KINETIC OR MECHANICAL VIEW OF NATURE. 35 



swarm of bodies, subject to a transverse movement in 

 straight lines like projectiles, and continually encounter- 

 ing each other on their way. The second line of research 24. 



Vortex 



in question is the study of bodies subject to rapid move- moti< >n. 

 ment round an axis, but immersed in a medium which 

 is itself movable like water, but not in a rotary but 

 merely in a flowing motion. The whole series of in- 

 vestigations which started by defining vortex or whirling 

 motion as distinct from transverse, flowing, or projectile 

 motion, and from vibratory to - and - fro motion, was 

 initiated by Helmholtz in 1857 in a purely mathe- 

 matical paper, and then applied and greatly extended 

 by Sir William Thomson in the conception of the vortex 

 atom. The third branch of research had its origin in 25. 



Faraday's 



experimental investigations carried on for many years on researches. 

 peculiar lines, and quite independently, by Faraday ; it 

 was put into mathematical language by Clerk Maxwell 

 in his celebrated treatise on electricity and magnetism 

 which appeared in 1872. It will be my object to show 

 in how far these different investigations have confirmed 

 and developed the kinetic view of natural phenomena. 

 But before doing this it will be well to realise what 

 specific problems presented themselves to theoretical . 

 physicists when once the undulatory conception of light 

 had taken hold of their minds ; what peculiar difficulties 

 were involved ; and into what distinct new lines of 

 reasoning they were conducted. 



We saw above that when the gravitational explana- 

 tion of a large class of phenomena had a century earlier 

 gradually gained ground, a great variety of researches 

 was suggested by it, and new lines of reasoning opened 



