ATTRACTION*. 



I* Robert Hooke, a man of singular inventive power, in 1671 entUa- 

 rouml to trace tht *oe of gravitation to waves propagated in a medium. 

 II' found thH bodiM floating on water agitated by waves were drawn towards 

 the centra of agitation*. He does not appear, however, to have followed up 

 that observation in such a way as to determine completely the action of waves 

 on an immened liody. 



Professor Challis has investigated the mathematical theory of the effect of 

 waves of condensation and rarefaction in an elastic fluid on bodies immersed 

 in the fluid. He found the difficulties of the investigation to be so great that 

 he baa not been able to arrive at numerical results. He concludes, however, 

 ^i the effect of such waves would be to attract the body towards the centre 

 ..I" agitation, or to repel it from that centre, according as the wave's length 

 is very large or very small compared with the dimensions of the body. Prac- 

 tical illustrations of the effect of such waves have been given by Guyot, Schell- 

 bach. Guthrie, and Thomson t. 



A tuning-fork is set in vibration, and brought near a delicately suspended 

 iiu'lit body. The body is immediately attracted towards the tuning-fork. If the 

 tuning-fork is itself suspended, it is seen to be attracted towards any body 

 placed near it 



Sir W. Thomson has shewn that this action can in all cases be explained 

 by the general principle that in fluid motion the average pressure is least 

 where the average energy of motion is greatest. Now, the wave-motion is 

 greatest nearest the tuning-fork, the pressure is therefore least there ; and the 

 suspended body being pressed unequally on opposite sides, moves from the side 

 of greater pressure to the side of less pressure, that is towards the tuning-fork. 

 He has also succeeded in producing repulsion in the case of a small body 

 lighter than the surrounding medium. 



It is remarkable that of the three hypotheses, which go some way towards 

 a physical explanation of gravitation, every one involves a constant expenditure 

 of work. Le Sage's hypothesis of ultramundane corpuscles does so, as we have 

 shewn in the article ATOM. That of the generation or absorption of fluid 

 requires, not only constant expenditure of work in emitting fluid under pressure, 

 but actual creation and destruction of matter. That of waves requires some 

 agent in a remote part of the universe capable of generating the waves. 



* Pothum<na Workt, edited by R Waller, pp. xiv. and 184 

 t l'Mosoi>kical Jfayatiru, June, 1871. 



