CHAP, iv.] ELECTRICITY AND MAGNETISM. 259 



the stratum of denser air between acting like the 

 glass of a Leyden jar in keeping the opposite charges 

 separate. If we could measure the electric potential at 

 different points within the thickness of the glass of a 

 Leyden jar, we should find that the values of the 

 potential changed in regular order from a -f value at 

 one side to a value at the other, there being a point 

 of zero potential about half way between the two. Now, 

 the air in fine weather always gives + indications, and 

 the potential of it is higher the higher we go to 

 measure it. Cavallo found more electricity in the air 

 just outside the cupola of St. Paul's Cathedral than 

 at a lower point of the building. Sir W. Thomson 

 found the potential in the island of Arran to increase 

 from 23 to 46 volts for a rise of one foot in level ; but 

 the difference of potential was sometimes eight or ten 

 times as much for the same difference of level, and 

 changed rapidly, as the east wind blew masses of cloud 

 charged with + or electricity across the sky. Joule 

 and Thomson, at Aberdeen, found the rise of potential 

 to be equal to 40 volts per foot, or i -3 volts per centi- 

 metre rise of level. 



During fine weather a negative electrification of the 

 air is extremely rare. Beccaria only observed it six 

 times in fifteen years, and then with accompanying 

 winds. But in broken weather and during rain it is 

 more often than + , and exhibits great fluctuations, 

 changing from to + , and back, several times in half 

 an hour. A definite change in the electrical conditions 

 usually accompanies a change of weather. " If, when 

 the rain has ceased (said Ceca), a strong excessive (-f ) 

 electricity obtains, it is a sign that the weather will 

 continue fair for several days." 



3O7. Methods of Observation. The older 

 observers were content to affix to an electroscope (with 

 gold leaves or pith -balls) an insulated pointed rod 

 stretching out into the air above the ground, or to fly a 



