18 



Mr. G. J. Burch. 



[Nov. 21, 



into a number of independent families of equal rank. These are 

 based on the study of skeletal structures, known facts of anatomy, 

 and pliylogenetic relationships. She arranges the families, according 

 to the lines of descent demonstrated in the present paper, as fol- 

 lows : — 



Zaphrentoidean Families : Zaphrentidse, Amphiastraeida?, Turbino- 

 lidas, Stylinidse, Oculinidae, Pooilloporidee, Madreporidee, Poritida?. 



Cyatliopliylloiclean Families : Cyathophyllida3, Astrasidse, Fungidap. 

 Eupsanmridse. 



V. " On the Calibration of the Capillary Electrometer." By 

 GEORGE J. Burch, M.A. Communicated by Professor 

 B. Price, F.R.S. Received June 24, 1895. 



In my papers* " On a Method of Determining the value of Rapid 

 Variations of a Difference of Potential," and " On the Time-Rela- 

 tions of the Capillary Electrometer," I showed that the photographic 

 record of an excursion of the capillary electrometer indicates electro- 

 motive changes not only qualitatively but quantitatively, even when 

 they last too short a time for the movements of the meniscus to be 

 completed. 



The movement of the mercury commences simultaneously with the 

 communication to it of the E.M.P., and ceases the instant it is with- 

 drawn, the velocity of the meniscus at any instant being proportional 

 to the difference between the P.D. of the source and the P.D. of the 

 charge in the electrometer. For the sake of brevity, this difference 

 will be referred to as the Acting P.D. 



I showed that if an excursion is recorded in the form of a curve, of 

 which the abscissae are proportional to the times and the ordinates to 

 the acting P.D.'s, the acting P.D. at any instant is given immediately 

 by the tangent to the curve at that point ; and the method of analysis 

 set forth in my papers was based on the determination of the tangent 

 or its equivalent. 



Since then the process has been applied to several hundred photo- 

 graphs, most of which were taken during the research on the 

 electrical phenomena of muscle and nerve in which I have assisted 

 Professor J. Burdon Sanderson. f 



In the apparatus finally adopted the sensitive plate is fixed to one 

 end of a balanced pendulum by which it is carried with uniform 

 velocity past a vertical slit. The image of the capillary is thrown 



* ' Roy. Soc. Proc.,' vol. 48, p. 89 ; and ' Phil. Trans.,' vol. 183, A, pp. 81—105. 

 f 'Physiol. Soc. Proc.,' June 24, 1893, in f Jo urn. of Physiol.,' vol. 16, p. 319, 

 and vol. 18, p. 171. 



