ELECTRICAL ACTIVITY OF ORE BODIES. 307 



Local action, however, in contrast to tlie foreg-oing-, would ]irobably 

 manifest itself locally in the imaginary surface, as a hillock or depres- 

 sion. It is to such anomalies that attention should subsequently be 

 directed, the electric activity of ore bodies, differences of potential of li([uids 

 in contact, and Matteuci's effects' constituting the salient points to be consid- 

 ered. In the interest of expeditious work all measurements should be made 

 electronietrically, the lines of survey radiating from a central point (]\ C.) 

 the potential of which is arbitrarily taken as zero." 



' Those uieutioued, p. 352. 



^Tho prosecution of the experiments described in this chapter was aided by the cordial coopera- 

 tion of Messrs. Pattou, Lamb, anil Ballard, of Virginia City, and Messrs. Rickard, Westcott, Harris, 

 and Bryan, of Eureka, Nevada. The work is also indebted to Professor Micliie, of West Point, for the 

 loan of a Rowland magnetometer made by Mr. Wm. Gruuow, of New York. 



