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R.\DIATION AT THE SURFACE OF THE EARTH 



127 



After completing the preliminary series of measurements just 

 described it was decided to undertake a series extending over a twenty- 

 four hour period. The readings were taken on December 18 and 19, 

 1908, in the Library of the Physical Laboratory. This was a large 

 well-ventilated room facing the south east, and in it the apparatus was 

 set up on a table close to one of the windows. During this set of 

 observations the barometer remained fairly steady, the maximum 

 change being from 746.3 to 749.5. The readings for this set of obser- 

 vations are given in Table X and a curve to illustrate them in Fig. 

 VIII. 



From an inspection of these readings and the curve, it will be seen 

 that there is no evidence of a regular diurnal variation in the ionisation. 

 The extreme deviations in the readings from the mean taken through- 

 out the whole period were not greater than 3%, and as the instrument 

 could not be used under the conditions of the experiment to give a 

 greater accuracy than 2 or 3%, one is forced to conclude from these 

 observations that at Toronto there are no daily variations in the 

 intensity of the penetrating radiation greater in magnitude than such 

 variations as came within the Umits of sensibility of the measuring 

 apparatus. 



In the experiments which have been described no special precau- 

 tions were taken to have the cylinder hermetically sealed. At a 

 number of joints minute openings were left unclosed and so there must 

 necessarily have been a constant interchange between the air inside 

 the cylinder and that outside. It is worthy of note to observe that 

 even with this interchange only very small variations in the conducti- 

 vity of the air occurred. 



TABLE X 



