642 Prof. McLennan and Mr. Macallum on Intensity of 



This diminution in intensity for an altitude of 64 metres is 

 practically the same (as Table I. shows) as that calculated by 

 Eve for a diminution in the earth's penetrating radiation. 



It must be remembered, however, in this connexion that 

 probably the walls of the tower itself were not without 

 influence. The clock-tower was a sandstone structure with 

 a square cross-section of about 50 square metres area, and 

 Avhile the building proper of the city hall was about 

 40 metres high, the tower extended up as a column to over 

 24 metres above the roof of the building. The clock-room 

 in which the measurements were made was provided with 

 four glass windows serving as dials for the clock, each with 

 a diameter of about 5 metres. The room itself had an attic 

 space above it and this again was covered with a tile roof. 

 It is presumable that the walls of the clock-room emitted 

 penetrating rays and also absorbed to a certain extent those 

 coming from the outside. However, the ionization observed 

 in the basement of the city hall was only 1 ion per c.c. per 

 second greater than that obtained on the university lawn. 

 Consequently the amount contributed by the tower and walls 

 oE the clock-room was probably not greater than 1 ion per c.c. 

 per second. The absorption by the glass windows too would 

 not be very considerable, and so one may perhaps, without 

 sensible error, set off the one effect against the other and 

 conclude that the readings obtained represent fairly well the 

 relative intensities of the penetrating radiation at the surface 

 of the earth at Toronto/and at a point 64 metres above it. 



II. Secondary Rays produced by the Eartlts Penetrating 

 Radiation. 



In the discussion which has preceded, it has been assumed 

 that 5'8 ions per c.c. per second represents the ionization 

 produced in air confined in a zinc receiver by the penetrating- 

 radiation present at the surface of the earth at Toronto, and 

 by other radiations which it may give rise to or be accom- 

 panied by. 



This value is somewhat higher than that obtained by 

 Wright * in 1908 at Toronto, whose numbers show a dif- 

 ference of only 3*8 ions per c.c. per second between the 

 readings taken on the ice of Toronto Bay, and those taken on 

 the university lawn. It must be remembered, however, that 

 in his experiments the lawn was covered with a layer of ice 

 and snow to a depth of 20 centimetres, while in the present 



* Proc. Roy. Soc. A. No. 577, p. 175 (1911). 



