﻿26 EQUIPMENT AND WORK OF AN AERO-PHYSICAL OBSERVATORY, 





Number of 

 observations. 



Mean value of potential. 





Bate. 



High. 



Low. 



138 metres. 



June 26 ; 



July 17 



July 20 



Sept. 21 



399 



60 

 107 



40 



94 



82 



97 



87 

 4(6) 



98 



15 



65 



13 



26 



54 



289 

 1,129 



389 



212 



586 



300 



435 



140 

 . 1,137 



943 

 —849 (c) 



458 



487 



413 

 1,825 



134 

 93 



70 

 107 

 192 

 108 

 112 



24(a) 

 265 

 248 

 -254 (c) 



36 

 4(d) 

 141 



89 



155 



1,036 



319 



105 



Oct. 4 



Oct. 5 



394 

 192 



Oct. 7 



323 



Oct. 14 



116 (a) 



872 



695 



Nov. 1.. 



Nov. 3 



Nov. 12......... 



Nov. 12 



—604 (c) 

 422 



Dec. 15 



Jan. 29 



483 (d) 

 272 



Feb. 9 



1 736 







(a) Denotes values at lowei- station below zero. 



(6) Observations not simultaneous; differing few minutes. 



(c) During rain ; negative values at both stations. 



We have, therefore, a mean value for the potential of the air, at an 

 elevation of 500 feet, of 637 volts. If we omit negative values and con- 

 sider only positive we obtain 543 volts, or, roughly, 4 volts per metre 

 elevation. We have no right, however, to omit the negative values, and 

 the true value for free air would be doubtless higher than the figure here 

 given. For the lower station we find the mean value of the potential 

 to be about one-fifth of that at the upper, while the elevation is about 

 one-eleventh. 



Thunder-storms. — As might be anticipated, there are some remarkable 

 variations in the potential during thunder-storms. We are able to record 

 the time of lightning by well-marked variation in the potential. The 

 accompanying diagrams show the potential variations during a thunder- 



^lectricocl Potentia-l of theJLir Fhoto^rccphioally recristered. at ttvo /St actions . 

 A- 194 m. above Secc-level an-(£ S - y^rn.oc^we jSe(:^-7eTrel. 



storm at both an upper and a lower station. For purposes of compari- 

 son, some characteristic fair-weather curves are given. Thunder-storms 

 cannot be studied to the best advantage until the electrometer comes into 



