360 SCIENTIFIC THOUGHT. 



Boscovich's or similar formulae, though the idea of action 

 at a distance between the minute particles of matter un- 

 derlies the theories by which Poisson, Navier, Cauchy, 

 Lame", and others calculated the effect of elastic forces in 

 solid bodies, or the phenomena of light passing through 

 transparent and crystalline substances. A different school 

 of physicists, starting from ideas of a different kind, with 

 which we shall become acquainted hereafter, have shown 

 that specific notions as to the molecular structure of bodies 

 are not required in order to deal with the phenomena 

 referred to. Nevertheless, the idea of action at a distance 

 governing the movements of immeasurably small, as it 

 seemingly does those of immeasurably large masses in 

 nature, received a great support by the development of 

 two other branches of science, which belong essentially to 

 the history of the present century. 

 34. The sciences of electricity and magnetism can be said 



Coulomb's _ , , 



measure- to have originated with Coulombs accurate measure- 

 ments. 



ments with the torsion -balance. With this instrument 

 he measured the attracting and repelling forces of 

 bodies, electrified or magnetised, by comparing them 

 with the mechanical forces required to twist a metallic 

 wire. In this way he fixed what have ever since his 

 time been termed the units of electricity or magnetism, 

 reducing these quantities to the same system of measure- 

 ment with which we measure the masses or inertia of 

 Extended by moving bodies. His methods were adopted and niodi- 

 weber!" 1 fied and greatly perfected by Gauss and Weber the 



prise par Coriolis et Poncelet pour I ment dans son grand et principal 

 base de la mecanique physique, ouvrage, mais dan* le scholie gen- 



n'est autre que celle de Newton lui- 

 meme, comme on le voit non seule- 



eral de sa non moins immortelle 

 ' Optique. ; " 



