the Luminiferous jffither. 395 



half potential and half kinetic*; in which case y = 2 ; and 

 therefore ^=3*3. 



The energy in a cubic foot of the aether at the earth being- 

 given by (l/ and (5), we have, by the aid of (4), 



^ 2 = i /^y 2 = __^_. (8) 



. 4xl'4 x2 _2 



* ' 3 x 10 7 x 6-6 x (186,300 x 5280) 2 " 35 x 10 24 ' * w 



which is the mass of a cubic foot of the aether at the earth, and 

 which would weigh at the place where <? = 32'2 about 



2 



W= TA24 °f a P 0Un( l, .... (10) 



compared with which Thomson's value is less than 4000 times 

 this value. Thomson remarked that the density could hardly 

 be 100,000 times as small — a limit so generous as to include 

 far within it the value given in (9). According to equation 

 (10), a quantity of the aether whose volume equals that of the 

 earth would weigh about i of a pound. If a particle describes 

 the circumference of a circle in the same time that a ray 

 passes over a wave-length X, the radius of the circle will be, 

 using equation (4), 



r "27r V y ' V -2ttV 43 ^ 

 or the displacement from its normal position will be about 

 ^ of a wave-length, or about 215 1 Q00 of an inch at the earth. 

 Eliminating V between (2) and (8) gives 



8 _ 4 

 *~3/*xl0 7 ~10 8 ••••■•• W 



for the tension of the aether per square foot at the earth, 

 and is equivalent to about 1*1 pound on a square mile. 

 The tension of the atmosphere at sea-level is more than 

 30,000,000,000 times this value. It somewhat exceeds the 

 tension of the most perfect vacuum yet produced by arti- 

 ficial means, so far as we are informed. Crookes produced 

 a vacuum of '02 millionth of an atmosphere t without reaching 



* Phil. Mag. 1855 [4] ix. p. 37. 



t " On the Viscosity of Gases at High Exhaustions," by William 

 Crookes, F.R.S., Phil. Trans. Roy. Soc. part ii. (1881) p. 400 : " Going up 

 to an exhaustion of '02 millionth of an atmosphere, the highest point to 

 which I have carried the measurements, although by no means the highest 

 exhaustion of which the pump is capable.'' 



