Proceedings, 27 



Ordinary Meeting, December 13th, 1887. 



Professor OsBORNE Reynolds, LL.D., M.A., F.R.S., 

 Vice-President, in the Chair. 



The Chairman referred to the death of the Society's 

 honorary member, Mr. J. B. Dancer, F.R.A.S., and Mr. 

 Faraday (one of the Secretaries) reported that he had been 

 waited upon by Mr. Abel Heywood, Junr., and Mr. H. H. 

 Howorth, M.P., with a request that he would bring to the 

 notice of the Society the desirableness of an effort being 

 made to obtain a grant from the Civil List for Mrs. Dancer. 

 Thereupon — 



It was moved by Mr. R. F. Gwyther, M.A., seconded by 

 Mr. Harry Grimshaw, F.C.S., and resolved, "That the 

 Officers of the Society be requested to consider, and, if 

 possible, to further the application for a grant from 

 the Civil List in behalf of the widow of the late 

 Mr. J. B. Dancer." 



Professor HORACE Lamb, M.A., F.R.S., made a short 

 verbal communication on reciprocal theorems in dynamics. 

 He called attention to a theorem recently given by 

 Helmholtz, which appears to include as particular cases 

 almost all the reciprocal relations as yet discovered in 

 Dynamics, and gave several illustrations from Optics and 

 Acoustics. In particular he referred to the principle of 

 acoustic reversibility formulated long ago by Helmholtz, to 

 the effect that if A and B be any two points in a homo- 

 geneous atmosphere at rest, the sound-intensity at B due to 

 a source at A is equal to the intensity at A due to an equal 

 source at B. The case when the air is in motion does not 

 come under the principle as here stated ; and in fact if B 

 be to the leeward of A, the intensity is greater in the first 

 case than in the second. The theorem referred to shows, 

 however, that the reciprocity still holds, provided that when 

 we transfer the source from A to B we also reverse the wind. 



