368 Lord Kelvin [April 27, 



of ether at the rate of 80 kilometres per second in lines all parallel to 

 the tangent to the earth's orbit round the sun. There is nothing 

 inconsistent with this in all we know of the ordinary phenomena of 

 terrestrial optics ; but, alas ! there is inconsistency with a conclusion 

 that ether in the earth's atmosphere is motionless relatively to the 

 earth, seemingly proved by an admirable experiment designed by 

 Michelson, and carried out, with most searching care to secure a 

 trustworthy result, by himself and Morley.* I cannot see any flaw 

 either in the idea or in the execution of this experiment. But a 

 possibility of escaping from the conclusion which it seemed to prove, 

 may be found in a brilliant suggestion made independently by 

 Fitzgerald f and by Lorentz $ of Leyden, to the effect that the motion 

 of ether through matter may slightly alter its linear dimensions, 

 according to which if the stone slab constituting the sole plate of 

 Michelson and Morley's apparatus has, in virtue of its motion through 

 space occupied by ether, its lineal dimensions shortened one one- 

 hundred-millionth |) in the direction of motion, the result of the ex- 

 periment would not disprove the free motion of ether through space 

 occupied by the earth. 



§ 11. I am afraid we must still regard Cloud No. I. as very 

 dense. 



§ 12. Cloud II. — Waterston (in a communication to the Royal 

 Society, now famous, which, after lying forty -five years buried and 

 almost forgotten in the archives, was rescued from oblivion by Lord 

 Rayleigh and published, with an introductory notice of great interest 

 and importance, in the Transactions of the Royal Society for 1892) 

 enunciated the following proposition : " In mixed media the mean 

 " square molecular velocity is inversely proportional to the specific 

 " weight of the molecule. This is the law of the equilibrium of vis viva." 

 Of this proposition Lord Rayleigh in a footnote^ says, " This is the first 

 " statement of a very important theorem (see also Brit. Assoc. Rep., 

 " 1851). The demonstration, however, of § 10 can hardly be defended. 

 " It bears some resemblance to an argument indicated and exposed 

 " by Professor Tait (Edinburgh Trans., vol. 33, p. 79, 1886). There 

 " is reason to think that this law is intimately connected with the 

 " Maxwellian distribution of velocities of which Waterston had no 

 "knowledge." 



§ 13. In Waterston's statement, the " specific weight of a mole- 

 quantities. The proper motions of the fixed stars assign to the apex a position 

 which may be anywhere in a narrow zone parallel to the Milky-way, and ex- 

 tending 20° on both sides of a point of Eight Ascension 275° and Declination 

 + 30°. The authentic mean of 13 values determined by the methods of Arge- 

 lander or Airy gives 274° and + 35° (Andre, ' Traite d Astronomie Stellaire ')." 



* Phil. Mag., December 1887. 



t Public Lectures in Trinity College, Dublin. 



X Versuch einer Theorie der electrischen und optischen Erscheinungen in 

 bewegrten Korpern. 



|| This being the square of the ratio of the earth's velocity round the sun 

 (30 kilometres per sec.) to the velocity of light (300,000 kilometres per sec). 



<fl Phil. Trans. A, 1892, p. 16. 



