = - km (X - U) (6) 



A y = - km (y ~ V) (7) 



. , (z — w ) /0 . 



A * = ~ km r s ( 8 ) 



Certain distributions of matter, called centrobaric, attract as though all of 

 the mass were concentrated at a single point and for field points not enclosed 

 by the mass may be considered as particles. Uniformly dense spheres and 

 spherical shells are examples of such bodies. The attraction of any body of 

 matter, not centrobaric, may be approximated by a particle of the same mass 

 if the distance from the field point to any point within the body is large com- 

 pared to the largest dimension of the body. Thus the anomaly computed for 

 a sphere of uniform density may be helpful in studying the observed anomaly of 

 a subsurface body whose depth of burial is large compared to its size. 



586 



