[MCLENNAN-MCLEOD] EARTH'S PENETRATING RADIATION 31 



Voyage II. on SS. Megantic. 



June 24th, 1913 River St. Lawrence 5-57 ions per cc. per sec. 



25th, " River and Gulf of St. Lawrence 5-26 



26th, " Gulf of St. Lawrence and Atlantic 5 • 07 



27th, " Atlantic Ocean 4-64 



28th, " " " 4-80 



29th, " " " 4-41 



30th, " " " 4-20 



July 1st " Off North Coast of Ireland and Irish 



Sea 4-44 



Mean of last three readings= 4-35 ions per cc. per sec. 



The readings taken on the first voyage it will be seen are quite 

 regular and uniformly low. Their average was 4-3 ions per cc. per 

 sec. The lowest reading 4-0 ions per cc. per sec. was that taken on 

 Sept. 21st, the day on which the steamer was passing from the Gulf 

 into the River St. Lawrence. 



The readings taken on the second voyage it will be seen started 

 at 5-57 ions per cc. per sec. and finally dropped to about 4-35, which 

 is the mean of the readings taken on the last three days. The high 

 initial readings were probably due to radioactive deposits which 

 settled on the ship when in port at Montreal, but it is interesting in 

 this connection to note that no important drop was observed in the 

 ionisation when the steamer left port in the first voyage. If the pre- 

 vailing winds at Glasgow were south-west ones it might afford an 

 explanation in part at least, of this result. 



It is interesting also to note that the final readings on the SS. 

 Megantic were practically the same as those taken during the voyage 

 of the Hesperian. This would go to show that the contribution of 

 the ionisation by radiation from the materials and contents of the 

 steamer was practically the same for both ships. Further as Simpson 

 and Wright* had previously obtained 4-1 ions per cc. per sec. for the 

 ionisation in a closed zinc vessel on a small skiff on the Atlantic, this 

 would go to shew that the amount contributed to the ionisation by 

 radiation from the ship was inconsiderable. 



The mean of the readings taken on the ocean with the Wulf 

 electrometer, viz., 4-33 ions per cc. per sec. it will be seen was 1-7 

 ions per cc. per sec. less than the mean of the readings previously 

 taken with the Wilson Electrometer and in view of the accuracy and 

 ease with which the readings could be made with the Wulf instru- 

 ment the value 4-33 ions may be accepted as closely representing 

 the true value of the ionisation obtainable on a large ship on the sea. 



*Simpson and Wright, Proc. Roy. Soc. Ser. A. Vol. LXXXV, p. 175, 1911. 



