6 PAUL WEAVER 
of the distance across the valley, variations in temperatures and den- 
sity of the air give rise to refraction, dust clouds decrease visibility, 
the party must continue because they have a limited time within 
which they must obtain water, and extended surveys are therefore 
not permitted. The engineer, however, must make some recommenda- 
tion, as the party must go on, and this is the situation which is gener- 
ally present in our geophysical problems of to-day. 
The first successful geophysical application to commercial work 
was the determination of depth to the ocean bottom, and this prob- 
lem is analogous to the measurement in the Colorado mountain be- 
cause the water between the measuring device and the object whose 
position is sought by the measurements has only slight variation in 
physical properties, and a calibration chart for these variations can 
be readily made. There are some geophysical problems where these 
conditions also exist, but they are not the ones to which our greatest 
effort is being devoted at this time. 
The principal geophysical methods which are being used are 
familiar to all of you. The measurements made by these methods are 
of the variation of gravity, of the variation of magnetic field strength, 
of the variation of electrical field strength, and of the variation in 
velocity of sound. They have been divided into methods which rep- 
resent the measurement of a natural field and those which give the 
measurements of an artificial field, but in considering the relation of 
geophysics to geology, a different classification is necessary, and the 
geophysical methods may be divided into two classes, those which 
measure a potential function or some of its components, and those 
which determine a point.’ 
3 The reader is referred for a complete discussion of a potential function to: W. D. 
MacMillan, The Theory of the Potential (McGraw-Hill Book Company, 1930). For the 
aid of those who are not at present interested further than in this present article, the 
following short summary may serve: 
Bodies in space exert force upon other bodies at a distance. The amount of the 
force varies in some way with the distance, usually inversely as the square of the dis- 
tance. Some kinds of force are attractive, some repellent. Now to define the potential, 
let us take one body, which we call A, at a fixed position. Let us move another body 
from a point 1 in space to a point 2, and let this second body be so small that every 
part of it can be considered the same distance from A, so we will call it P, meaning a 
particle. It will require work to move it from r to 2 if they are at different distances 
from A. Then, the amount of this work is equal to the difference in the values of the 
potential function for z and for 2. 
The potential function is a number, since work is a number. It has no direction, 
but it changes in general from every point in space to the next one, and at different 
rates. We express this by saying that the potential function is a scalar function of posi- 
tion, and that its derivatives are the components of a vector. 
Gravity, magnetic, and electric forces each vary inversely as the square of the 
distance, but differ in that the gravity force is always one ef attraction, whereas the 
magnetic and electric may be either attractive or repellent. The potentials for these 
forces, therefore, “differ from one another only in the constant factor of proportionality 
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