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with the other hold a sharp-pointed needle, and keeping the pointdirected towards the knob of the jar, proceed gradually near it,until the point of the needle touches the knob. This operationdischarges the jar entirely; and you will either receive no shockat all, or so small a one as can hardly be perceived. The pointof the nedle, therefore, has silently and gradually drawn all thesuperfluous fluid from the inside surface of the electric jar.
The Electrified Cotton.
Take a small lock of cotton, extended in every direction asmuch as conveniently can be done; and by a linen thread aboutfive or six inches long, or by a thread drawn out of the same cot-ton, tie it to the end of the prime conductor: then turn the winchof the machine, and the lock of cotton, on being electrified, willimmediately swell, by repelling its filaments from one another,and will stretch itself towards the nearest conductor. In this situ-ation let the winch be kept turning, and present the end of yourlinger, or the knob of a wire, towards the lock of cotton, whichwill then immediately move towards the linger, and endeavour totouch it; but take with the other hand a pointed needle, and pre-sent its point towards the cotton, a little above the end of the fin-ger, and you will observe the cotton immediately to shrink up-ward, and move towards the prime conductor. Remove theneedle, and the cotton will come again towards the finger. Pre-sent the needle, and the cotton will shrink again, which clearlyproves that the needle draws off the electric lluid from the cotton.
The Electrified Bladder.
Take a large bladder well blown, and cover it with gold, silver,or brass leaf, slicking it with gum-water: suspend it at the end ofa single thread, at least six or seven feet long, banging from theceiling of the room; and electrify the bladder, by giving it astrong spark with the knob of a charged bottle: this done take aknobbed wire, and present it to the bladder when motionless; andyou will perceive, that as the knob approaches the bladder, thebladder also moves towards the knob, and, when nearly touchingit, gives it the spark which it received from the charged phial, andthus it becomes unelectrified. Give it another spark, and, insteadof the knobbed wire, present the point of a needle towards it, andthe bladder will not be attracted by, but rather recede from, thepoint, especially if the needle be very suddenly presented towardsit. This is one of Mr. llenly’s experiments.
The Dancing Balls.
Fix a pointed wire upon the prime conductor, with the pointoutward ; then take a glass tumbler, grasp it with your hands, andpresent its inside surface to the point of the wire upon the primeconductor while the machine is in motion: the glass in this man-ner will soon become charged; for its inside surface acquires theelectricity from the point, and the bands serve as a coating for theoutside. This done put a few pith balls upon the table, and co-ver them with this charged glass tumbler. The balls will imme-diately begin to leap up along the sides of the glass as represented,tig. 1,” Plate LXLV. and will continue their motion fur a consider-able time.
Thf. Electric Flies.
These flies are composed of small brass wires, figs. 2 and 3,Plate I.XIV, fixed into a cap of brass, easily moveable upon anaxis of the same metal, and exactly balanced, so that they may1 urn with the smallest force. The ends, which ought to be verysharp, are all bent one way, with regard to one another, as thosebelonging to a, b, in the figure; though tiie two sets of points con-stituting the two flies there represented, are contrary to eachother; so that the whole flies must have a contrary motion. Fix-ing the axle with the two flies upon the prime conductor, andworking the machine, botli will begin to turn very swiftly, each ina direction contrary to that of the points. In this manner, with apowerful machine, several flies may be made to turn either in thesame or in contrary directions; and by their gradual increase ordecrea.-e in size may represent a cone or other figure; for (hecourse of each will be marked by a line ot lire, and thus the wholewill exhibit a beautiful appearance in the dark. The light is saidto be more brilliant when the ends are slightly covered with seal-ing-wax, grease, or other electric matter, J he flies, in this expe-
riment, turn the same way whether the electricity lie positive ornegative; the reason will easily be conceived from the theoryabove laid down, viz. that in positive electricity the fluid issuesfrom the. body electrified, and that in negative electricity it entersinto it. In the former case, the force of tiie fluid, which actsequally oil tiie air and on tiie point from whence it is.-ues, mustcontinually put the point the contrary way; and in negative elec-tricity, when tiie point solicits a continual draught of electric mat-ter from tiie air, tiie direct impulse of tiie former must also pro-duce a motion in the point in the course in which the lluid itselfmoves. In vacuo no motion is produced; because there is no air onwhich the fluid may act when it issues from the point. In like man-ner, when air is inclosed in a glass vessel, tiie motion of the electricfly soon stops; because the air and the glass soon becomes so muchelectrified, that the escape of the fluid from tiie point is prevented,and therefore its motions are impeded, and in a little time entirelystopped. On applying a conductor to the outside of the glass,tiie fly renews its motion; because an opportunity is now given tothe fluid to escape, by running through the glass. But this mustsoon cease, because, when tiie glass is thus charged, no more canbe carried from the outside. Fig. 4 shews another fly whichturns perpendicularly by the same electric power. By means ofthe stream of electricity issuing from a point, many other amusingexperiments have been contrived: such is the electrical orrery,shewing tiie motion of the sun, earth, and moon . The sun andearth go round the common centre of gravity between them in asolar year; and tiie earth and moon go round the common centreof gravity between them in a lunar month. These motions arerepresented by an electrical experiment as follows: Tiie ball S,fig. a, represents tiie sun, E the earth, and M tiie moon , connect-ed by wires a c and b d : A is the centre of gravity between theearth and moon. These three balls, and their connecting wires,are bung and supported on the sharp point of a wire A, so tiiat tiiewhole may turn on the point «. In like manner E and M aresupported at the point b, round which they turn. And as E andM are light when compared with S and E, there is much less fric-tion on the point b, Ilian on tiie point a. Tiie weights of the ballsmay be adjusted so, that E and M may go twelve times round b,in the time that S and E go once round a. This affords an amus-ing experiment in electricity ; but it is so far from proving thatthe motions of the planets in the heavens are owing to a like cause,that it plainly proves they are not; for the real sun and planets arenot connected by wires or bars of metal, and consequently therecan be no such metallic points as c and d between them. Andwithout such points, the electric fluid would never cause them tomove ; for, lake away these points in the above-mentioned expe-riment, and the balls will continue at rest, let them be ever so-strongly electrified. Fig. 6 represents a water-mill for grindingcorn, turned by a stream of electricity. A is tiie water-wheel,B the cog-wheel on its axis, C Ilia trundle turned by that wheel,and D the running mill-stone on the top of the axis of thetrundle. It may easily lie contrived and turned also by electri-city, if instead of the round plate 1) for the mill-stone, there is ahorizontal wheel on the trundle C, with spur-cogs, which willturn two trundles placed on its opposite sides: and on the top ofeach of these trundles’ axis, may be a round plate representing amill-stone; so that this model lias all the working parts of a dou-ble water-mill, turning two mill-stones. Set the mill near theprime conductor, and place the crooked wire so that its point maylie directed towards tiie uppermost side of the great wheel A,Then turn the glass globe by tiie winch, and the stream of firethat issues from the point of die wire will turn the wheel, and con-sequently all the other working parts of the mill. Small boats orswans are sometimes made of light wood or cork, and they maybe attracted and made to swim in any direction, by applying alinger towards them; or they may have tiie addition of sails, andwill then be made to sail briskly before an electrical gale from awire held in the hand of the operator.
The Thunder House.
Fig. 9, Plate LXIV. is an instrument representing the side of ahouse, either furnished with a metallic conductor, or not; bywhich both tiie bad effects of lightning striking upon a house notproperly secured, and the usefulness of metallic conductors, maylie clearly represented. A is a board about three quarters of an
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