5 



observations ; and we seem, therefore, entitled to conclude that the 

 diurnal movements of the two horizontal magnetometers are accounted 

 for by electric currents traversing the upper strata of the earth. 



There is one point of diiference, to which it important to draw at- 

 tention. It will be seen that the calculated curves are, for the most 

 part, above the observed. The reason of this will be evident upon a little 

 consideration. The zero from which the calculated results are measured 

 is the mean of the day ; whereas that of the observed results is the true 

 zero, corresponding to the absence of all current. JN'ow, the chief deflec- 

 tions of the galvanometer needle (as appears from the latter curves) are 

 those in which the sun is above the horizon ; and the zero line, conse- 

 quently, divides the area of the diurnal curve unequally, being conside- 

 rably nearer to the night observations than to those of the day. If the 

 calculated curves be displaced by a corresponding amount, their agree- 

 ment with the observed will be much closer. 



The difference here noted is one of considerable theoretical impor- 

 tance. Magnetometric observations furnish merely differential results, 

 the magnitude and the sign of which have reference solely to an arbi- 

 trary zero. We are accordingly ignorant even of the relative values of 

 the effects, and are unable to compare them with their physical causes, 

 whether real or supposed. In these respects the galvanometric observa- 

 tions have the advantage. In them positive and negative are physically 

 distinguished by the direction of the currents ; and this, as well as the 

 absence of all currents, is indicated by the instrument itself. The re- 

 sults, therefore, furnish the measures of the forces by which they are 

 produced. 



The next, and most important, ctep in this inquiry will be to assign 

 the physical cause of these phenomena. The existence of electric currents 

 traversing the earth's crust has hitherto been maintained as an hypothesis, 

 on account of its supposed adequacy to explain the terrestrial magnetic 

 cha,nges. Now, however, their existence is proved, not only to be a 

 fact, but also a fact sufficient to explain the phenomena. It remains^ 

 therefore, only to ascertain their source ; and it will be for those who 

 deny that the sun operates by its heat in producing the phenomena of 

 terrestrial magnetism, to assign to these currents a more probable 

 origin. 



Peopessoe William K. Sullivan read the following paper, written 

 by himself and Joseph P. O'Eeillt, C.E. : — 



On the Hydbocaebonates and Silicates of Zinc of the Peovince 

 or Santandee, Spain. 



eEOLOGICAL conditions UNDEE WHICH THE OEES OF ZINC OCCTJE. 



The district of country comprised by the province of Santander lies be- 

 tween the prolongation of the Pyrenees, which, under various names, tra- 

 verses the north of Spain, and the Bay of Biscay — ^the mountains forming 



