176 



NATURE 



[December 26, 1907 



sibility of an Atlantic cable was in the early 'fifties a 

 much-discussed question : and the mathematical inves- 

 lij,^ation which Lord Kelvin made of the conditions 

 governing' the propagation of signals in long sub- 

 marine cables proved to be the most important contri- 

 bution to the practical solution of that problem. He 

 .showed that the retardation must be proportional to 

 the square of the length of the cable ; and, further, he 

 applied the theorems of Fourier to predict the degree 

 of attenuation of the impulses on their arrival at the 

 distant end. This was followed by the invention of 

 the mirror galvanometer, and later by the siphon re- 

 corder, with both of which instruments his name 

 will ever be associated. The final success of the 

 Atlantic cables of 1S65 and 1866 was a triumph for his 

 inventive ingenuity no less than for his mathematical 

 skill and insight. He had likewise been brought in- 

 timately into nautical matters, leading him to devise 

 the method of taking flying soundings, and to publish 

 a set of tables for facilitating the use of Sumner's 

 method at sea. To heighten his public fame he also 

 re-modelled the mariner's compass by radical improve- 

 ments which quickly established its superiority to all 

 earlier forms. 



.AH this was duly recounted in the article of 1876. 

 and might well suffice to place him in the very first 

 rank of physicists had he achieved nothing' more. 

 Von Helmlioltz, summing up his intellectual attain- 

 ments at 'that date, had remarked upon his method 

 of treating the problems of mathematical physics how 

 he had " striven with great consistency to purify 

 matheinatical theory from hypothetical assumptions 

 wliicli were not a pure experience of facts." He main- 

 tained that " the gift to translate real facts into mathe- 

 matical equations, and vice versa, is by far more rare 

 than that to find the solution of a given mathematical 

 problem "; adding, " And in this direction Sir William 

 Thomson is most eminent and original." Happily for 

 science this gift continued to be exercised for thirty 

 vears after von Helmholtz penned this appreciation of 

 his friend. .As the years went on Lord Kelvin con- 

 tinued with marvellous activity of mind and body to 

 add to his long list of scientific labours. 



It has been noted above at how early a date, namely, 

 in 1842, Lord Kelvin had published the germ of his 

 theories about the age of the earth. This was in a 

 paper on the linear motion of heat which ap- 

 peared in the Camhridge and Dublin Matheinatical 

 Journal. This same subject he had made the topic of 

 his inaugural lecture in 1846 on taking up his pro- 

 fessorship at Glasgow. He returned to it in 1876 as 

 the theme of his address as president of the Physical 

 and Mathematical Section of the British .Association 

 at Glasgow. To the geologists who demanded un- 

 limited time for the operation of these formative actions, 

 which, on the abandonment of catastrophic notions, 

 they had assumed to proceed with constant uniformity, 

 Lord Kelvin announced with the utmost confidence 

 that they must hurry up their phenomena, since the 

 .•ige of the earth as a habitable planet, so far from 

 being unlimited, could not possibly exceed four hun- 

 dred millions of years, and was more probably within 

 twenty millions of years. The proposition was sup- 

 ported by several converging lines of argument. The 

 surface temperature could not be what it was, con- 

 sidering the average conductivity of rocks and the 

 gradient of temperatures found underground, if the 

 cooling process had proceeded from an unlimitedly long 

 anterior date. The heat of the sun itself must be 

 constantly dissipated, and its temperature sinks; and 

 with the cooling of the sun the earth also cools. Its 

 form, in relation to centrifugal forces, was incom- 

 patible with the hypothesis of an unlimited time since 

 it was a fluid mass. The controversy which arose, as 



NO. T991, VOL. yy] 



the biologists and geologists endeavoured to combat 

 these arguments, lasted for a quarter of a century; and 

 the end is indeed not yet. 



Hydrodynamics is a branch of natural philosophy 

 in which the Cambridge school under Stokes has 

 always been strong; and Lord Kelvin, as a pupil and 

 friend of Stokes, worked much at it. Hydrodynamics 

 was indeed continually in Lord Kelvin's thoughts. 

 His brilliant speculation of the vortex-atom remains — 

 if we are to except recent electric theories of matter 

 — the one and only hypothesis of the ultimate struc- 

 ture of matter that has yet been found to hold its own 

 against destructive criticism. It has not yet been 

 shown to be impossible or self-contradictory. Apart 

 from this, his otlier investigations into hydrodynamics 

 have been most fruitful. He discussed the conditions 

 of stability of fluid motion in a large number of cases,, 

 some of them of practical importance. Within the 

 past two years he contributed to the Royal Society of 

 Edinburgh a series of papers on deep sea waves, 

 papers which are full of characteristic applications, 

 of Fourier's theorems, and show unabated keenness 

 in following out an intricate analysis. In elasticity 

 and the kindred problems of dynamics, the influence 

 of the master's hand is no less evident. The article 

 which he contributed to the " EncyclopEedia Britannica "' 

 on elasticity will remain a classic of science for many 

 years. Those who are intimate with Lord Kelvin's 

 work generally will know how much in this article 

 there is that lies behind his other studies. His con- 

 tinual reference to the analogies which he found be- 

 tween the phenomena of magnetism and of electricity 

 and those of elastic solids shows the working of his 

 mind, and the fundamental views which he held on 

 elasticity dominate alike his Baltimore lectures of 1SS4 

 and the papers on molecular physics of his latest 

 years. 



To the science of electricity, Lord Kelvin's contri- 

 butions have been no less notable. Imbued with 

 admiration for Faradav's e.xperimental work, Lord 

 Kelvin early set himself to ascertain whether the 

 phenomena of electromagnetism can be explained on 

 an elastic solid theory. .Although it was left to Max- 

 well to carry to fruition this part of the subject, it was 

 Kelvin's merit to have first applied mathematical 

 analysis to the facts revealed by Faraday's researches. 

 It was in 1847 that he first proposed a mechanical 

 representation of magnetic force ; and to this subject 

 he returned in iSgo, in an article first published in 

 the third volume of his collected mathematical and 

 physical papers. It was in the early days, too, that 

 he investigated the conditions of the discharge of a 

 Leyden jar in circuits possessing self-induction, pre- 

 dicting mathematically the fact that under certain 

 conditions these discharges would be found to consist 

 of electric oscillations. This discovery was published 

 in 1853. Later, Fedderson and others observed these 

 oscillations experimentally ; and in the 'eighties this 

 abstract research of Lord Kelvin's became the start- 

 ing point of the investigations of Sir Oliver Lodge 

 and of the lamented Heinrich Hertz, leading directly 

 to wireless telegraphy. 



In 185 1 Lord Kelvin, impelled by the characteristic 

 precision of his scientific character, and urged by the 

 needs of exact measurement in telegraphy, had 

 already adopted the absolute system of measurement 

 initiated by Gauss, and extended by Weber. In Lord 

 Kelvin's hands the absolute system of measurement, 

 and with it the adoption of the metric system of 

 standards, became almost an article of creed. In 

 season and out of season he urged the superiority of 

 the decimal measures over the ordinary British ones; 

 and, consistently, he strove to bring all scientific 

 measurements into terms of the fundamental metric 



