Nov. 29, 1883] 



NATURE 



113 



in popular language. The physical the ■ries of astiMiiomy, light, 

 and sound involved even more mathematical complexities than 

 thermodynamics, but these subjects bad been rendered popular, 

 and this to the great improvement of the theories. 



What rendered the subject of thermodynamics so obscure 

 was that it dealt with a thing or entity (heat) which, although 

 its effects could be recognised and measured, was yet of such 

 a nature that its mode of operation could not be perceived by 

 any of our senses. Haddocks been a work of nature, and had 

 the mechanism been so small that it was absolutely imper- 

 ceptible, Galileo, instead of having to invent a machine to 

 perform a definite functiin, would have had, from the ob- 

 served nntion of the hands, to discover the mechanical prin- 

 ciples and actions involved. Such an effort would have been 

 strictly parallel to that required for the discovery of the 

 mechanical principles of which the phenomena of heat were 

 the result. 



In the imagined ca=e of the clock, the discovery might have 

 been made in two ways. By the scientific method : from the 

 observed motion of the hands the fact that the clock depended 

 on a uniform intermittent motion would have led to the discovery 

 of the principle of the uniformity of the period of vibrating bodies ; 

 and on this principle the whole the iry of dynamics might have 

 been founded. Such a theory of mechanics would have been 

 as obscure but not more obscure than the theory of thermo- 

 dynamics based on its two laws. But there was another method ; 

 and it was by this thit the theory of dynamics was brought to 

 light — to invent an artificial clock, the action of which ciuld be 

 seen. It was from the actual pendulum th,it the principles of 

 che constancy of the periods of oscillating .and revolving bodies 

 were discovered, whence followed the dynamical theories of 

 astronomy, of light, and of sound. 



As regarded the action of heat, no visible mechanical con- 

 trivance was discovere I which would afford an example of the 

 mechanical principles and motions involved, so that the only 

 apparent method was to di^cuver by experiment the laws of the 

 action of heat, and to accept these as axi jmalic laws without 

 fonning any mental image of their dynamical origin. This was 

 what the present theory of thermodynamics purported to be. 



In this form the theory was purely natheinatical and not fit 

 fi r the subject of a lecture. But as no one who had studied 

 the subject doubted for one moment the mechanical origin of 

 these laws. Prof. Reynolds would be follow ing the spirit if not 

 the letter of bis subject, if he introduced a conception of the 

 mechanical actions from which these laws sprang. This he 

 should do, alth jugh he doubted if he should have so ventured, 

 had it not been that while considering this lectur-e he hit upon 

 certain mechanical contrivances, which he would call kinetic 

 engines, which afforded visible examples of the mechanical action 

 of heat, in the same sense as the pentjulum w'as a visible example 

 of the same principles as those itivolved in the phenomena of 

 light and sound. Such machines, thanks to the ready help of 

 Mr. Foster his assi tant in constructing the apparatus, he shoul I 

 show, and he could not but hope that these kinetic engines 

 might remove the source of the obscurity of thermodynamics on 

 wliich he had dwelt. 



The general action of heat to cause matter to expand was 

 sufficiently obvious and popularly known ; also that the ex- 

 panding matter could do work was sufficiently obvious. But 

 the part which the heat played in doing this work was very 

 o bscure. 



It was known that heat played two, or it might be said three, 

 distinct mechanical parts in doing this work. 

 These parts were :-- 



1. To supply the energy necessary to the performance of 

 work. 



2. To give to the matter the elasticity which enabled it to 

 expand — to convert the inert matter into an acting machine. 



3. To convey itself, i.e. heat, in and out of the matter. 



This third function was generally taken for granted in the 

 theory of thermodynamics, although it had an important place 

 in all applications of this theory. 



The idea of making a kinetic engine which should be an 

 example of action such as heat, had no sooner occurred to him 

 than variuus very simple means presented themselve . Heat 

 was transformed by the expansion of the matter caused by heat. 



At first he tried to invent .some mechanical arrangement 

 which worrld expard when promiscuous agitation was imparttd 

 to its parts, but contraction seemed easier — this was as good. 

 All that was wanted was a mechanism which wou'd change its 



shape, doing work when its parts were thrown into a state of 

 agitation. 



