AURORA BORE A1IS, ETC.] 



METEOROLOGY. 



1185 



above the surface of the earth. In the early part of 

 August, 18G3, the usual annual meteoric display was ex- 

 ceedingly great, attaining the maximum on or about the 

 llth of the month, when as many as 200 "shooting 

 stars" were noticed in about three hours. In the con- 

 stellation of Cassiopeia, we perceived, at 9 p. M., an 

 incessant discharge south-westward, lasting for above an 

 hour, none of the balls having a great size, and the dis- 

 tance travelled by each being from three to five degrees : 

 the elevation above the surface of the earth seemed to be 

 great, judging from the comparatively small velocity 

 and apparent magnitude of each meteor. Most observers 

 remarked, that the paths of the meteors, about that date, 

 chiefly lay from north-east to south-west. We ob- 

 served but few instances in which any tail was shown, 

 although the evenings were generally favourable for 

 observation. 



The extremely rapid course of these bodies renders it 

 very difficult either to calculate their elevation, or to use 

 means of testing the nature of the light they emit, as 

 can be done with the suu, <fec., by spectrum analysis.* 

 Two observers at distant stations, may, by chance, simul- 

 taneously observe one ; but such an occurrence seldom 

 presents itself ; hence the varying estimates which are 

 made respecting their height. There can be little doubt, 

 from their periodicity, that they are solid bodies, and 

 not of electric origin, as was formerly supposed on, per- 

 haps, the slightest possible grounds. The different 

 theories which have been suggested, to account for these 

 meteoric phenomena, have been already alluded to in an 

 earlier portion of this section ; and we regret that, at pre- 

 sent, nothing of a more definite nature has been arrived 

 at.f 



With respect to the Aurora Borealis, which has also 

 been spoke. i of, there can scarcely be a doubt as to its 

 origin. Its great altitude, fleeting nature, and general 

 appearance, all bespeak an electric origin. Few persons 

 residing southward of Scotland can have any idea of 

 the beauty of this phenomenon, which may bo seen in 

 great perfection, even in our own isles, during the 

 winter season. In the western and northern districts of 

 that country, we have often observed the entire heavens 

 illuminated ; the north, north-western, and north-easterly 

 parts shining, of course, with the greatest brilliancy. Oc- 

 casionally the coruscations proceed from those quar- 

 ters extending far beyond the zenith, in a southerly 

 direction, so that the whole sphere overhead is brilliantly 

 lit up. At other times, but more rarely, the zenith 

 itself seems to be the centre of action, and, ray-like, 

 thousands of broad sheets of flame, of many colours, dart 

 on all sides. Generally speaking, when an Aurora is 

 thus developed, the flames assume a lambent or phos- 

 phorescent appearance, are comparatively ill-defined, and 

 rarely more dense in one place than another. At times 

 we have seen a hike, five miles long, and four broad, illu- 

 minated from the northern side by an Aurora, to an 

 extent quite equal to that it usually presented under the 

 full moon, and the shadows from the over-hanging 

 mountains were deeply cast on the water. During several 

 years' observations, often carried on nightly, we noticed 

 that the Aurora was always better seen inland than near 

 the sea ; our places of observation ranging between the 

 east and west coast of Scotland, on a lino extending 

 north of the Clyde and Forth rivers, as far as the coun- 

 ties of Ross and Cromarty. 



In the experiments which we undertook, some years 

 ago, on the spark in vacuo, from the Rhuinkorf coil, and 

 which are related at a previous page,J we frequently no- 

 ticed the peculiar effect resulting from the introduction 

 of a minute quantity of various vapours into the highly 

 exhausted receiver. So long as the remaining atmo- 

 spheric air was pure, the colour of the spark was a rich 

 reddish purple, becoming bluer in proportion as the ex- 

 haustion was increased. A more dense state of the 

 air, effected by its gradual admission, only made the 

 spark of a redder tint. If, however, the vapour of bi- 

 sulphide of carbon were passed in, in exceedingly minute 



Bn ante, p. 168. 4 See ante, p. 1150. 



