ELECTRICAL ACTIVITY OF ORE BODIES. 315 



a speculation is, therefore, remote, artificial, and forced, and, in cases where 

 there is a better hypothesis, deserves only very secondary consideration. 



Suppose now that, in connection with an ore body, witli reference to 

 which experiments are being conducted, electric action actually does occur. 

 In the consideration of these currents we are at once confronted by the impor- 

 tant fact that insomuch as electric action has been going on for an indefinite 

 period of time the currents must have become constant both in intensity 

 and direction, and that therefore the equipotential surfaces corresponding to 

 this flow will have fixed and probably well-definable positions. 



In view of the fact that with most geological readers the consideration 

 of electric phenomena will be merely an incidental matter, it may be well to 

 be more explicit than would otherwise be necessar}'. By far the greater 

 number of electrical phenomena can be explained by regarding electricity 

 as in the nature of an incompressible fluid. The analogy is, in fact, very 

 complete, and extends even into further detail than need be noticed here. 

 We speak of a liquid as having a tendency to flow from a higher to a lower 

 level ; of electricity, as flowing from an equipotential of greater to one of 

 less value. In the former case the "levels" are approximately spheroidal 

 surfaces — "geoids'' — parallel to the normal surface of the earth; in the lat- 

 ter they may be closed, or may extend to infinity; they may be quite simple 

 or exceedingly complex. In order to exhibit the topography of a countr}^ in 

 detail, it may be represented graphically by the aid of a series of ecjuidistant 

 earth levels. In electricity an analogous problem is similarl}- solved, those 

 surfaces being chosen for which the potential value from surface to surface 

 increases by a definite amount.^ If a reservoir, the water in which is con- 

 stantly at a level, p, be joined by a pipe with one in which the water-level 

 is constantly q (both |) and q being measured vertically upwards from some 

 fixed datum, and^>3), the quantity of liquid traversing any right section 

 of the pipe in the unit of time would art. jiar. be dependent on the dimensions 

 of the latter and upon 2)—q- If a point on an equipotential of the value ^J 

 be connected by a thin wire with a point on one of the value q, analogous 

 remarks may be made with reference to the quantity of electricity (1) flowing 



'Neither level uor potential imply the presence of matter or of electricity, respectirely, at a 

 given point. 



