494 



ELECTRICITY. 



vanished, there was heard a noise similar to that of 

 thunder. These globes sometimes leave behind them 

 a luminous track ; at other times they are dispersed in- 



have been ar< 



with a shower of meteoric 



Tin vi fire balls has someti 



computed at 5 miles in a second, at other timc- 



to divided sparks ; sometimes they are separated into miles in a second. The li.llnwing is a list of the alti 

 ^Su* globes of a smaller sixe; and on some occasions tliey tudes of some of the most iiii|xirtant. and ]mdue 



I'iJi:-. 



tbooting 



Fire of St 



electricity, 



Another species of meteors, called falling or shooting 

 stars, are of very frequent occurrence. They have been 

 observed at all seasons of the year, and during every 

 kind of weather ; but they certainly occur most fre- 

 quently at the season when the aurora boreal is is visi- 

 ble, and are always seen in a lower region of the atmo- 

 sphere. Mr Singer has counted no fewer than SO of 

 them in a single hour, and sometimes nearly twice that 

 number. 



The fire of St Elm, or Castor and Pollux, as it has 

 been called, is a bright light, which often appears at the 

 tops of ship masts, on the points of bayonets, and on 

 the summits of spires. This phenomenon is obviously 

 nothing more than the electricity of the atmosphere dis- 

 charging itself into the earth by a pointed body eleva- 

 ted in the air. 



Several authors have employed the electric fluid, 

 for the purpose of accounting for volcanoes, earth- 

 quakes, fogs, and water-spouts ; but no facts have yet 

 been established, which entitle us to consider these 

 phenomena as of electrical origin. See Bergman, O/>ux- 

 cula, torn. v. p. 291. Dalton, Metenro/o^ical Essays. 

 Ronayne, Pliil. Trans. 1772, vol. Ixii. Cavallo' s Elec- 

 tricity, vol. ii. ; Cavallo's Natural Philosophy, torn. iv. 

 p. 359. Bertholon, De I' Electricitt des Mfteoret, tom. ii. 

 and Singer's Elements of Electricity, p. 261. See ME- 

 TEORIC STONES. 



CHAP. III. 



On the Effects of Electricity. 

 SECT. I. On the Mechanical Effects of Electricity. 



IN the transmission of the electric fluid through bo- 

 dies that have the power of conducting it with facility, 

 its passage is not marked with any mechanical effect; 

 but when electricity is accumulated to such a degree 

 as to be capable of passing through a non-conducting 

 substance, it experiences such a resistance in its nas- 

 sage, that it can only be overcome either by expanding 

 or shattering the substance itself. It is thus that the ac- 

 cumulated electricity of the atmosphere is carried off 

 in silence and safety when it strikes upon a rod of iron, 

 while a beam of wood or a tree is shivered into a 

 thousand fragments. The mechanical effects of elec- 

 tricity arc exhibited in its power of impelling and dis- 

 persing light bodies, of perforating, expanding, com- 



pressing, tearing, and breaking to pieces, all conduct- 

 ing substances Uirough which it is sufficiently power- 

 ful to force its passage. 



Exp. I. If a light wheel, having its vanes made of j_ 



card paper, be made to turn freely upon a centre, it of light 

 will be put in motion when it is presented to an elec- bodies, 

 trifled point. The wheel will always move from the 

 electrified point, whether its electricity is positive or 

 negative. In this experiment, the current seems to be 

 produced by the recession of the similarly electrified 

 air in contact with the point ; and, therefore, the cir- 

 cumstance of the wheel turning in the same direction 

 when the electricity is negative, cannot, as Mr Singer 

 has remarked, be considered as any proof of the exist- 

 ence of a double current of the electric fluid. 



Exp. 2. Having formed a groove, either by bending 

 a piece of clean card piper, or by hollowing out a piece 

 of baked wood, or by placing parallel to each other 

 two straight sticks of sealing wax, lay the groove upon 

 the plate of Henley's universal discharger, * and place 

 a large pith ball, about half an inch in diameter, so as 

 to be at equal distances from the two brass knobs of the 

 discharger. The distance of these knobs should be 

 about four inches, and the groove placed on the line 

 joining the knobs. If one of the wires is connected 

 with the outside of a charged jar, while the knob of 

 the jar is brought into contact with the other wire of 

 the discharger, so that a small spark may pass from 

 the one knob of the discharger to the other, the pith 

 ball will be impelled from the positive to the negative 

 knob; that is, to the knob which communicates with 

 the negative side of the jar. Mr Singer recommends 

 the use of points instead of the knobs employed by Ca 

 vallo ; and he has shewn, we think satisfactorily, that 

 the transmission of the charge is prevented by the use 

 of knobbed wires; and that the motion of the ball is, in 

 that case, produced by attraction, whereas, with point- 

 ed wires, die effect is produced solely by a continued 

 current of electricity. 



Exp. 3. Place upon an insulating stem a light wheel 

 of card paper, properly impended upon pivots, as re- 

 pn-.-cnted in Fig. 5. Plate CCXLV1. and introduce it PLATE 

 between the pointed wires A B of the universal dis- ' ' 

 charger, placed exactly opposite to each other, and at 

 the distance of little more than an inch from the upper 

 vanes. Then having connected the wire A with the 

 positive conductor, and the wire B with the negative 

 conductor of an electrical machine, the little wheel will 

 revolve in the direction AB ; and if the wire B is con- 



* 8e Put II. of thii article for a description of this ingenious instrument. 



