FINENESS OF LIGHT. 12? 



another one from the opposite or a cross direction 

 is much greater than when the one carriage is stand- 

 ing still ; and that again is greater than if they are 

 both moving one way, and the swift one runs up 

 on the slow. In all these cases, and in every 

 case that we can examine^ the swifter of the two 

 impinging substances produces a proportionally 

 greater effect than the other. Soft iron can be made 

 to move so rapidly as not only to cut hardened steel 

 as freely as a steel saw cuts soft timber, but it can 

 be made to burn the steel as easily as if that were 

 the most inflammable of substances. The purest 

 water of the brooks and streams wears away their 

 channels ; and the winds, which are but the thin air 

 in motion, level the abodes of man with the earth, 

 and sweep the productions of the earth into the sea : 

 nor is there the least doubt that if a spider's thread 

 of sufficient length, and no thicker than those threads 

 generally are, could be borne onward against the 

 globe with sufficient velocity, it would cleave the 

 globe asunder more easily, and in less time, than 

 the arrow of Tell cleft the apple on the head of his 

 son. 



If light and heat (for in the beams of the sun they 

 come together, and have many curious combinations 

 in them) be matter, they must be matter in a state 

 of far more minute division than we can ever observe 

 with all our artificial helps. Were they of the size 

 of the most minute grains of dust, even of that 

 which floats invisibly when the immediate rays of 

 the sun are not admitted into the best-lighted room, 

 but which those direct rays disclose as " the light 

 motes that dance in the beam," they would tell upon 

 the earth like cannon-shot, and it would have been 

 long ere now pounded to dust ; but instead of that, 

 they are the most kindly as well as the swiftest 

 messengers that visit our abodes ; and though they 

 bring us no matter that we can know by bringing it 

 to the test of matter, they bring us active energy, 



