



NATURAL PHILOSOPHY. 



THE NATURE OF FORCE. 



The following abstract of a lecture recently delivered before the 

 Royal Institution, London, by Professor Tyndall, presents most clearly 

 some of the remarkable facts that have been recently developed 

 respecting the nature and correlation of mechanical force and heat. 

 A substance suspended at a height of sixteen feet above the earth's 

 surface, and allowed to fall, reaches the surface in one second of time, 

 its velocity, which has been regularly accelerated, being then at the 

 rate of thirty-two feet per second. Suppose that, instead of being 

 pulled downward by gravity, the weight is cast upward in opposition 

 to the force of gravity, with what velocity must it start from the 

 earth's surface in order to reach a height of sixteen feet ? With a 

 velocity of thirty-two feet a second. This velocity imparted to the 

 weight by the human arm, or by any other mechanical means, would 

 carry the weight up to the precise height from which it has fallen. 



Now, the lifting of the weight may be regarded as so much mechan- 

 ical work. I might place a ladder against the wall, and carry the 

 weight up a height of sixteen feet ; or I might draw it up to this 

 height by means of a string and pulley, or I might suddenly jerk it 

 up to a height of sixteen feet. The amount of work done in all 

 these cases, as far as the raising of the weight is concerned, would be 

 absolutely the same. The absolute amount of work done depends 

 solely upon two things : first of all, on the quantity of matter that is 

 lifted ; and, secondly, on the height to which it is lifted. If you call 

 the quantity or mass of matter ?n, and the height through which it is 

 lifted A, then the product of m into h, or mh, expresses the amount of 

 work done. 



Supposing, now, that instead of imparting a velocity of thirty-two 

 feet a second to the weight, we impart twice this speed, or sixty-four 

 feet a second ; to what height will the weight rise ? You might be 

 disposed to answer, " To twice the height ; " but this would be quite 

 incorrect. Both theory and experiment inform us that the weight 

 would rise to four times the height ; instead of twice sixteen, or thirty- 

 two feet, it would reach four times sixteen, or sixty-four feet. So, 

 also, if we treble the starting velocity, the weight would reach nine 

 times the height ; if we quadruple the speed at starting, we attain 

 sixteen times the height. Thus, with a velocity of one hundred and 

 twenty-eight feet a second at starting, the weight would attain an 

 elevation of two hundred and fifty-six feet. Supposing we augment 



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