[cLiNE] RADIATION AT THE SURFACE OF THE EARTH 138 



When this point was made certain the readings were commenced. 

 These were taken on Dec. 18 and 19, 1908. They are given in Table 

 XI and a curve drawn from them is shown in Fig. IX. 



This curve also, it will be seen, shews no regular periodic variation. 

 Further, the greatest deviation from the mean value was not more than 

 4%. The capacity of the measuring system was greater in this set of 

 measurements than in the previous one but the same voltage was 

 applied to the sliding condenseil- as in the previous measurements. 

 The result of this change was to reduce the sensibihty of the apparatus 

 slightly, and this would account for the 4% variations in the readings 

 apart altogether from any variation in the conductivity. 



A point of special interest in connection with the readings is the 

 gradual drop which they indicate in the conductivity of the enclosed 

 air. This drop, which has been observed by others in similar measure- 

 ments, was no doubt due to the gradual removal by the field of the 

 suspension particles designated by Langevin' as " large ions " and first 

 investigated by him. When the air was introduced into the receiver it 

 was passed through a tube filled with cotton wool. This filter it was 

 thought would remove all the dust and fine particles in suspension, but 

 it was found after the completion of the measurements that when air 

 filtered in the same way was led into an expansion cloud chamber of 

 the C. T. R. Wilson type it still contained suspension particles in 

 abundance which acted as cloud nuclei. During these measurements 

 the barometer only changed from 744 to 752 mm. 



On account of the higher radioactivity in the walls of this receiver 

 the conductivity of the air in this case was about double its value in the 

 early measurements. This enabled the writer to take a greater num- 

 ber of observations in a given time, but on account of the failure to 

 increase the voltage of the condenser tube it did not contribute any- 

 thing to the sensitiveness with which the readings were taken. 



v.— EXPERIMENTS. 



Series III. 



The experiments which have been described up to the present were 

 all conducted indoors, and as the walls of the Physics Building very 

 probably reduced in some degree the intensity of the penetrating 

 radiation, it was thought best to complete the series of observations by 

 placing the receiver of the apparatus outside and so exposed directly 

 to the atmosphere. 



^Comptes Rendus, 140, pp. 232, 1905. 



