1886.] on Thomas Young. ^71 



whole doctrine of material caloric. He gives appropriate illustrations 

 of the manner in which he supposed the molecules of bodies to be 

 shaken asunder by heat. " All these analogies," he says, " are cer- 

 tainly favourable to the opinion of the vibratory nature of heat, which 

 has been sufficiently sanctioned by the authority of the greatest 

 philosophers of past times and by the most sober reasoners of the 

 present." In anticipation of Dr. Wells, Young had observed and 

 recorded the fact, that a cloud passing over a clear sky sometimes 

 causes the almost instantaneous rise of a thermometer placed upon the 

 ground. The cloud he assumed acted as a vesture which threw back 

 the heat of the earth. Eadiant heat and light are here placed in the 

 same catagory. William Herschel had already shown their kinship, 

 by proving that the most powerful rays of the sun were entirely non- 

 luminous. Subsequent to this, the polarization of heat, by Principal 

 James Forbes, rendered yeoman's service in the propagation of the 

 true faith. 



The passage in which Young introduces and defines the term 

 energy is so remarkable, that I venture to reproduce it here. 



" The term energy may be applied, with great propriety, to the 

 product of the mass or weight of a body into the square of the number 

 expressing its velocity. Thus, if a weight of one ounce moves with a 

 velocity of a foot in a second, we may call its energy 1 ; if a second 

 body of two ounces have a velocity of three feet in a second, its 

 energy will be twice the square of three, or 18. This product has 

 been denominated the living or ascending force, since the height of 

 the body's vertical ascent is in proportion to it ; and some have con- 

 sidered it as the true measure of the quantity of motion ; but although 

 this opinion has been very universally rejected, yet the force thus 

 estimated well deserves a distinct denomination. After the con- 

 siderations and demonstrations which have been premised on the 

 subject of forces, there can be no reasonable doubt with respect to 

 the true measure of motion ; nor can there be much hesitation in 

 allowing at once that since the same force, continued for a double 

 time, is known to produce a double velocity, a double force must also 

 produce a double velocity in the same time. Notwithstanding the 

 simplicity of this view of the subject, Leibnitz, Smeaton, and many 

 others, have chosen to estimate the force of a moving body, by the 

 product of its mass into the square of its velocity ; and though we 

 cannot admit that this estimation of force is just, yet it may be allowed 

 that many of the sensible effects of motion, and even the advantage 

 of any mechanical power, however it may be employed, are usually 

 proportional to this product, or to the weight of the moving body, 

 multiplied by the height from which it must have fallen, in order to 

 acquire the given velocity. Thus, a bullet moving with a double 

 velocity, will penetrate to a quadruple depth in clay or tallow; a 

 ball of equal size, but of one-fourth of the weight, moving with a 

 double velocity, will penetrate to an equal depth ; and, with a smaller 

 quantity of motion, will make an equal excavation in a shorter time. 



