Penetvatimj Radiation from the Earth^s Surface. 403 



given by Milne of usually from 2 to 6 minutes* between the 

 shock and its echo is exactly that between the beats. On the 

 15 sec. diagram the interval is about three minutes for a 

 boom period 16*5. With a nearer approach to isochronism 

 the interval would be longer until, if the periods are 

 sufficiently near, the free vibration will have been damped 

 out before opposition in phase can occur. This is evident 

 in the 16 sec, 17 sec, and 18 sec diagrams. 



The object of this paper is to point out that on ordinary ele- 

 mentary theory it is erroneous to derive information regarding 

 the movement of the earth from the measurement of the 

 boom of a Milne seismograph as at present constructed, as 

 we have no knowledge to what extent synchronism may 

 affect the result. To obtain the necessary knowledge the 

 tape must be driven at a higher speed, or it might be obtained 

 in some cases from the interference effects. In cases where 

 there is a near approach to isochronism between the boom 

 period and the wave period, the amplitude of swing depends 

 largely on the damping effect of friction (equation 1), which 

 is entirely ignored at present in this connexion. 



LIII. A Penetrating Radiation from the Earth^s Surface. 

 By H. Lester Cooke, M.A.^ Demonstrator of Physics^ 

 McGill University^ Montreal^. 



C. T. R. WILSON t has examined the rate of discharge 

 of an insulated charged conductor placed inside a 

 closed vessel, the inclosed gas not being exposed to any 

 known ionizing agent. The effect of different gases and 

 varying pressures was studied. He found that the rate of 

 discharge was approximately proportional to the pressure and 

 also to the density of the gas employed. This discharge 

 takes place through the gas, and must be ascribed to a pro- 

 duction of ions proportional to the pressure and density of 

 the gas ; in other w^ords, to the amount of gaseous matter 

 present. 



To account for this there are several explanations possible. 

 The ionization may be due to : — 



(1) a radiation or active emanation from the walls of the 

 inclosing vessel ; 



* Britisli Association Reports, p. 72 (1900). 



t Communicated by Prof. Rutherford, F.R.S. A preliminary account 

 of these experiments was given before the American Physical Society, 

 Washington, Dec. 1902. An abstract of the paper was published in the 

 ^Phvsical Review.' See Rutherford and Cooke, Phvs. Rev. Ixxxiv. 

 p. 183. 



X C. T. R. Wilson, Proc. Roy. Soc. Ixviii. p. 151 Ixix. p. 277. 



2 D 2 



