1877.] On Resistance to the Passage of an Electric Current. 401 



transmission in a perpendicular direction across a crystalline plate cut 

 otherwise than perpendicular to a principal plane, I have since worked 

 out the differential equation (between two parameters) of the lines of 

 curvature of the wave-surface, the discussion of which shows that there 

 are no umbilici out of the principal planes. Hence the four directions 

 determined by equations (15) are the only ones perpendicular to which 

 if a plate be cut one of the images is free from astigmatism. — October 

 1877. 



XI. " Notes on Physical Geology. — No. III. On the probable 

 Age of the Continent of Asia and Europe ; and on the Abso- 

 lute Measure of Geological Time/' By Rev. S. Hattghton, 

 M.D., D.C.L., F.R.S., Professor of Geology in the University 

 of Dublin. Received June 11, 1877. 



This paper was withdrawn by the Author after the reading, for correc- 

 tion of a numerical error, and will appear as amended next Session. 



The Society then adjourned over the Long Vacation, to Thursday, 

 November 15. 



<( On the Increase in Resistance to the Passage of an Electric 

 Current produced on certain Wires by Stretching/'' By Her- 

 bert Tomlinson, B.A., Demonstrator of Natural Philosophy, 

 King's College, London. Communicated by Prof. W. G. 

 Adams, F.R.S. Received November 14, 1876. Read Decem- 

 ber 21 * 



The object of this iuquiry was (1) to determine the relation between 

 increased resistance to the passage of an electric current and stretching- 

 force ; (2) to ascertain how much of the increased resistance in each case 

 is produced by mere increase of length and diminution of section. 



In order to determine the increase of resistance from stretching, the 

 wires were each divided into two parts about 14 feet or more in length. 

 One end of each part was fastened to a stout hook, firmly fixed into a 

 block of wood, the two hooks used being about 8 inches apart, and the 

 block of wood in which they were fixed securely fastened across two 

 strong uprights, which were placed resting against the wall, so that the 

 weights attached to the wires might swing clear of the table on which 

 the uprights were placed. A loop was made at the other end of each part 



* SeeProc. Roy. Soc. vol. xxv. p. 451. 



