E. R. Hall— Effect of Magnetic Force, etc. 131 



prise me if the final verdict of science shall place the Potomac 

 formation, at least the lower member in which the plants occur, 

 within that geologic system. While the remaining types point 

 strongly in this direction, I do not regard the Dicotyledons as 

 at all negativing, but even more strongly suggesting this view. 



Still it may be admitted that according to the ordinary modes 

 of arguing from similar statistics, the sum of all the facts 

 here presented would make the Potomac, consideied from the 

 point of view of the flora alone, homotaxially equivalent to the 

 Wealden of England and North. Germany, now usually included 

 in the Cretaceous system. If the vertebrate remains are Ju- 

 rassic and the flora Cretaceous we only have here another con- 

 firmation of a law exemplified in so many other American de- 

 posits, that, taking European faunas and their correlated floras as 

 the standard of comparison, the plant life of this country is in 

 advance of the animal life. This law has been chiefly observed 

 in our Laramie and Tertiary deposits, but is now known to apply 

 even to Carboniferous and Devonian floras. It is therefore to 

 be expected that we shall find it to prevail during the Mesozoic 

 era. If, therefore, it be finally settled that the fauna of the 

 Potomac series is homotaxially Jurassic, and we take our start- 

 ing point from Old World geology, there will be no more objec- 

 tion to regarding the Potomac flora as Jurassic than there is 

 now in contemplating the Laramie flora as Cretaceous. In 

 fact, so far as the character of the flora is concerned there is 

 much less difficulty in the case of the Potomac than in that of 

 the Laramie, since I have shown the Potomac flora, viewed in 

 all its bearings, cannot be said positively to negative the refer- 

 ence of the formation to the Jurassic upon the evidence of the 

 plants alone. 



I do not, however, desire to be understood as arguing for 

 the Jurassic age of the Potomac formation. The most that it 

 is intended to claim is that, if the stratigraphical relations and 

 the animal remains shall finally require its reference to the 

 Jurassic, the plants do not present any serious obstacle to such 

 reference. 



Akt. XIY. — Experiments on the Effect of Magnetic Force on 

 the Equipotential lines of 'an Electric Current; by E. H. 

 Hall, Instructor in Physics at Harvard College. 



The experiments of which the following article will give 

 some results have been made in the Jefferson Physical Labora- 

 tory of Harvard College at occasional intervals during a period 

 of more than three years. Some of the results have been an- 



