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ELECTRICITY

inch thick, ami shaped like the gable end of a house. This boardis fixed perpendicularly upon the bottom board B, upon whichthe perpendicular glass pillar, C I), is also fixed in a hole abouteight inches distant from the basis of the board A. A square holel L M K, about a quarter of an inch deep, and nearly one inchwide, is made in the board A, and is filled with a square piece ofwood; which is made nearly'of the same dimensions, because itmust go so easily into the hole, that it may drop off by the leastshaking of the instrument. A wire L K is fastened diagonally tothis square piece of wood. Another wire III of the same thick-ness, having a brass ball II, screwed on its pointed extremity, isfastened upon the board A; so also is the wire M N, which isshaped in a ring at O. From the upper extremity of the glasspdlar C D, a crooked wire proceeds, having a spring-socket F,ihrough which a double knobbed wire slips perpendicularly, thelower knob G of which falls just above the knob II. The glasspillar DC must not be made very fast into the bottom board;but it must be fixed so as it nvav be pretty easily moved round itsow n,axis; by which means the brass ball G may be brought neareror farther from the ball H, without touching the part IS K G.Now when the square piece of wood L M I K (which may repre-sent the shutter of a window or the like) is fixed into the hole so,that the wire LK stands in the dotted representation I M, thenthe metallic communication from II to O is complete, and the in-strument represents a house furnished with a proper metallic con-ductor: but if the square piece of wood L M I K is fixed so, thatthe wire LK stands in the direction LK, as represented in thefigure, then the metallic conductor II O, from the top of the houseto its bottom, is interrupted at I M, in which case the house is notproperly secured. Fix the piece of wood LMIK so, that itswire may be as represented in the figure, in which case the nie-fallie conductor II O is discontinued. Let the ball G be fixed atabout half an inch perpendicular distance from the ball II; then,by turning the glass pillar D C, remove the former ball from thelatter; by a wire or chain connect the wire E F with the wire Qof the jar P, and let another wire or chain, fastened to the hookO, touc h the outside coating of the jar. Connect the wire Q. withthe prime conductor, and charge the jar; then, by turning theglass pillar D C, let the ball G con-.e gradually near the ball H ;and when they are arrived sufficiently near one another, the jarwill explode, and the piece of wood LMIK will be pushed outof the hole to a considerable distance from the thunder house.Now the ball G, in this experiment, represents an electrifiedcloud, which, when it is arrived sufficiently near the top of thehouse A, the electricity strikes it; and as this house is not secur-ed with a proper conduc tor, the explosion breaks off a part, i. e.knocks off the piece of wood 1 M. Repeat the experiment withonly this variation, viz. that this piece of wood I M is situated so,that the: wire J, K may stand in the situation I M, in which casethe conductor II O is not discontinued ; and then the explosionwill have no effect upon the piece of wood L M, this remainingin the hole unmoved; which shews the usefulness of the metallicconductor. Again, unscrew the brass ball II from the wire II I,so that litis may remain pointed. With this difference only in theapparatus, repeat both the above experiments; and you will findthat the piece of wood I M is in neither case moved from itsplace, nor any explosion will be beard; which not only demon-strates the preterence of the conduc tors with pointed terminationsto those with blunted ones; but also shews that a house furnishedwith sharp terminations, although not furnished with a regularconductor, is almost sufficiently guarded against the effects oflightning. This apparatus is sometimes made in the shape of abouse, as represented fig. 10, where, for the sake of distinctness,(he side and part of the roof next the eye are not represented.The gable end A C represents that of the thunder house, and maybe used in the same manner with that above described, or morejeadily by the following method. Let one ball of the dischargingrod touch the ball of the charged jar, and the other the knob A ofthe conductor AC of the thunder house: the jar will then ofcourse explode, and the time] will act upon the conductor justmentioned. The conducting wire at the windows h h must beplaced in a line. The sides and gable, AC, of the house, areconnected with the bottom by hinges; and the building is kepttogether bv a ridge on the roof. '1 o use this modi 1, fill the smalltube a v.itii gunpowder, and ram the wire c a little way into the

