ATMOSPHERIC ELECTRICITY. 293 



in this way we should especially mention Cavallo/" After many trials 

 this philosopher found the ordinary kites with which children amuse 

 themselves as useful as those which had been carefully made by 

 himself, provided he took the precaution to cover them with varnish, 

 or saturate them with boiled linseed oil, in order to protect them from 

 the rain. Those which he used in his numerous experiments were 

 four feet long by two broad. Kites of these dimensions appeared to 

 him the best, because they could be managed with ease, and are suffi- 

 ciently large to sustain the weight of the cord at a convenient height. 

 Those made of cloth or tafieta he found less useful, because they needed 

 more wind to raise them. He discovered, also, that the best cord was 

 formed of two strands of hemp twisted with a copper wire.f Lastly, 

 he tried further to increase the conducting power of the cord by 

 coating it with lamp black, charcoal powder, powdered emery, or 

 other similar substances diffused in gum water ; but the effect was 

 scarcely perceptible and of slight duration, because these substances, 

 not sticking to the cord, were easily rubbed off. In this respect Nairne 

 obtained a greater effect by simply steejjing the cord in water satu- 

 rated with salt. In this way it became a very good conductor of elec- 

 tricity, on account of the diffused humidity of the air, which was 

 strongly attracted by the salt. 



§ 3. 0/" electroscopes^ electrometers j and rlieometers.X — To ascertain 

 the presence of electricity in the rods which they had raised in the 

 air, observers at first drew the sparks with the finger, or with some 

 other conducting body. When the electric tension was too feeble to 

 give out a spark, they placed near the rods a little saw-dust, or some 

 very fine down or cotton, and then observed the attraction which was 

 exercised on these substances. The Abbe Mazeas§ attempted by this 

 method to measure the intensity of atmospheric electricity from the 

 relative distances at which this attraction was manifested. These 

 rude and inconvenient means induced philosophers to have recourse to 

 other more delicate ones, especially when it became necessary to ob- 

 serve electricity of feeble tension, with which the electro-atmospheric 

 apparatus is charged in clear weatlier. As early as 1752, NoUetjfj to 

 attain this object, em{)loyed his electrometer composed of two simple 

 threads, which were separated from each other by the effect of repul- 

 sion when the conductor witli which they were connected was electri- 

 fied. He was the first who reduced the measure of the intensity of 

 atmospheric electricity to the observation of an angle. A little 

 after this Canton, T[ in his experiments^ terminated the two threads of 

 the French philosopher's electrometer by little balls of the pith of 

 elder, very nearly the same time that Franklin**, used with advantage 

 in the electric chime to study the electricity of the clouds. 



- Traits complet. d'Electricitc^, par Tibfere Cavallo, p. 274, et suiv. Paris, 1785. 



f Charles used a metallic cord of wire, and M. Peltier a simple wire of (copper?) of mm. 

 .5 diameter — (See the Traite de rhyBi(iue Experimentale et Matheiaatique of M. Biot, 

 torn. II, p. 444, aud the work of JI. Peltier entitled " Observations et llecherchcs Expe'ri- 

 mentales sur les Trombes," introduction, p. 7. Paris, 1840.) 



X The term rheometer is used in this article to denote a needle galvanometer. — Tr. 



tj Hist, dc Electricity, par Priestley, tom. II, p. 222. 



II Lettres eur I'Electricittf, tom. I, p. 175. Paris, 1753. 



fj Transact. Philos. for 175.3, 1st part, p. 356, 1753. 



•-'" Experiments and Observations ou Electricity, p. 112. 