In order to raise a bucket from a well either the rope was 

 pulled or the w indlass wound — such a machine did not act by 

 promiscuorts agitation ; but if the rope was a heavy one (a chain 

 was better) aird it was made fast at the tip of the well so that 

 it just suspended the bucket, then if it w as shaken from the top 

 waves or wriggles would run down the rope until the whole 

 chain had assumed a continually changing sinuous form. And 

 since the roiie could not stretch, it conld not reach so far down 

 the well with its sinujsities as when straight, so that the bucket 

 would be somewhat raised and work rlone by promiscuous 

 agitation. The chain would have changed its mechanical cha- 

 racter, ani from being a rigid tie in a vertical dir-ection would 

 possess kinetic elasticity, «.c. elasticity in virtue of the motion of 

 its parts, causing it to conti"act its vertical length against the 

 weight of the bucket. Now it was easy to see in this case that 

 to perform this operation the work spent in shaking the rope 

 performed the two parts of imparting energy of motion to the 

 chain and raising the bucket. A certain amount of energy of 

 agitation i 1 the chain would be necessary to cause it to raise a 

 butket of a certain weight through a certain distance, and the 

 relation which the energy of .agitation bore to the work done in 

 raising the bucket followed a law which if ex|jressed w'ould coin- 

 cide exactly with the sec rnd law of thermodynamics. The 

 energy of agitarion imparted to the chain was virtually as much 

 spent as the actual work in raising the bucket, that was to say, 

 neither of these energies could be used over again. If it was 

 wanted to do further work the raised bucket was taken off, and 

 then to get the chain down again it must be allowed to cool, i.e. 

 the agita'ior must be allowed to die out ; then attaching another 

 bucket, it would be necessary to supply the same energy over 

 again. 



He had other methods besides the simple chain which served 

 better to illustrate the lecture, but the principle was the same. 



In one there was a complete engine with a working pump. 

 By mere agitation the bucket of the pump rose, lifting 5 lbs. of 

 water one foot high ; before it would make another stroke the 

 agitated medium mirst be cooled, i.e. the energy which caused 

 the elrsticity must be taken out, then the bucket de cended, and, 

 being agitated again, made another sti'oke. 



He felt that there was a childish simplicity about these kinetic 

 engines, which might at fir'st raise the feeling of " Abana and 

 Pharpar " in the minds of some of his hearers. But this would 

 be only till they realised that it was not now attempted to make 

 the be,-t machine to raise the bucket, but a machine that would 

 raise the bucket by shaking. These kinetic engines were no 

 mere illu-trations or analogy of the action of heat, but were 

 instances of the action of the same principles. The sensible 

 energy in the shaking rope only differed in scale from the energy 

 of heat in a metal bar. The temperature of the bar, ascertained 

 from absolute z;ro, measured the mean square of the velocity of 

 its parts multiplied l<y some constant depending on the mass 

 of these parts. So the mean square of the velocity of the links 

 of the chain multiplied by the weight per foot of the chain really 

 represented the energy of visible agitation in the chain. 



The waves of the sea constituted a soitrce of energy in the 

 form of sensible agitation ; but this energy could not be used 

 to work continuously one of these kinetic machines, for exactly 

 the same reason as the heat in the bodies at the mean tem- 

 perature of the earth's surface could not be used to work heat- 

 engines. 



A chain attached to a ship's mast in a rough sea would 

 become elastic with agitation, but this elasticity could not be 

 used to raise cargo out of the hold, because it would be a 

 constant quantity as long as the roughness of the sea lasted. 



Besides the waves of the sea there w as no other source of 

 sensible agitation, so there had been no demand for kinetic 

 engines. Had it been otherwise, they would not have been left 

 for him to discover — or, had they been, he might have been 

 templed to patent the inventions. But there had been a demand 

 for what might be called sensible kinetic elasticity to perform for 

 sensible motion the part which heat elasticity performed in the 

 thermometer. 



And it had not been left for him to invent kinetic mechanism 

 for this purpo-e, although it might be that its semblance to the 

 thermometer had not been recognised. The principle was long 

 ago applied by Watt. The common form of governors of a 

 steam-eirgine acted by kinetic elasticity, which elasticity, depend- 

 ing on the speed at which the governors were driven, cau-ed 



