the Eartlis Penetrating Radiation on Land and Sea. 523 



0. T. R. Wilson compensating condenser gold-leaf electro- 

 meter. The receiver R, which had a capacity o£ abont 

 30 litres, was made of a sample of zinc about *5 mm. in thick- 

 ness, which on comparison with a number of different samples 

 was found to contain the least amount of radioactive im- 

 purities. The electrode E was provided with an earthed 

 guard-tube at I where it entered the receiver, and the 

 ionization in the receiver was measured by applying satu- 

 rating voltages to the latter by a battery of dry cells connected 

 as shown in the figure. 



Extreme care was taken before commencing the measure- 

 ments to have the receiver R hermetically sealed so that there 

 could be no possibility of air containing active emanations 

 entering it during the course of the observations. A small 

 aneroid barometer was supported inside the receiver with its 

 face directly in front of a glass window sealed into the wall 

 of the vessel, and tests made with it from time to time 

 showed that the sealing remained secure and intact through- 

 out all the measurements. 



In making these tests the receiver was carried from a cool 

 room either to a warm one or out into the bright sunshine. 

 This procedure was always followed by a rise in the baro- 

 metric reading owing to the air in the vessel becoming 

 heated. While the receiver was kept in the warmer position 

 the high reading was always steadily maintained, but on 

 taking it back to the cooler place the reading just as regularly 

 rapidly fell to its former value. This beha\iour showed that 

 no air escaped from the receiver during the period when the 

 pressure was greater than that of the atmosphere outside. 



A summary of the results obtained in these measurements 

 at a number of points of observation is given in Table I. 

 (p. 524). 



The final readings given in the table are the means of a 

 number taken at each observation point. Those at Cambridge 

 were made in a small brick cottage, the property of the 

 Cambridge Scientific Instrument Co., and the thanks of the 

 writer are due to the Manager, Mr K. S. Whipple, for 

 placing this building at his disposal and for very kindly 

 assisting in taking practically all of the readings in that place. 



Those at Bowland were taken in a room in the residence 

 of Mr. William Ramsay, a large stone structure, while those 

 in the Meteorological Observatory, a stone building, were 

 made with the kind permission of the Director, Mr. R. F. 

 Stupart. The Physical Laboratory at Toronto, in which one 

 of the sets of readings was made, is constructed of brick. 



From inquiries it was learned that in none of these four 

 observing stations was there any radioactive contamination 



