OCR Text |
Show 516 Of E/eélrz'c Fluid moving in vacuo. mination of the heavens, " though feen in places Iooo Icagues " afunder, this might as well be owing to its being very Cxtenfivc, " as very high ;"--Which are nearly words ufed by Dr. Franklin to Mr. Bartram, upon the fubjeét of an aurora fuppofed to have been vilible both in Philadelphia and London. (See Cotes's Lee- tures, Ulloa's Voyage, The accounts given by Meflieurs Smeaton and Nairne of their air-pumps, Fergul‘on's Atlronomy, article 175, Muffchenbroek's Introd. ad Phil. Natur. art. 2502, and Philoft Tranf. for 1752, p. 4.74.) E.] - ['* § 23. " The vacuum above is a good conduflor."--By a va~ cuum here, we are not perhaps peremptorily to underfland the abfence of air; but a fituation only where the air is thought intirc fufli- ciently rare to fufl‘er the fluid to pafs ealily, rapidly, and conpr cuoufly along. Under this head, the following {hort remarks may poflibly ufe.-When a given fubflance conducts the fluid away be of from a charged body, the attraction afforded by the conductor is aflifled by the repulfion of the fluid; the fluid running through the conduflo from one particle to another,till an equilibrium is obtained-Wher, n this 0 eration takes place in Air, the chief difiiculty confifls, not in ma ing the fluid fenflble of the attraction of the condué‘to r, but in getting the better of the oppofition given to its puffage by the air.--But in Vacuum: where the latter difliculty is almofi wholly re- moved, then an improper difpofition of the attracti ng fubflances {oon begins to be felt; f0 that for inflame, it will be much eafler for the fluid to pafs a given dillance if thrown into a dozen {mall intervals with conductt rs between, than to pafs the fame firm of diflance thrown into me large interval with the conduct ing fub-» fianccs all at the hither end : Jull as it is eafier for a man (who has only a certain aflivity) to leap twelve ditches of ten feet each fucceflively, than to leap one large ditch of 120 feet at once-It is difficult however to determine by common experime nts to what diltance thefe intervals in given cafes in vacuo, are to be limited. When Meflieurs Walfh. and De Luc for inflance found that in an arr/5211 double-barometer, in which the quickfilver had been carefully boiled, in order to purge it from air, no lhock or fpark could be tranfmitted; it might have been obferved that attrac‘tion aeling in flraight lines, or at leall in uniform directions, the curve of the barometer would prevent the operation, there; for how could the. quickfilver in the leg B ‘ move the fluid (fuppofe d low down) On the furface of the quickfilver in the leg A? It could not draw it through the fides of the tubes; nor could it aft upon it round by, 9 See the Plate, 1:. 522.. way Oftbe Seafon 9f t/Je Tmrfir the Aurora. 517' way of the curve; for then it mull attract in oppoftte directions, firf'i up the leg A, and then down the leg B: lts powers of attraction therefore being thrown away, the fluid remains at refl with refpeft to them. But when a few conduc‘tin g particles become in time introduced * into the tube, the fluid is attrac ted round from particle to particle, till it turns the corner and falls in the way of the other leg-'5 atlraélion.-So alfo, when, in aflraz 'gbt barometer (exhaulled with the fame care) an attempt to charge the top of it by a. coating on the outlide, is faid to fail from the too~pe rfe€t vacuum giving no afliftance within ; may it not be fuppofctl owing to the inctiualit" of the contention between attracting particles that are fmall enough to routinurflz/pczzdnl in {o rare a medium, when oppofed to the 5:. traction of the maiTy glafs? 1n the brawn: however, where there is neithe r curved tube, nor perhaps much difpropcrtion in the forces of the attracling bodies, tha- mution of the fluid may be expeéicd to take place at greater intervals : And it may be thought perhaps that the mere "pa/flan of the fluiJ, without much help from the few attrac ting bodies that are to be found there, is fullieient for a difpe rfion to enfue; efpeci - ally as the fluid may be faid originally perhaps rather to have been expelled, than to have been tonduc'lea' thither. E.] [ 1- § 26. " Would not the auroras become more frequent after " 1/): apprmrls aft-winter ~9"»Muffchenbroek reckons up 750 appearances of the aurora, which he had obfcrved in 29 years; and it feems, from his table, (taking it without any comments as itllands) that they are mofl: frequent at the clofe of winter; They are the next frequent at the clofe of fummer; fewer in winter; and fewefi at the winter and fummer {olllicesz 7', of the whole number averaged, having appeared in the May months, and only :‘5 in the June and the December months each. But thefe meteors being made firmly to depend on a degree ofinfulation in the earth, .&c. on the one hand, and of moillure in the air on the other; it does not, {eem enough perhaps to look only to the perlec‘lion of the ice, 54c. and its after-latuiation, with other circumllances, in the north ; but we mull ztlfo attend to the moillure to be ruifetl and imported from the fouth; Which moiflure coming fometimes perhaps along. the higher regions of the atmofpheie from more diltunt parts, and fometimes along the furfacc of the earth from more contiguous parts, to the places Vthere the infulation takes place, the leulons of appearance may hence be affected-Hie auroras Iwould oblerve, ap- pear moft at the periods when the moilture or. cold refpeétively may "‘ [In our common experiments, Mr. Nairne has proved that much vapour is: left or generated in what is called avacuum; and indeed the fafl is perhaps to be. explained upon theory. 13.] he: |