Modes of Physical Force. 93 



tingham in 1866. " A train of multiplying wheels ended 

 with a small metallic wheel, which, when the train was 

 put in motion, revolved with extreme rapidity against 

 the periphery of the next wheel, a wooden one. In the 

 metallic wheel was placed a small piece of phosphorus, 

 and as long as the wheels revolved the phosphorus 

 remained unchanged ; but the moment the last wheel 

 was stopped, by moving a small lever attached to it, the 

 phosphorus burst into flames. My object was to show 

 that while motion of the mass continued heat was 

 not generated, but that when this was arrested, the force 

 continuing to operate, the motion of the mass became 

 heat in the particles. The experiment differed from that 

 of Rumford's cannon-boring and Davy's friction of ice in 

 showing that there was no heat while the motion was 

 unresisted, but that the heat was dependent on the 

 motion being impeded or arrested. We have now 

 become so accustomed to this view, that the moment 

 we find motion resisted we look to heat, electricity, or 

 some other force, as the necessary and inevitable result." 

 And so, to use again the author's own words, " Mo- 

 tion will directly produce heat and electricity, and elec- 

 tricity, being produced by it, will produce magnetism 

 a force which is always developed by electrical cur- 

 rents, at right angles to the direction of these currents, 

 Light, also, is readily produced, to all appearances, by 

 motion directly, when accompanying the heat of friction, 

 or mediately, by electricity resulting from motion, as in 

 the electric spark, which has most of the attributes of 

 solar light, differing from it only in those respects in 

 which light differs when emanating from different sources, 

 or when seen through different media ; for instance, in 

 the position of the fixed lines in the spectrum, or in the 

 ratio of the spaces occupied by rays of different refrang- 



