Mr. J. Croll on Geological Time. 363 



The quantity of the alternating current is determined by A ; 

 and the value of a only affects the epoch of maximum current. 

 If we make p = 0, the effect is that of closing the circuit of x, 

 and we find 



"W + LW 



This expression shows that the condenser has no effect when the 

 current is closed. 



If we make p = co , the effect is that of removing the con- 

 ductor y, and thus breaking the circuit. In this case 



M¥ 



A 2 = 



R 2 -f 



(--A)' 



This expression gives a greater value of A than when the circuit 

 is closed^ provided 2CL?z 2 is greater than unity, which may be 

 ensured by increasing the capacity of the condenser, the self- 

 induction of the electromagnetic coil, or the velocity of rotation. 

 If CLw 2 = l, the expression is reduced to 



A Mn 



A =TT 



This is the greatest effect which can be produced with a given 

 velocity, and is the same as if the current in the coil had no 

 " electromagnetic momentum." 



If the electromagnet has a secondary coil outside the primary 

 coil so as to form an ordinary induction-coil, the intensity of the 

 secondary current will depend essentially on that of the primary 

 which has just been found. Although the reaction of the se- 

 condary current on the primary coil will introduce a greater 

 complication in the mathematical expressions, the remarkable 

 phenomenon described by Mr. Grove does not require us to enter 

 into this calculation, as the secondary sparks observed by him 

 are a mere indication of what takes place in the primary coil. 



XLIII. On Geological Time, and the probable Date of the Glacial 

 and the Upper Miocene Period. By James Croll, of the 

 Geological Survey of Scotland*. 



IT is nearly a century ago since Lagrange's determinations of 

 the superior limits of the excentricity of the planetary 

 orbits were publishedf . The results at which he arrived were 



* Communicated by the Author. 



f Memoirs of the Berlin Academy for 1782. 



