NEWTON ON LINES OF FORCE. 441 



But he does not stop here. To follow him further we 

 must look backward to find the ladder he is climbing; for 

 Newton has a way of not leaving his ladders readily avail- 

 able, and sometimes he is charged with pulling them up 

 after him. 



When Peregrinus placed his bit of iron in different posi- 

 tions on the lodestone globe, he saw it stand upright at the 

 poles, and at various inclinations between poles and equa- 

 tor. Gilbert, three centuries afterward, observed the same 

 thing; but neither perceived that a line drawn lengthwise 

 through the needle in all its positions would be curved and 

 extend between the poles. Porta, multiplying the piece 

 of iron many times in the form of filings sprinkled about 

 the stone, saw them branch out from the poles like hairs, 

 but not in continuous curves; while to Cabseus they seemed 

 to fall into lines more plainly curved, but still not arching 

 from pole to pole. Then came Descartes, who found what' 

 all had missed, namely, that not only did the filings fall 

 into regular curved lines from pole to pole, but that their 

 arrangement in such lines in that intervening space must 

 be the effect of some force there existing and acting on 

 them. This Christopher Wren had also seen, and Sprat, 

 in recording his experiment, even refers to the "lines of 

 directive force." Not only did Descartes note these lines 

 arching between opposite poles of the same magnet, but 

 as extending between the poles of two magnets and seem- 

 ingly connecting them. These curves, which the filings 

 traced for Descartes, occupy the magnetic field or Gilbert's 

 orb of virtue, and, when so rendered visible, map it. And 



on the hypothesis that the mechanical action observed between electri- 

 fied bodies is exerted through and by means of the medium, as in the 

 familiar instances of the action of one body on another, by means of the 

 tension of a rope or the pressure of a rod, we find that the medium must 

 be in a state of mechanical stress. . . . 



The nature of this stress is, as Faraday pointed out, a tension along 

 the lines of force combined with an equal pressure in all directions at 

 right angles to these lines." Maxwell: A Treatise on Electricity and 

 Magnetism. 3d ed. London, 1892, vol. I., 63. 





