April i, 1922] 



NATURE 



405 



. — All instance of change 

 rotational inertia. 



rise, the wire now making an angle with the vertical 

 whilst the disc rotates about an axis through its 

 centre and perpendicular to its plane, the centre 

 I I remaining vertically 



below the fixed point 

 of suspension. At a 

 still higher speed the 

 disc will begin to 

 whirl about the point 

 of suspension with 

 the wire and its own 

 plane both practically 

 horizontal. " The 

 changes in inertia of 

 the rotating disc oc- 

 cur at definite speeds 

 as it receives energy, 

 and it is not so im- 

 probable as at first 

 sight it may seem, 

 therefore, that the 

 changes in form 

 which occur at per- 

 fectly definite tem- 

 peratures (as, for 

 instance, when heat 

 is added to ice to 

 turn it to water, and to water to turn it to 

 steam) may really take place in a similar manr^er by 

 some corresponding change in disposition in the 

 particular internal or ionic movement which we 

 imagine to constitute heat ; in other words, that this 

 motion may enjoy some form of increased freedom 

 which may be regarded as an increase of 

 thermal inertia." 



The conception of entropy as incurred 

 thermal inertia is worked out in detail in 

 the later chapters, and an interesting 

 mechanism is described which affords a 

 parallel to the behaviour of the working 

 substance in an engine (Fig. 2). In this 

 model the actual working substance is 

 represented by an arrangement consisting of 

 I short shaft carrying a pair of heavy 

 • governor " balls mounted on bell-crank 

 levers controlled by varying tension springs, 

 ('hanges in the rotational inertia of the re- 

 volving masses represent changes in the 

 thermal inertia of the working substance. 

 With the apparatus illustrated in the 

 it is possible to go through a cycle of operations, 

 such as the usual steam-engine cycle (Rankine's 

 cycle) or Carnot's cycle, and examine the analogies 

 between thermal and mechanical processes in detail. 

 NO. 2735, VOL. 109] 



Teachers and students of thermodynamics would be 

 well advised to study this volume. 



In connection with such mechanical analogies our 

 attention has been directed to an address delivered 

 before the Institution of Civil Engineers in 1883 by 

 Prof. Osborne Reynolds. The lecturer referred to 

 the work of Rankine, who assumed the thermal motion 

 to be rotatory, and, when compelled to abandon the 

 theory of " molecular vortices," called on all those who 

 taught the subject of thermodynamics to try to find 

 some popular means of illustrating the second law. 

 " The call was made twenty years ago ; but I believe 

 up to the present no such illustration has been forth- 

 coming." " The communication of heat to matter 

 means the communication of internal agitation — mob 

 agitation. If, then, we are to make a machine to act 

 the part of hot matter, we must make a machine to 

 perform its work in virtue of internal promiscuous 

 motion amongst its parts." As an illustration, 

 Osborne Reynolds instances the possibility of raising 

 a bucket by violently shaking the upper end of a 

 heavy rope or chain. A modification of the illustration 

 is afforded by a kind of chain composed of a series of 

 parallel horizontal bars of wood connected and sus- 

 pended by two strings. " By giving a circular oscilla- 

 tion to the upper bar, the whole apparatus is set into 

 a twisting motion (agitation) ; the strings are continu- 

 ally bent, and the vertical length of the whole system 

 is shortened." Osborne Reynolds refers also to the 

 governor of a steam-engine, which acts by kinetic 

 elasticity depending on the speed. " The motion of 

 the governor is not of the form of promiscuous agitation, 

 but, though systematic, all the motion is at right angles 





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Fk;. 2. — Mechanism ilhistrating the l)ehuvi()iir of the working; substai 



book 



to the direction of operation, so that the principle of its 

 action is the same." Here we have the germ of the 

 model discussed in the book under review. We may 

 venture the opinion that the development of this 

 model owes not a Httle to the late Prof. A. M. 



r I 



