484 



ELECTRICITY. 



Rnoi- 



iBdXja 



fmvillo. 



Singular 

 example of 

 powerful 

 electricity 



thunder 

 . torm. 



imperceptible, but when the fogt were situated high in 



tlu- air the reverse took place. Mr Henley t'oiuul from 



xperiinrmx that the electricity of a thick fog ncpa- 



ratf>d tfte IwiHsof n rlertrometer from ! to * of nn inch. 



'^' r l ava " made great number of valuable experi- 

 mrnU on atmospherical d'-ctricity, lx>th \vit!i mi eleo- 

 trical kite, anil another instrument of hi< own inveii- 

 tion, which he calls an atmospherical electrometer. * 

 These cx|x*riments with the electrical kite afforded the 

 following general result* : 



1. The nir apjx-ars to ho nt. all times charged with 

 vo electricity, which is stronger in frosty than in 

 warm weather, and is by no means less in the night 

 than in the clay time. 



'-'. The electricity of the kite is generally diminished 

 t>y the presence of clouds. They very seldom increase 

 it, and produce sometimes no effect whatever. 



S. The electricity of the kite is generally negative, 

 and very seldom positive in the time of rain. 



4. The electricity of the kite is not affected by the 

 aurora borealis. 



5. The electrical spark from the string of the kite, 

 or any insulated conductor connected with it, is sel- 

 dom longer tlian a quarter of an inch, but is extreme- 

 ly pungent. When the index of the electrometer 

 do*s not indicate more than 20, the person who re- 

 ceives the shock feels its effect in his legs, and the effect 

 of it resembles more the discharge of the Leyden phial, 

 than a spark taken from the prime conductor. 



6. The electricity of the kite is in general stronger 

 or weaker, according as the string is longer or shorter. 



' n tne course of the experiments from which these 

 conclusions were deduced, Mr Cavallo met with a sin- 

 gular example of a powerful electricity existing in the 

 atmosphere, when there was neither thunder nor light- 

 ning, and when there was none either for three days 

 before or three days after the observation. Indepen- 

 dent of the importance of the observation itself, it affords 

 a. useful lesson respecting the precaution which ought 

 at all times to be taken by those who are engaged in 

 examining the electricity of the atmosphere. As the 

 account of it, given by Cavallo, will not admit of 

 abridgment, we shall present it in his own words. 



" Octol>er the 18th. After having rained a great 

 deal in the morning and night before, the weather be- 

 came a little clear in the afternoon, the clouds appear- 

 ing separated, and pretty well defined. The wind was 

 west, and rather strong, and the atmosphere in a tem- 

 perate degree of heat. In these circumstances, at three 

 P.M. I raised my electrical kite with three hundred 

 nd sixty feet of string. After that the end of the 

 string had been insulated, and a leather ball, covered 

 with tinfoil, had been hung to it, I tried the power 

 and quality of the electricity, which appeared to be po- 

 itive, and pretty strong. In a short time a small cloud 

 passing over, the electricity increased a little ; but the 

 cloud being gone, it (\i-t reaped again to its former de- 

 gree. The string of the kite was now fastened by the 

 silk lace to a post in the yard of the house wherein I 

 lived, which was situated near Islington, and I was re- 

 peatedly charging two coated phials, and giving shocks 

 with them. While I was so doing, the electricity, which 

 was still positive, began to decrease, and in two or 

 three minutes time it became so weak, that it could be 

 hardly perceived with a very sensible cork ball elec- 

 trometer. Observing at the same time that a large and 



black cloud was approaching the zenith (which, no 

 doubt, cau.-cd the decrease of the electricity) indicating 

 imminent rain, I introduced the end of the string 

 through window, in tt first-floor room, when 

 fa-tcncil it by the silk lace to an old chair. The qua- 

 drunt electrometer was set upon the same window, and 

 was, by means of a wire, connected with the hiring of 

 the kit''. PII iiiLj now three quarters of n hour after 

 three o'clock, the electricity was absolutely unperceiv- 

 able ; however, in about three minutes time, it became 

 gain perceivable, but now upon trial was found to be 

 negative ; it is therefore plain, that its stopping ws 

 nothing more than a change from positive to negative, 

 whicli was evidently occasioned by the approach of the 

 cloud, part of which by this time had reached the ze- 

 nith of the kite, and the rain also had begun to fall in 

 Irops. The cloud came farther on, the rain in- 

 creased, and the electricity keeping pace with it, the 

 electrometer soon arrived to 15. Seeing now that the 

 electricity was pretty strong, I began again to charge 

 the two coated phials, and to give shocks with them ; 

 but tlie phials had not been charged above three or four 

 times, before I perceived that the index of the electro- 

 meter was arrived at 35, anil was keeping still increa- 

 sing. The shocks now being very smart, I desisted 

 from charging the phials any longer ; and, considering 

 the rapid advance of the efectricity, thought to take 

 off the insulation of the string, in case that if it should 

 increase farther, it might be silently conducted to the 

 earth, without causing any bad accident, by being ac- 

 cumulated in the insulated string. To effect this, as I 

 had no proper apparatus near me, I thought to remove 

 the silk lace, and fasten the string it.-elf to the chair; 

 accordingly I disengaged the wire that connected the 

 electrometer with the string, laid hold of the string, 

 untied it from the silk lace, and fastened it to the chair ; 

 but while I effected this, which took up less than half a 

 minute of time, I received about a dozen or fifteen very 

 strong shocks, which I felt all along my arms, in my 

 breast, and legs, shaking me in such a manner, that I 

 had hardly power enough to effect my purpose, and to 

 warn the people in the room to keep their distance. As 

 eoon as I took my hands off the string, the electricity 

 (in consequence of the chair being a bad conductor) 

 began to snap between the string and the shutter of the 

 window, which was the nearest body to it. The snap- 

 pings, which were audible at a good distance out of the 

 room, seemed first isochronous with the shocks which I 

 had received, but in about a minute's time oftener ; so 

 that the people of the house compared their sound to 

 the rattling noise of a jack going when the fly is off. 

 The cloud now was just over the kite ; it was black, 

 and well defined, of almost a circular form, its diame- 

 ter appearing to be about 4-0 3 ; the rain was copious, 

 but not remarkably heavy. As the cloud was going off, 

 the electrical snapping began to weaken, and in a short 

 time' became unaudible. I went then near the string.and 

 finding the electricity weak, but still negative, I insu- 

 lated it again, thinking to keep the kite up some time 

 longer; but observing that another larger anil denser 

 cloud was approaching apace towards the zenith, as I 

 had then no proper apparatus at hand, to prevent every 

 possible bad accident, I resolved to pull the kite in ; ac- 

 cordingly a gentleman who was by me began pulling 

 it in, while I was winding up the string. The cloud was 

 now very nearly over the kite, and the gentleman, who 



A description of UICK instrument* will be found in Put II. of the present article. 



