14 ABSTEACTS: ATMOSPHERIC ELECTRICITY 



effect of the potential gradient is simply to lower the value of the 

 maximum potential to which it is allowable to charge the central system. 

 In the case of the conductivity of the positive ions, while the ionic paths 

 are influenced, neither the conductivity nor the maximum potential 

 allowable are influenced on the whole. 



The effect of making measurements of conductivity on the top of 

 a high tower is discussed in the above hght and it is shown that, if suit- 

 able precautions are taken, the proper conductivity will be obtained 

 for the air at that altitude, in spite of the influence of the charge on 

 the tower. There is an advantage in making measurements on the air 

 far removed from the ground, since the number of ions present are then 

 uninfluenced by the effects due to the potential gradient referred to 

 in the first abstract. The question of shielding the instruments to 

 protect them from the potential gradient is discussed. It is only in 

 the case of the Ebert apparatus that shielding of this kind is necessary. 

 It is important to ascertain, however, whether the avoidance of charge 

 on the earthed portions of the apparatus by shielding is not counter- 

 acted by the effect of the charge on the shield. It is shown that a 

 properly arranged shield is on the whole effective. 



W. F. G. S. 



ATMOSPHERIC ELECTRICITY.— T/ie atmospheric-electric observa- 

 tions made on the second cruise of the Carnegie, 1909-1913. C. W. 

 Hewlett. Terr. Mag., 19: 127-170. 1914. 



The atmospheric-electric work on board the Carnegie has been con- 

 fined entirely to observations of the specific conductivity, the potential- 

 gradient, and the radioactivity of the atmosphere, the greater part of 

 the observations consisting of the first two quantities named. The 

 observations are divided naturally into three principal groups, accord- 

 ing to the observer who made them. From New York to Colombo, 

 E. Kidson conducted the observations; for the route from Colombo 

 to Manila, owing to breakage in the instruments and the impossibility 

 of having the requisite repairs made, there are no observations; from 

 Manila to Tahiti, H. F. Johnston conducted the observations, and from 

 Tahiti to New York, the work was carried on by C. W. Hewlett. For 

 an account of the instruments and methods used, as wefl as for the 

 detailed results obtained, reference must be made to the original article. 



From a summary of the various results thus far obtained at sea, 

 the following deductions in regard to the mean values of the elements 

 may be drawn: The potential-gradient is of the same order of mag- 



