PEOSPECTING. 147 



"By an extension of the same reasoning, it is clear that if the surface 

 were level and horizontal, and if a complete thermal survey of the surface 

 were to be made, the result might be expressed by a series of isothermal 

 contours analogous to those by which topographical features are ordinarily 

 presented, and that the summit of an elevation on the thermal map would 

 lie vertically above the hot body. 



"Similar methods of procedure and expression are applicable to a center 

 of electrical excitation. If the body were electrically active, an electrical 

 survey would result in the determination of a series of equi-potential con- 

 tours separated by a fixed difference of potential, and these would culminate 

 above the ore body. In short, replace temperature by potential, isothermal 

 by equi-potential, and the consideration made in reference to the hot body 

 will apply to an ore body, only that in the case of electrical excitation we 

 have to do with circumstances vastly more complex, with a body, as it were, 

 in part hot, in part cold, or one over which heat is irregularly distributed. 

 "The remarks made on the surface manifestations of a subterranean hot 

 body apply readily to any imaginary line or any imaginary plane lying be- 

 neath the surface and sufficiently near the hot body. To make the case per- 

 fectly general, however, we should have to consider the isothermal surfaces 

 themselves in their actual position and contour. The first of these would 

 completely envelop the hot body; whereas, subsequent ones intersect the sur- 

 face of the earth until finally they would become indistinguishable from the 

 normal terrestrial isothermal, as shown by the dotted lines in the figure. 

 Similarly, in the case of a detailed electrical investigation, it would be nec- 

 essary to trace the equi-potentials as surfaces surrounding and intersecting 

 the electrically active ore body. The presence of an ore body is evidently 

 manifested throughout the whole superficial and subterranean region in 

 which the equi-potential surfaces are traceable or in which an electrical 

 disturbance due to the presence of an ore body exists, and the applicability 

 of the electrical method of prospecting consists in the fact that the indica- 

 tions of the existence of an ore body occupy a space greatly in excess of 

 the size of the body itself, namely, the whole region of sensible electrical 

 excitation. 



