A U R 



except in the night, although its brightnefs is fuch as mud 

 fometimts make it vifiblc to us did it rtally exift in the day- 

 time. 



" In anrwer to this it muft be obferved, that though the 

 paflage of cleftricity through a good conduclor is inftanta- 

 neou?, yet through a bad conductor it is obferved to take 

 fome time in paffing. As our atmofphere therefore, unkfs 

 very violently heated, is but a bad conduftor of cleftrieity ; 

 though the equilibrium in it is broken, it can by no means 

 be inllantaneoufly rellored. Add to this, that as it is the 

 aftion of the fun which breaks the equilibrium, fo the 

 fame aftion, extending over half the globe, prevents almoft 

 any attempt to reftore it till night, when flalhes arife from 

 various parts of the atmofphere, gradually extending 

 themfelves with a variety of undulations towards the 

 equator." 



It has been obferved, that the ftreams of the aurora bo- 

 realis do not always move with rapidity ; but they fometimes 

 appear for a confulerable time quite Rationary, and they are 

 fometimes carried in differait direftions with a ilow motion. 

 In order to account for thefe circumflances, it fhould be 

 confidered, that weak eleftric lights have been fometimes 

 obferved to exhibit the fame appearance at the furface of the 

 earth, and we may therefore fuppofc them much more capa- 

 ble of doing fo at great heights above it, where the con- 

 ductors are fewer in number, and much more imperfeft. 

 From inftances that might be adduced, we may reafonably 

 conclude, that fmall portions of our atmofphere may by 

 various caufes he fo much eleftrified as to fhine, and likc- 

 vife be moved from one place to another, without parting 

 with the eleftricity they have received for a confiderable 

 time. In this manner we may account for the corona, or 

 circle, which is often formed near the zenith by the aurora 

 borealis, when any of its parallel dreams of light that (hoot 

 upwards, and by the laws of perfpeftive, appear to con- 

 verge towards a point, are over our heads, and actually come 

 to a point. As this corona is commonly ftationary for fome 

 time, it would ferve as a mark by which to determine the 

 diftance of the objeft ; e. g. let two perfons, one at Lon- 

 don, and the other at Edinburgh, oblerve an aurora bore- 

 alis ; then by noting the ftars among which the corona was 

 obferved at each place, its true altitude from the furface of 

 the earth may eafily be determined by trigonometry. Al- 

 though the true height of the aurora borealis has never yet 

 been determined, there is no fufficient reafon for fuppofuig 

 that it is higher than a meteor, feventy miles above the fur- 

 face of the earth, which m.eteor, both by its fplcndor and 

 figure, feems to prove that the air pofTeffes a confiderable 

 degree of denfity at that diftance from the earth. Beiides, 

 if its ftreams refemble the paffage of eleftric light through 

 a vacuum, it cannot be hence inferred, that the air is in a 

 ftate fimilar to the vacuum of an air pump in thofe places 

 where the aurora boreahs is produced ; becaufe we have in- 

 ilances of fimilar appearances that are produced in very 

 denfe air. " The plate of an eleftrophorus is often fo 

 highly eleftrified, as to throw out fiafties from different parts 

 as foon as it is lifted up, and by proper management, it 

 may be always made to emit long and broad flaflies, which 

 Ihall fcarcely be felt by the finger ; inftead of fmall, denfe, 

 and pungent fparks ; fo that, though long flnfhes may be 

 produced in rarefied air, it by no m.eaiis follows, that the 

 fame may not alfo be produced in denfer air. As little can we 

 infer any thing from the colours, for we obferve the eleftric 

 fpark fometimes white, fometimes blue, and fometimes pur- 

 ple, in the very fame ftate of the atmofphere, and from 

 the fame fubftance." Mr.Littlc, the inventor of an air-pump 

 of a new conftniftion, ftating the boundaries of the atmo- 



A U R 



fphere within which the aurora borealis, confidered as art 

 elcftrical phenomenon, is vifible, conceives that it cannot 

 appear in air lefs rarefied than near 4000 times, and confe- 

 quently that its neareft diftance from the earth is about 45 

 miles, according to Dr. Halley's table of the air's refraftion 

 at different altitudes ; and that in air rarefied more than 

 26000 times, it would not be vifible, and therefore its 

 greateft diftance is about fifty miles, by the fame table. 

