564 



NATURE 



[January 21, 191^ 



lines similar to those ol A\ he shows that there are 

 no forces tending to deform the guides of B, and 

 if we imagine them to disappear the flow is un- 

 changed and remains similar to the flow in A. 

 But since he proves so much, he ought to prove 

 that there is no other way in which the flow can 

 occur in B, and I am afraid that this cannot be 

 done except by Kirchoff's use of higher mathe- 

 matics. Indeed, Kelvin showed that there might 

 be two answers to all such problems, one of them 

 being unstable. 



The first of these fluid motion papers (1852) 

 deals with the free vortex. The example to which 

 he most frequently referred his students was that 

 of water leaving a basin by a central hole. The 

 nearly circular motion of every particle is such 

 that the speed is inversely proportional to distance 

 from the axis, and he satisfied himself that our 

 simple theory as to pressure, circular speed v, and 

 height was correct. But he noticed that particles 

 of dust on the lower surface of the basin moved 

 in towards the hole nearly radially ; he arrived at 

 the conclusion that surface friction destroyed v, 

 and therefore destroyed the centrifugal force, and 

 therefore destroyed the balance of pressures, and 

 therefore created a radial flow. This simple prin- 

 ciple gave the key to the atmospheric phenomena 

 of great forest fires; it enabled him to explain 

 what occurs at river bends and why a river through 

 alluvia! ground tends to become more and more 

 crooked; it enabled him to explain the phenomena 

 of cyclones, and, most important of all, it enabled 

 him in 1857 to give his simple explanation of what 

 had puzzled many clever scientific men for two 

 hundred years, the grand currents of atmospheric 

 circulation. That short paper is easy to under- 

 stand. The Bakerian lecture of the Royal Society 

 with the same title, delivered in 1892 two months 

 before his death, added nothing to that simple 

 explanation, then thirty-five years old, but in it 

 he gave at some length the history of the problem, 

 Hadley, in 1735, explained the trade winds in 

 latitudes 30 S. to 30 N., but in all the numerous 

 writings by distinguished men before and after 

 Hadley until 1857 there was only slipshod reason- 

 ing and no explanation of the prevailing S.W. 

 winds north of latitude 30 N. There is a whirl of 

 the atmosphere there from the west which would 

 produce no northerly flow of the air, only that 

 there is friction against the earth ; this diminishes 

 the speed of the air and upsets the balance of 

 pressures, producing a northerly flow close to the 

 earth's surface — exactly the basin phenomenon ! 

 His proof, in 1849, that pressure lowers the melt- 

 ing point of ice consists in subjecting a mixture of 

 ice and water to a Carnot cycle. He assumes with 

 Carnot that no heat disappears when work is done, 

 NO. 2256, VOL. 90] 



but he states quite clearly, as nobody else had 

 ever done, what the third part of the cycle would 

 be if Carnot were wrong and if less heat were given 

 out than what had been received. 



Using Regnault's experimental results for 

 steam. Lord Kelvin had in 1848 calculated the 

 value of Carnot 's function, and James Thomson 

 used the result, which was this : — " We 

 find that the quantity of work developed 

 by one of the same thermal units descending 

 through one degree about the freezing-point is 

 4'g7 foot-pounds." This enables him to find that 

 the lowering of the melting-point is 000000355 p 

 where p is the increase of pressure in pounds per 

 square foot. This paper and I'Celvin's paper and 

 their connection with the vexed question, "Who 

 discovered the second law?" are exceedingly 

 interesting. Kelvin's paper of 1851 first estab- 

 lished the second law on a logical basis irrespec- 

 tive of assumed properties of matter, and Kelvin 

 was too generous in giving credit to Clausius and, 

 indeed, to Rankine also. But these four men 

 and Joule himself were all very close to the 

 discovery in the three years 1848 to 1851. I know 

 of no more interesting reading than what I find in 

 Prof. Silvanus Thompson's life of Lord Kelvin 

 during these years. No one of Plutarch's heroes 

 " played the game " more nobly than the 

 Thomsons. 



James Thomson reasoned out from the above 

 principle the cause of the flow of glaciers and the 

 plasticity of ice and other curious ice phenomena, 

 as well as the influence of stress' on crystallisation 

 generally, in a series of papers and letters until 

 i88g. In 1862 he had made a model of a surface 

 showing how p, v and t for carbonic acid vary, and 

 had thought of conditions of instability. Dr. 

 Andrews's Bakerian lecture of the Royal Society in 

 1869 caused him to revert to his previous study of 

 the discontinuities of his surface, to complete his 

 model and to write papers of 1871 on the abrupt 

 changes at boiling and condensing. He reasoned 

 out the existence of the triple point for ice, water 

 and steam in 1872 and 1873 in the same way as 

 that of his 1849 paper on ice. The one p, t 

 curve for saturated steam drawn on copper by 

 Regnault is really two curves the slopes of which at 

 0° C. are not the same, being in the ratio dp/dt 

 for ice-steam-;- dp/dt for water-steam = i' 13. 

 These matters are familiar to all readers of 

 Maxwell's book on heat, but the student will be 

 interested in the letters and notes from 1862 which 

 describe how Thomson was led to his results. He 

 used to tell his students, with some glee, how his 

 eye detected in Regnault's curve the discontinuity 

 at 0° C. which nobody had noticed before. 



His \aluable papers on the strength and elas- 



