IkingJ PEXETRAri-VG RADIATIO-s FROM THE EARTH 103 



of the gradients in the earth can be obtained. Corrections for the 

 finite size of the box may be made by means of an analysis similar to 

 that given by the writer for a circular plate*, in which case the for- 

 mulae are comparatively simple: this corresponds to the making the 

 box referred to circular in shape. 



(iii) Owing to difficulties attending measurements from structures 

 or from balloons it might be advantageous to keep the electroscope at 

 various heights by means of a small pilot balloon capable of reeled 

 in quickly enough that the rate of leak during ascent or descent may 

 be neglected or at any rate allowed for: active deposit at various 

 heights should be tested for by means of a dummy electroscope simi- 

 larly exposed and a correction made if possible. At the time of the 

 observation measurements of the electrostatic potential gradient 

 should be made and a record kept of other meteorological conditions. 



July 10, 1913, 



McGill University, 

 Montreal. 



4ving, loc. cit., p. 249. 



