and especially in Scotland. 159 



the distance or interval at which the spark would pass from 

 the one ball to the other. From these experiments, he de- 

 duced the two following laws. 



(1.) ** The respective quantities of electricity requisite to 

 pass a given interval, varied in a simple ratio of the density 

 of the air. When the density was one-half as great, the dis- 

 charge occurred with one-half the quantity accumulated ; — 

 that is to say, with one-fourth of the intensity or free action.'* 



(2.) " The distance through which a given accumulation 

 could discharge, was found to be in an inverse simple ratio of 

 the density of the air, the intensity or free action being sup- 

 posed constant. In air of one-half the density, the discharge 

 occurred at twice the distance.''* 



According to the general laws thus deduced, electricity from 

 the earth will have a greater tendency to flow into the atmo- 

 sphere, when the atmospheric pressure on the earth's surface 

 is diminished ; and if, at the same time, the atmosphere, as well 

 as the ground, be saturated with moisture, then the flow of 

 electricity will be still more facilitated. 



The general law established by Mr Harris, and the application 

 of it, now proposed, to the electrical condition of the earth, com- 

 pletely accords with the views before alluded to, of some recent 

 writers who maintain, that, when the atmosphere is in a moist 

 and highly electrified state, the barometer must necessarily be 

 low. ** If," says Mon. de Tessan, " a mass of air placed at the 

 surface of the earth is moist and electrified, the barometer 

 ought, CfBteris paribus, to shew a less pressure than if it were 

 not in that mass." t This is not the place to explain the grounds 

 of the opinion now referred to ; it is sufficient to notice the fact, 

 that it is entertained, and by persons whose character and re- 

 searches entitle them to respect. 



* Snow Harris on some elementary laws of Electricity. Lond. Roy. Soc. 

 Trans; for 1834, p 228. 



t L'lnstitut, 10th June 1841. 



