640 Prof. McLennan and Mr. Macallum on Intensity of 



earth in Paris, and u q" = 3'5 at a height of 300 metres on 

 the Eiffel Tower. 



In addition to these results some observations made by 

 Gockel — who noted a moderate decrease in the saturation 

 current of an electroscope which he carried in a balloon 

 ascent of some 4 kilometres — seem to be the only ones which 

 throw any light on the question of a diminution of the inten- 

 sity of the penetrating radiation with the altitude. 



In order to see if such a diminution as that indicated by 

 Eve's numbers was observable at Toronto, a series of 

 measurements was made at different points, both on the 

 ground and on buildings at different heights above the 

 ground. 



The intensity of the penetrating radiation at different 

 places was obtained by measuring with a 0. T. R. Wilson 

 electrometer *, carrying a compensating electrical condenser, 

 the saturation current through a mass of air confined in an 

 hermetically sealed zinc receiver of about 30 litres capacity. 

 The zinc of which the receiver was constructed was of the 

 ordinary commercial variety and was not selected with any 

 idea of being specially free from radioactive impurities. Jt 

 was about 0*5 mm. in thickness. 



As V. E. Pound f has shown that 7 mm. of aluminium 

 are required to completely absorb all the different types of 

 /3 radiation emitted by a sample of radium in equilibrium, it 

 follows that the walls of the receiver used in these measure- 

 ments were not sufficiently thick to exclude all radiations of 

 the /3 type which might accompany, and be possibly caused 

 by, the penetrating radiation. 



From observations made at Toronto at different points in 

 the month of March last, selected sets of consecutive readings 

 are given in Table II. 



From the table it will be seen that the value obtained for 

 " q" in a room in the physical laboratory was about the same 

 as that obtained in the open space on the university lawn, 

 viz., slightly over 15 ions per c.c. per second. The measure- 

 ments made on the ice on Toronto Bay, however, gave a 

 much lower value, " q }, = d'3, than those made on the lawn, 

 and confirmed the observations made by one of us and 

 Mr. C. S. Wright J in 1907 and 1908, when there was 

 found an exceptionally low value for " q " over the water of 

 Lake Ontario. 



* Proc. of Roy. Soc. of Canada, p. 85, 1908, and Phil. Mag. Feb. 

 1909, p. 310. 



t Proc. of Roy. Soc. of Canada, p. 53, 1908, and Phil. Mag. Jan. 1909, 

 p. 126. 



X Wright, Phil. Mag. Feh. 1909, p. 310. 



