312 Mr. C. S. Wright on Variations in the 



the last series o£ observations. A careful set of measurements 

 was therefore undertaken, having for its object a determination 

 of the relative decrease in the values of " g " over water, from 

 those obtained in the laboratory, when freshly cleaned re- 

 ceivers of lead, zinc, and aluminium were used in turn as the 

 containing vessels. In the case of the lead cylinder the con- 

 ductivity was measured at a larger number of points, to 

 determine if possible in what way the ionization was influenced 

 by external conditions such as a change of soil. 



The results, which are in many cases the mean of a number 

 of observed values obtained on different occasions and differing 

 but slightly from one another, are given in Table XI. 



From the observations with the lead cylinder it will be 

 seen that the ionization obtained over water of different 

 depths was practically constant, the value for u q " being as 

 before but slightly less than that obtained above a sandy soil. 

 It is important to note also that the ionization on top of the 

 University tower was the same as that found below on the 

 campus, indicating that the atmosphere could not be the source 

 of the penetrating radiation which gave the variations noted 

 in the values of u q " for air confined in a closed metallic 

 receiver. 



From the values of " q " given in Table XI. for the three 

 receivers, we obtain for the difference between the ionization 

 in the laboratory and over the water the numbers 6*7, 7 "4, 

 and 6*0 for the receivers of lead, zinc, and aluminium respec- 

 tively. These values then may be taken as giving a measure 

 of the relative ionization in the three receivers due to that 

 portion of the radiation from the earth which was cut off by 

 the water, together with the secondary rays induced by this 

 radiation in the different cylinders. That the actual numbers 

 obtained for " q " at any observing station were not in the 

 same ratio as these decreases, is a conclusive proof that the 

 ionization measured at these stations was not due entirely to 

 the radiation from the earth. 



It might be well to call attention again to the fact that 

 these decreases in the receivers of lead, zinc, and aluminium, 

 of 6*7, 7*4, and 6'0, which are in the ratio of 1*1 to 1*23 to 

 l'OO, must give a true measure of the total ionization in the 

 three receivers which is due to a portion at least, if not the 

 whole of the penetrating radiation from the earth. If, then, 

 we could obtain cylinders of lead, zinc, and aluminium free 

 from active impurities and possessing no intrinsic activity, 

 we should expect the values for " q " at every point on the 

 surface of the earth to be in this ratio. 



Emphasis might also be laid upon the extremely low values 



