234 Mr.lI.ThniMpsi.il. On Correlation of [Mar. 1, 



The dimensions of the iron rings and their boxwood jackets were 

 as follows : 



Ring I. Ring II. 

 Rings 



External diameter 6'9 cm. 6*1 cm. 



Width 2-9 3-0 



Thickness 0'4 0'35 



Mean radius 3'25 2'82 



Boxwood jackets 



External diameter 7' 7 6*9 



Width 3-7 3-8 



Thickness 1-3 M5 



Convolutions 515 473 



Gauge of wire 0*91 mm. 0'91 mm. 



The rings were formed from rectangular bars with welded joints, 

 and were turned in the lathe to the above dimensions. 



The magnetic fields were calculated from the formula II = 2nt"/r 

 where n is the number of convolutions, i the C.G.S. current, and r the 

 mean radius. 



The current was derived from a battery of twenty-seven storage 

 cells used for lighting the house. 



IX. "On Correlation of certain External Parts of Palwmon 

 geri'atus." By H. THOMPSON. Communicated by Professor 

 WELDON, F.R.S. Received January 25, 1894. 



In 1890 Professor Weldon published (' Roy. Soc. Proc.,' vol. 47, 

 p. 445) the results of measurements of certain organs of the common 

 shrimp, with a view to establish by accurate data the degree of varia- 

 tion existing in those organs. In a later paper (' Roy. Soc. Proc.,' 

 vol. 51, p. 2) he determined, by Mr. Gallon's method there described, 

 the degree of correlation existing between four organs of the same 

 animal, and in a recent paper (' Roy. Soc. Proc.,' vol. 54, p. 318) 

 similar determinations have been worked out by him for certain 

 organs of Carcinus mcenas. 



Some time ago it was suggested to me by Professor Weldon that I 

 should determine the values of correlated variations in a number of 

 parts of the hard exoskeleton of the common prawn (Palwmon 

 serratus). Accordingly, 1000 adult female prawns, chosen at random, 

 were procured from Plymouth and measured. Twenty-two measure- 

 ments were made of each prawn, except in the case of two or three 

 measurements which were made on part only of the sample. 



The parts measured were the following : 