* See the Section on MagnetUm, p. 256, Eip. 15, tt uy. \ Ante, p. 20. 

 VOL. I. 



quantities, a sudden flash of delicately whitish-yellow 

 light was produced, exactly resembling that of the dif- 

 fused Aurora we have shortly before alluded to. With- 

 out describing a great variety of experiments, we may 

 remark, that every tint of the Aurora may thus be pro- 

 duced by choosing proper vapours (chiefly of ethers), 

 and simultaneously regulating the vacuum, which should 

 vary, in the air-pump, from about 27 to 29 inches, the 

 external air having a barometric height of not less than 

 29-30. 



Now a repetition of such experiments cannot fail to 

 suggest, to any one who has seen the Auroral phenomena 

 we have just described, their great similarity, or, to us, 

 identity of appearance. If electricity be assumed as the 

 cause of Auroras, precisely such circumstances may occur, 

 in the elevated regions of the atmosphere, as we refer to 

 in the above remarks namely, a minute quantity of solid 

 colouring matter, diffused in a highly attenuated atmo- 

 sphere. Rapid chemical combination would constantly 

 occur, attended with the production of flame, between 

 the solid matter and the oxygen of the atmosphere ; and 

 if it be objected that the quantity of oxygen present 

 would be excessively small, we reply, that the results ob- 

 tained in the air-pump receiver, are produced under 

 exactly similar conditions ; for the vacuum very nearly 

 approaches that existing in the regions in which the 

 Aurora is supposed to exist, assuming that to lie within 

 the limits of our atmosphere. 



Whilst offering this as an explanation of certain 

 Auroral appearances, we do so under the conviction that 

 it can be but a suggestion, the value of which is depend- 

 ent on analogy alone. It is by no means certain that the 

 Aurora is at all caused by electric action ; indeed, such is 

 only inferred from the disturbance of the magnetic needle 

 generally attending the displays of Aurora ; and, again, 

 the relation of this fact to electrical science is also one 

 of inference, depending on the knowledge we possess of 

 the mutual production of the electric and magnetic 

 forces, so far as our experimental determinations have 

 carried us. 



THB METEOROLOGICAL INFLUENCE OF DRAINAGE. 

 We have incidentally spoken of the various local causes 

 which influence the meteorological character of some 

 districts, and we shall devote a short space to the con- 

 sideration of that subject ; more especially in reference 

 to the great advances which have been made in the 

 drainage, &c., of some parts of this country, and to the 

 permanent improvement of extensive tracts of land, in 

 a meteorological, agricultural, and sanitary point of 

 view. 



It is familiar, in the experience of every one, that a 

 low, flat, and marshy tract of land is generally a region 

 of fog, and often of rain. In England, for example, 

 the land between the Thames and Boston Wash a length 

 of about 100 miles, and of variable width is chiefly of 

 this kind. The superincumbent air is loaded with 

 moisture, and, as far as our experience has gone, there 

 is a greater frequency of thunder-storms, and, for the 

 moment, heavier showers of rain, than in the districts 

 flanking it. The same remarks are equally applicable to 

 extensive tracts in France, Holland, Russia, India ; 

 Africa, as at Sierra Leone ; Florida, Georgia, and various 

 places in North and South America ; with many others 

 too numerous to mention here. The meteorological 

 characteristics we have just referred to, of course vary, 

 according to the annual average temperature ; being in 

 excess ic low latitudes and about the equator, and 

 gradually diminishing as we approach the pole, where the 

 moisture of the air arrives at its minimum. If these local 

 peculiarities were due solely to the presence of water, 

 then the ocean should be equally liable to them ; but that 

 such is not the case is well known ; and the cause of the 

 difference is easily explained. 



In the Section on Heat, we called attention to the 

 different radiating powers which bodies possess ; pointing 

 out that those of a rough nature radiate quickly, whilst 

 polished surfaces are poor radiators. Now just such a 

 difference subsists between the surface of the ocean, or 

 any extensive sheet of water, and that of land saturated 



7 M 



