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The general action of a wedgo is well known. It torces apart surfaces between which it is drivcn. If, on the other hand, it has smooth sidos, tho reciprocal actiou may take place. When placed betwen two opposing faces that are preasod together by a constant force, and left to itself, it may, by their action, be violently cxpelled. The school-boys's trick of shooting an orange seed frora between liia finger and thumb by compressing it violently, so that it nies out, is a good, though not very refiaod, illustration of thia principie. The slippery sides of tho seed help tlio action. Holmes also alludes to it in describing the toughness ol tho "BCttler's elm:" "The wertfros flew Trom betwoen lts lips, Ttaelr blunt ends i'rizzled liko colery tips." The reciprocal has como tobe well recognized ae ita original. If, honever, a wed ge is placed between two surfaces that constantly tend to approach each other, and the conditions ar so arrangod that when the wedgo moves apparently the wrong way the surfaces Wil coma closer together, then the wedge will act ia Uiia parad oxical raanner. The conditione may be obtained by the arrangement shown in the cut. Twopieces oí wood are hinged togetlier, so as to opon nnd shut like a book. A eliip oí leathei answers as well as a hinge. These are t; represent the surfaces that tend to ap proach each other. A wedge of abont the proportions of the one in the drawing must be provided, and alao two colindara or rollers. Cotton spools, or even two peneils, may be used as rollers. The apparatus is now put together as shown ia the illustration. The wedge is introduced betwecn the hinged boardi, a rolter being placed abové and below it. lt is placed so far withintheir opening thot theanglc tbat tht boards maksvith eaih otherisgreatcr, orof more degrees, than is the angle of the wedge. Tho thin end of the wedge, supported in on hand, projects outward trom the opened boards, and pressure is applid to tho upperraost one. iDaccordancewith general rules, this, it would secm, sbould draw tho wedge in ward. But, on the contrary, it will be found that it drives it outwftrd, in opposition to the normal movement. The lattor is forced outward, so that its thicker parta are brought lictween the rollers. The ordinary action of the wedge is coraplicated by theinclination ol the boards to each otlier, and by the rollers. Iftheangla included between the boardB is greater than that betwoen the eides ol the wedge, then the boards will approach moro by the rollers movlng outward, than the wedeein correepouding moveinent can drive them apart. The consequence is that tho rollers rove OHtward, carrving with them the wedge, and, until the angles become equal, the motion continue, the Mgrebraic suni ol tho wodg and roller action indicating an approach of the opposing faces. Whenthe aaglos bocome cqual, tho conditions of reposo are reached. Thisaction of the wodge, when seen, appcarg most curious. It in a good Ilustration of the short road totruth thatisoften afforded by experimenta. The other cut shows a simple apparatus for exhibiting one of the laws of projectod bodics. If a body is projected or thrown in a horizontal direction, it will take no longer to rcach the ground than one Lhat is dropped at the same moment. Thua, il tito marbles are pimultaneously relcaBcd over a levcl floor, and if to one of tbem a strong horizontal impulsa is pivcn, so as to tlirow it a considerable distance, both will reach the ground at the same instant, altbough one travels a miich greater distauce than does t lic other, and thev will make but ouc sound as they strike, providcd all the adjugtmenta are correct. A board abont ten inches long and fivo incues wide is mouoted on two pioces, one at each end. On its forward end, at one side, a Jlttlo platform or trap door, two inches square, is hinged. Aa India rubber band extends diagonatlv from the outer corner ol this trap door downward to the main piecc. Thus tho door is drawn stronftly downward. With a center bit a hole is made partly through the platform. The end of the board at ita other side is free. On its upper Burface, running Imck from this plate, two strips are aeenred, making a groove about au Inch wide. Withln this a block of wood, two inche long, südes, fitting closcly, but movintt with little friction. A band of India rubber is fastened to and extends forward (rom its front end, and is secured to tho board near its front. These two parts ars intended to receiye two marbles. One reste on tho trap door. The holugivcs it a good resting place. Tho other rui in tho groovo in front of th movablo block, and can bo diseharged by it a distance of ten fcet or more. Aim catch is arranged so as to hold up tho trap door, but is rèleawd whra a hall s tlirown boni tho groove. lts arrangement is clear from tho eut. Tho eatch is set so as to hold up tlictrap door, and a marlilois placed llipreon. Another marble s placed in tho Rroove, the block bcing ])reviously dniwn back. Tho niarblo renta in front of and in contact with the sliding block. It iu wcll to have a trigger arrangement for holilin tlio block in placo, and for releasing it when all i reudv. Such can orhíI v' l1 tlivisoil by any one. The apparatos being Brmly securod und pctféctlj hori.uutul, thu block is leaHod. It shoot.-. tho rnnrhle in front ol it I acroBg tlio room. As thumarblc lcavcs tho prroovc it. ttnicliPSÜM eatrh and releases tho trap door. The elastic hand jork thia down, so that the other l:ill fulla vortically and perfeotlv froely from tho platform. It no elastic wore used to pull t he trap down, theballwould be Mtaifed in ite pulling. lf tlic adjiLstmoute ara correct, both bails will striki' the Hoor togother, making onljr ono report. Tlriw is more tban an Ulustration of a law of projection. It also Ilustra tes tho fact that different (ones art on a body irrespective of each othor. Tluia the action of pravity is exertod to an equal exteut on both ballH, tliongh one is perfectlv free and the ot hor isactod onby ast rong horizontal force. It vmy b UBed :ie an examploofa broad principie of nature which h enunciated in one of Newton's laws ot force. -


Ann Arbor Courier
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