 He is alfo of opinion, that it is air burnt and exploded 

 in its paflage, wliich makes the eleftric matter vifible, and 

 that without air, if it could pafs at all, it would not be lu- 

 minous. Upon the whole he concludes, that the aurora 

 borealis is confined within our atmofphere. Irifta Trauf. 

 vol. vi. p. 387. 



Dr. Franklin fuppofes, that the elcftrical fire difcharged 

 into the polar regions from many leagues of vaporifed air 

 raifed from the ocean between the tropics, accounts for the 

 aurora borealis ; and that it appears firft, where it is firlt 

 in motion, i. e. in the moft northern part, and the appear- 

 ance proceeds fouthward, though the fire really moves north- 

 ward. Franklin's Exper. and Obf. 1769, p. 49. Phil. 

 Tranf. vol. xlviii. part i. p. 358, part ii. p. 784. lb. vol. li. 

 parti, p. 403. lb. vol.lxxii. p. 15. Lettere dell'EIlcttri- 

 cifmo, p. 269.; or Prieftley's Hift. of Eleftricity, vol. i. 



Mr. Kirvvan ( Irifti.Tranf. 1 788, p. 70, &c.) fuppofes, 

 that the rarefaftion of the atmofphere in the polar regions 

 proceeds from the aurora: boreahs and auilralis, and that 

 thefe are produced by a combuftion of inflammable air, 

 caufed by eleftricity. This inflammable air is generated, 

 particularly between the tropic?, by many natural opera- 

 tions, fuch as the putrefaftion of animal and vegetable fub- 

 ftances, volcanoes, &c.; and being lighter than any other, 

 occupies of courfe the h'gheft regions of the atmofpheie. 

 Confequently, this kind of air is chiefly thrown off towards 

 the poles, and occafions the northern lights, which are the 

 higheft of all meteors, though they fometimes extend pretty 

 low into t!ie atmofphere. Mr. Kirwan further adds, that 

 after the appearance of an aurora borealis, the barometer 

 commonly falls, and tliat it is generally followed by high 

 winds, proceeding ufiially from the fouth; all which fafts 

 ftrongly prove a rarefaftion in the northern regions. To 

 the fame purpofe, it is obferved by Mr. Winn (Phil. 

 Tranf. vol. 73.), that the appearance of an aurora borealis 

 is a certain fign of an hard gale of wind from the fouth or 

 fouth-weft. Tiiis occurred, without fail, in twenty-three 

 inftances; and he thinks that the fplcndor of meteors will 

 enable the obferver to form fome judgment concerning the 

 erfuing tenipeft. If the aurora is bright, the gale will 

 come on within twenty-four hours, but will be of no long 

 duration; if the Hght is faint and dull, the gale will be lels 

 violent, and longer in coming on, but will laft longer. 

 His obfervations were made in the Enghfli Channel, where 

 fuch winds are very dangerous; and by attending to the 

 auron-E, he fays, that he often efcaped, when others were 

 nearly wrecked. Obfervations of this kind would ferve to 

 leflcn the dangers of navigation. 



" That the aurora borealis ought to be fucceeded by 

 winds, may be eafily deduced from the hypotlicfis above- 

 mentioned. If this phenomenon is occafioncd by the vail 

 quantity of eleftric matter conveyed to the equatorial parts 

 of the earth, it is certain that the earth cannot receive any 

 great quantity of this matter at one place without emitting 

 it at another. The eleftricity, tiierefore, which is con- 

 ftantly received at the equator, muft be emitted nearer the 

 poles, in order to perform its courfe; otherwife there would 

 not be a conftant fupply of it for the common operations of 

 nature. It is to be obferved, that eleftrified bodies are 



always 



