ATMOSPHERIC ELECTRICITY. 355 



during the wliole time a storm lasts. Volta,* who directed his atten- 

 tion to these variations, states that he observed as many as fourteen 

 changes of sign in the space of one minute by exposing his electro- 

 meter to the action of thunder clouds ; and, according to De Saussure,t 

 these changes follow each other in some cases with such rapidity that 

 we hav^e not time to record them. Schilblerl has also made some ob- 

 servations, which define these electric variations better than any one 

 had done before him ; they teach us that the electricity of the electro- 

 meter gradually increases in proportion as the storm draws near ; that 

 it reaches its maximum when the storm is nearest to the point of ob- 

 servation. Independently of these gradual changes, it also under- 

 goes others, which are exhibited in a sudden manner at the moment 

 of the appearance of every flash of lightning ; finally, that this last 

 phenomena produces, also, in the electricity a sudden passage from 

 one state to another of an almost equal intensity. To study the 

 course of the electricity during storms, Schlibler made a series of ob- 

 servations at very close intervals of time, and he represented the 

 result by a curve, as in his observations on the electricity of rain and 

 snow. 



To show how complicated are the electric phenomena, we here 

 quote the detail^ of observations made during two storms — one of 

 which passed at a distance^ the other appeared directly over the zenith 

 of observation : 



"April 11, ISOG. — In the day the electricitj' of the iiir is feebly positive, and tlio t.e:a- 

 pertiture ro?e to 1 1° R. 



"C o'clock, evening. — It began to rain; tlie electricity became negative. 



" 7 o'clock, evening. — The rain ceased; dark cloud covers the sk}-, and a storm shows itself 

 in the horizon on the southeast. The electricity is still negative; it decreases suddenly 

 ■with the flashes of lightning, every one of them producing, in the straws of an electro- 

 meter, an approach, followed by a new divergence, but less than that which took place 

 the moment before. 



"7 o'clock li minutes, evening. — A flash of lightning suddenly reduces the electricity to 

 zero, but causes it to pass for an instant to the positive state ; it afterwards becomes 

 negative. 



"7 o'clock 18 mimdes, evening. — The electricity becomes positive, and remains so from this 

 instant till that in which the storm appears to be neurerest; the positive electricity goes on 

 increasiug ; v/itli every flash of lightning the straws diverge, to fall back almost immedi- 

 ately, yet preserving every time a greater divergence than what they before had. 



"7 o'clock 45 minutes, evening. — The storm moves in the direction of the northeast ; the 

 positive electricity decreases, and the flashes of lightning are more rare. 



" 8 o'clock 4 minuks, evening. — Tlie electricity is at zero ; but a rain which follows causes 

 it anew to pass to the ntgative state ; nothing more is perceived of the storm. 



" J/-yy 14, 1813. — During the day the temperature rose to -|-1G° 3 R ; the baromct^jr 

 is several lines below its mean ; the wind blows lightly from the SE., and the electricity 

 has a feeble positive tension. 



" 4 o'clock 40 minutes, evening — A storm rising on the horizon in the SW. ; the positive 

 electricity increases with every flash of lightning, but more rapidly than in the fust storm ; 

 great drops of water fall. 



" 4 o'cUtck 47 minutes, nening. — The storm is nearer ; its distance appears about 4,000 feet. 

 A very vivid flash of lightning, followed by a peal of thunder, suddenly changes the 

 positive to negative electricity, of a nearly eijual intensity ; at the same time there 

 occurrs a heavy shower. From thi.s point, and wJiile the storm passed to the NW., the 

 negative electricity presented a course opposite to that taken by the positive electricity. 

 The rain continued. 



" 3 o'clock 4 minutes, evening. — The electricity became positive anew. 



t Journal de Physique, tome LIX, page 296. 1809. 

 + Voyages dans les A'fes, tome II, § 801, page 220. 

 § Journal de Sclr.veigger, tome IJ, page 377. 



