January Z, 1908] 



SCIENCE 



invented a sounding apparatus using steel 

 piano wire instead of rope, with a depth 

 indicator depending on the pressure of the 

 water, and a compass, both of which are 

 to-day in universal use. The principle of 

 the compass was again that of lightness for 

 sensitiveness, the card being supported by 

 silk strings. He also elaborated the 

 method of correction of the compass for 

 the ship's magnetism. Later on came in- 

 struments for the measurement of the large 

 currents and potentials used in present-day 

 practise. 



The culmination of Thomson's applica- 

 tion of Fourier mathematics, and perhaps 

 his most sensational contribution to science, 

 was his estimate of the age of the earth, 

 based upon the time it has taken to cool, 

 our knowledge being derived from the 

 measurement of the rate of increase of 

 temperature as we go below the surface of 

 the earth. His conclusion was that the 

 earth had required from one to two hun- 

 dred million years to cool from its molten 

 state to the present, a conclusion which 

 was a violent shock to the geologists, who 

 required a far greater period for the 

 formation of the rocks. 



None of the above-mentioned subjects 

 was, however, Thomson's favorite subject 

 of research, the place of which must un- 

 doubtedly be given to his speculations on 

 the nature of the ether, and the constitu- 

 tion of matter. In fact, he stated at his 

 jubilee that there had not been a day dur- 

 ing the last forty years when he had not 

 devoted some time to the consideration of 

 these subjects, but that the total result 

 must be summed up as failure, inasmuch 

 as he knew no more of their true nature 

 now than at the beginning. This dis- 

 couraging admission must be taken with 

 several pailfuls of salt, and attributed to 

 that true scientific humility which is the 

 characteristic of great minds, for there is 

 no one who has contributed more to our 



knowledge of both ether and matter than 

 Thomson. His fondness for this subject 

 was a symptom of his devotion to and 

 mastery of the principles of mechanics, 

 whether in its applications to rigid bodies, 

 to elasticity, or to hydrodynamics. In 

 connection with his contributions to geol- 

 ogy may be mentioned his powerful re- 

 searches on the tides, of both fluid and 

 solid spheroids, and his conclusion from 

 the motion of the earth that it is nearly as 

 rigid as a sphere of steel. Thomson and 

 Tait's "Treatise on Natural Philosophy" 

 (roguislily referred to by Clifliord as 

 T -\- T') was an epoch-making woi-k, con- 

 ceived on a scale never before attempted, 

 and destined to be completed only in the 

 first instalment devoted to mechanics, on 

 which subject it constituted a wonderfully 

 inspiring guide. This work, which Hehn- 

 holtz thought enough of to translate into 

 German, is difficult of characterization, 

 but we may mention its insistence on the 

 value of Newton's ideas, and its exposi- 

 tion of the power of Lagrange's gen- 

 eralized methods in dynamics. The por- 

 tion which perhaps best shows the orig- 

 inality of Thomson's genius is the chapter 

 on systems containing rotating gyrostats, 

 of whose peculiar action it gives the com- 

 plete key. The recent application of these 

 principles in the Brennan mono-rail rail- 

 way and the Schlick gyrostat for prevent- 

 ing the rolling of ships is no more interest- 

 ing than Thomson's use of them to con- 

 struct from rigid materials a model of an 

 elastic atom, in his "Steps toward a 

 Kinetic Theory of Matter," read at the 

 British Association meeting in Montreal in 

 1884, or his model of a gyrostatic ether 

 whose elasticity was to be similarly ex- 

 plained. 



This occasion of Thomson's first visit 

 to the United States was otherwise 

 signalized by the deliverance at the 

 Johns Hopkins University of a remarkable 



