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Show METE 0 R S. NoTE I. 4 . ihock taken through inflammable air It was obferved by J?r: ~riefl:l~y t~~~t~~e~~e~;~~mfiances perhaps fome o~ the colours was red, in common air lt IS bluelfh' . thou h the denfity of the medmm through of the northern lights may bear analog~ ' lou; as is well explained by Mr. Morgan. which light is feen mufi principally ~ary lt_s co is red when feen through a dark clou~, or Phil. Tranf. Vol. LXXV. Hence ltght~~~~le ra s cannot permeate fo denfe a me~mm. near the horizon; becaufe the ~or\~efr~- ~t as t~ey are generally fecn on clear mghts. But the ihooting fl:ars confifl: 0 w_ lte I~ .' r ht is probably too faint to come to us. and nearly vertical: in other fituauons t /~~ ~~orthern lights, as in March, 1716, all But as in fome remarkable appearances o r e d ch other thefe appear to have been fi · kly to 1uccee ea • {i d the prifmatic colours were een qmc f h . te pofed medium could not be fuppo e owing to real combufl:ion; as the denfity o h t{i e mlo:rs mufl: have been owing to different to change fo frequently ; and therefore t ~ ehco ry of combufl:ion. In Smith's Optics,. degrees of heat accor d"m g t o Mr. Mo. r-gadn s t etioo ns of the northern lights are d e! ie n" b e d P· 69. the prifmatic colours, and opttcal ecep by Mr. Cotes. . . Cl:1 fee from air, is faid by Mr. Morgan and The Torricelhan vacuum, If perfe TYh_r . nance therefore would preclude the !" .n. duCl:or IS ClrCUffill t others to be a penecL non-con . fi h But as Mr. Morgan did not try o electric fl:reams from rifing above the atmo Pd ere •. . or fomething containing air, fur-k h h a vacuum an as alr, h Pafs an eleCl:ric ihoc t roug ' rr to the production of light, t e con-fi f l a icity may be neceuary 1 . h rounding the tran It o e e r . If h the fl:rearns of the northern tg ts were clu!ion may perhaps fiill be dubiOus. th:w::e;ld only be vif\ble at each extremity of fuppofcd to rife above our atmofphere, y . immerged into the atmofphere i but r l they emerge from, or are agam . their coune ; w 1ere fi the abfence of eleC\:ric light In a vacuum not in the.i r J. Ourney through the vacuum ; . or . h d k . the t f fhaking a barometer m t e ar , d by the common ex penmen o . 1 . "f is fufficiently prove . . I . n the glafs at its top, IS ummous l eleCtricity, produced by the fnCl:IOn oft le me~curyl~ l t ·r the vacuum be complP.te. · 1 · · · t • but there IS no 1g 1 1 the barometer has a htt e air m J , • y ingenioufly accounted for by Dr. b ]' or northern dawn, IS ver The aurora orca Js, . . H . {i s the following electric phenomena: . . l of deetncJty e premr e 'rh Franklin on prmctp es . fi . I etricity Handing on its furface. 2 • ..._ at r ll 1- o v has much po mve e c l. 1. that all new- Ia en n I I ed with a crufl: of eterna ICe,. f I t' tude round the po es are cover {i about twr lve dc~rees o a 1 a.· fl "d That the denfe part of the atm.ofphere ri es which is impervious to the ele .~ JC h u~- . 3· ts of it the electric fluid wi!l pafs to almo!l:· but a few miles high; and that Ill t e larer par any difl:ance. 1 t"on of pofltiveeleetric matteron the {i h fl be a great accumu a 1 Hence he fupp~ est ere mu . . h' ch not being able to pafs through the crufl of frefh-fallen fnow m the p_ola: regtons, w .~ f the u er pa.rts of our atmofphere, which. icc into the earth, mufi ufe wto the rarem·alr 0 d ppthe equator defcend again into the fl: fi fl: · 1 afiage . and pa mg to war s will the lea re 1 s p '. I . fil t i1 earns. And that many of the <lcnfer atmvf-phere, and thence mto the eart 1 Ill I en r . . of the-attending thefc lights are optical deceptions, OWlng to the fituatJOn :~~~~:~n~;l:olds them ; which makes all afcending parallel lines appear to converge to a point. NoTE 1'. METE 0 R S. 5 The idea, above explained in note on 1. l23, of the exifience of a fpher~ of inflammable gas over the aerial atmofphere would much favour this theory of Dr. Franklin; becaufe in that cafe the denfe aerial atmofphere would rife a much l~fs height in the polar regions, diminilhing almofl: to nothing at the pole itfelf ; and thus give an eafier paffagc to the afcent of the eleCl:ric fluid. And from the great difference in the fpecific gravity of the two airs, and the velocity of the earth's rotation, there mufl: be a place between the poles and the equator, where the fuperior atmofphere of inflammable gas would terminate ; which would account for thefe fl:reams of the aurora borealis not appearing near the equator; add to this that it is probable the electric fluid may be heavier than the magnetic one ; and will thence by the rotation of the earth's furface afcend over the magnetic one by its centrifugal force; and may thus be induced to rife thr~ugh the thin firatum of aerial atmofphere over the poles. See note on Canto II. 1. 193. 1 fhall have occafion again to mention this great accumulation of inflammable air over the poles ; and to conjeCture that thefe northern lights may be produced by the union of inflammable with common air, without the affifl:ance of the electric fpark to throw them into combul1ion •. The antiquity of the appearance of 110rthern lights has been doubted; as none were recorded in our annals fince the remarkable one on Nov. 14, 1574, till another remarkable one on March 6, 1716, and the three following nights, which was feen at the fame time in Ireland, Ruffia, and Poland, extending near 30 degrees of longitude and from about the 5oth degree of latitude over almofl: all the north of Europe. There is however reafon to believe them of remote antiquity though inaccurately defcribed; thus the following curious paffage from the Book of Maccabees, (B. II. c. v.) is fuch a defcription of them, as might probably be given by- an ignorant and alarmed people. " Through all the city, for the fpace of almofi forty days, there were feen horfemen runniHg in the air, in cloth of gold, and armed with lances, like a band of foldiers; and troops of horfemen in array encountering and running one againfl: another, with fhaking of ll1ields and multitude of pikes, and drawing of fwords, and cafl:ing of darts, and. glittering of golden ornaments and harnc[s." • |