146 ON THE MOTION OF SOLIDS THROUGH A LIQUID. [CHAP. V. 



be greater on the side nearer A than on that which is more 

 remote. Hence by (18) the average* pressure on the former 

 side will be less than that on the latter, so that B will experience 

 on the whole an attraction towards A. As practical illustrations 

 of this principle we may cite the apparent attraction of a 

 delicately-suspended card by a vibrating tuning-fork, and other 

 similar phenomena studied experimentally by Guthrief and ex 

 plained in the above manner by Thomson . 



The same principle accounts for the indraught of a light 

 powder, strewn on a vibrating plate, towards the ventral segments. 



* Since is by hypothesis a periodic function of t, the term -^ in (18) con 

 tributes nothing to the average effect, 

 t Proc. R. S. 

 Reprint, Art. XLI. 