1 tube; then connect the tube c with the bottom of a large jar, ori battery. When the jar is charged, from a communication troinj the hook at C, on the outside, to the top of the jar, by the cl is-,charging rod; the discharge will fire the powder, and the explo-sion of die latter will throw off the roof, with the sides, back, andfront, so that they will all fall clown together. Fig. 11 representsa small ramrod for the tubecc, and fig. 12 a pricker for the touch-hole at C. Fig. 13 represents a mahogany pyramid, by meansof which the same experiment may be exhibited. It is used in amanner similar to that just now described, the piece at a beingthrown out by the discharge; by which iheans the upper part fallsdown in three pieces. Mr. Jones of Ilolborn makes the front ofthe common thunder houses, as well as the powder house abovedescribed, with two pieces of wood or windows h It, which, bybeing placed in proper situations, the one to conduct and the otherto resist the fluid, will illustrate by one discharge the usefulness ofgood conductors for securing buildings or magazines from the ex-plosion of thunder, as well as the clanger of using imperfect ones.

To shew that the Electric Fi.uid prefers a Short Passagethrough the air, to a Long one through good conductors, Dr.Priestley invented the following experiment: Bend a wire aboutlive feet long in the form represented in fig. 14, Plate LX IV. sothat the parts A and B may approach within half an inch of eac hother; and connect its extremities with the hook of the battery,and the discharging rod. Upon sending the explosion of a bat-tery through it, a spark will be seen between A and B, which. proves that the electric fluid, prefers the short passage through theair to the long one through or along the wire. The whole charge,however, does not run from A to B, but part of it runs throughthe wire, which is proved by placing a slender wire between Aand B; for with only this addition to the apparatus, the small w irewill be scarcely made red-hot; whereas if the large bent wire be:cut at C, so as to interrupt the circuit ACB, the small wire willbe melted, and even exploded, by the same c harge which, beforethe wire was cut, hardly made it red hot. By this experimentDr. Priestley says, the different degrees of conducting power indifferent metals may be tried, by using metallic circuits of equallength and thickness, and observing the difference of the passagein each through the air. Dr. Van Martini, by the grand electricalmachine and battery in Teyler’s Musa-uin at Haarlem , tried tocommunicate magnetism to needles made out of watch springs ofthree, and even six, inches in length ; as well as to steel bars nineindies long, from a quarter to half an inch in breadth, and about tin:twelfth part of an inch in thickness. The result was, that when thebar or needle was placed horizontally in the: magnetic meridian,which ever way the shock entered, the end of the bar that stoodtoward the north acquired the north polarity, and the opposite endacquired the south. If the bar, before it lceeiveel the shock, hadsome polarity, and was placed with its poles contrary to the usualdirection, then its natural polarity was always diminished, andoften reversed ; so that the extremity of it, which in receiving theshock looked towards the north, became the north pole, Ac.A hen the bar or needle was struck standing perpendicularly, its low-,est end became the north pole, in any case, even when the bar hadseme magnetism before, and was placed with the south pole down-wards. All other circumstances being alike, tbe bars seemed toacquire an equal degree of magnetic power, whether they werestruck while standing horizontally in the magnetic meridian, orperpendicular to the horizon. When a bar or needle was placedin the magnetic equator, whichever way the shoc k entered, itnever gave: it any magnetism ; but if the shock was given throughits width, then the needle acquired a considerable degree of mag-netism, and the end of it which lay towards the west became thenorth pole, and the other end the south pole. If a needle or bar,already magnetic, or areal magnet, was struck in any direction,its power was always diminished. For this experiment they triedconsiderable large bars; one being 7.08 inches long, 0.20 broad,and 0.05 thick. When (he shock was so strong, in proportion toI the size of the needle, as to render it hot, then the needle gene-I rally acquired no magnetism at all, or very little. To determine| whether electricity and the act upon melals in the same manner,i Dr. Van Marum caused wire's of different metals to be: drawnj through the same hole, of one-thirty-eighth part of an inch in dia-| meter, and observed how many inches of each could be melted byI the explosion of his battery ; taking care, in all these experiments,

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