430 Prof. McLennan and Mr. Murray on Residual 



given by Wright * in a paper " On variations in the con- 

 ductivity of air enclosed in metallic receivers/' and that of 

 the latter in a paper by McLennan and McLeod f on 

 " Measurements on the Earth's penetrating radiation with 



a Wolff electrometer/' there is no need of repeating them 

 in this place. In the experiments the conductivity of air 

 was measured at three stations, the one being on the ice on 

 Toronto Bay, where the water was about 20 feet deep, the 

 second in the Library of the Physics Building, and the third 

 in a brick house free from radioactive contamination on land 

 at a point about two miles from the shore of Lake Ontario. 

 Three different types of measurements were made. In the 

 one the air was confined in a vessel of clean zinc of about 

 30 litres capacity; in the second it was enclosed in an air-tight 

 zinc Wolff electrometer of about two litres capacity ; and in 

 the third it was confined in a vessel of ice whose capacity 

 was about 30 litres. Three distinct ice vessels were used, 



* Wright, Phil. Mag-, xvii. p. 295 (1909). 

 t McLennan and McLeod, loc. cit. 



