L. V. Pirsson — Geology of Conanicut Island. 363 



of a strong magnetic field and are able to substantiate his 

 result ; for no effect of tlie field was observed. The function 

 of the magnetic field and the air blast therefore appears to 

 reside in the blowing out of a species of voltaic arc which 

 would tend to prevent the full charge, and the quick discharge 

 of a condenser. 



While we have dwelt in this paper principally upon the 

 point of the difference between impulse and electrical oscilla- 

 tion and upon the possibility of transforming one and of the 

 limits of transformation of the other, we are led also to speak 

 of the importance of adjusting the rate of frequency of the 

 impulses of the generator to the time of charging the con- 

 denser which is employed. It is evident that the rate of the 

 generator should not be higher than the time taken to charge 

 the condenser : for in that case the quantity of discharge would 

 be less than that which might be obtained with a slower rate 

 of alternation in the generator.* For spectrum analysis we 

 have found that a generator giving 400 alternations per second 

 gave better results with various dielectrics than an alternator 

 giving 1000 alternations per second. 



Jefferson Physical Laboratory. 



Art. LI. — On the Geology and Petrography of Conanicut 

 Island, R. 1. ; by L. V. Pirsson. 



Conanicut Island, lying in E"arragansett Bay in Rhode 

 Island and crossed by the meridian 71° 23' of west longitude 

 and the parallel 41° 30' of north latitude, is about nine miles 

 long in its greatest extent and nowhere much over a mile in 

 breadth. As will be seen by reference to the map of fig. 1 it 

 consists in reality of two portions, one large, the other much 

 smaller, connected by a curving sand beach about an eighth of 

 a mile long and nowhere much above a hundred yards in 

 breadth. 



The island consists in the main of extremely fissile soft 

 shales, which weathering easily have determined the topog- 

 raphy and given rise to smoothly rounded slopes and low 

 hills. The highest point is about 135 feet above sea level. 

 These smooth and undulating surfaces afford therefore no 

 exposures of the underlying rocks and it is only along the 

 shores where the constant gnawing of the waves has cut 

 into the soft shales, making in places rugged shore lines, that 



* Indeed the rate of the alternator may be so rapid that a large condenser in 

 the circuit can be made to appear of small capacity. And under these condi- 

 tions the condenser will not serve to annul self induction. The time of the re- 

 sidual charges must also be taken into account. The charging of a condenser, 

 therefore, by means of an alternator is far from a simple phenomenon. 



