Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/104

Rh that certain kinds of cirro-stratus are also regarded by meteorologists as sure indications of rain and wind. Dr Richardson frequently observed the lower surface of nebulous masses illuminated by polar lights, a fact illustrative of the comparatively low situation of these auroras. Biot, also, in the island of Unst, observed many auroras that could not be higher than the region of clouds. Sir John Franklin in like manner observed low auroras. &quot;The important fact,&quot; says he, &quot;of the existence of the aurora at a less elevation than that of dense clouds was evinced on two or three occasions this night (13th February 1821, at Fort Enterprise), and particularly at 11 hours 50 min., when a brilliant mass of light, variegated with the prismatic colours, passed between a uniform steady dense cloud and the earth, and in its progress completely concealed that portion of the cloud which the stream of light covered, until the coruscation had passed over it, when the cloud appeared as before.&quot; Captain Parry, as stated in his third voyage, observed aurorae near to the earth s surface. It is said that while Lieutenants Scherer and Ross and Captain Parry were admiring the extreme beauty of a polar light, they all simultaneously uttered an exclamation of surprise at seeing a bright ray of the aurora shoot suddenly down ward from the general mass of light, and between them and the land, which was only 3000 yards distant. The ray or beam of the polar light thus passed within a distance of 3000 yards, or less than 2 miles, of them. Further, Mr. Farquharson observed in Aberdeenshire an aurora borealis not more than 4000 feet above the level of the sea. Fitzroy believed that aurora in northern latitudes indicates and accompanies stormy weather at a distance, and that straining and cracking of the ice may cause the hissing and whizzing sounds.

M. Silbermann (Comptes Rendus, Ixviii. p. 1051) notes facts which strongly confirm the connection of aurora with some form of cirrus cloud He says (of the aurora of 15th April 1869), &quot;At 11 hours 16 min. the phenomenon disappeared in a singular fashion. It appeared as if the columns of the aurora were still visible, but the stars were hidden, and it soon became obvious that fan-like cirrus clouds, with their point of divergence in the north, had taken the place of the aurora. Between 1 and 2 in the morning these clouds had passed the zenith, and let fall a very fine rain. On stretching out the back of the hand one felt a pricking of cold, and now and then there were minute scintillations in the nearest strata of air, like a hail of tiny crystals of ice, which afterwards turned to a rain of larger and larger drops. At 4 o'clock in the morning the cirrus of the false aurora was still visible, but deformed towards the top, and presenting a flaky aspect. One interesting point is, that the cirrus never appeared to replace the aurora either from the right or the left, but to substitute itself for it, like the slow changes of a dioramic view.&quot; &quot;I had previously observed a fall of small ice crystals on the 30th April 1865. At 6 Paris seemed enveloped in a cirrus of vertical fibres, recall ing those of amianthus, and more or less wavy. It was a rain of little sparkling prisms. At the same time I heard a rustling or crepitation, and on extending my hand I felt a pricking sensation of cold, and distinguished the crystals which fell and melted immediately.&quot;

In a later memoir (Ibid., p. 1120) he remarks that many storm-clouds throw out tufts of cirri from their tops, which extend over a great portion of the sky, and resolve themselves into a very fine and cold drizzle, which frequently degenerates into a warmer and more abundant rain. Usually the fibres are more or less sinuous, but in much rarer cases they become perfectly rectilinear, and surround the cloud like a glory, and occasionally shine with a sort of phosphorescence, As an illustration he quotes his obser vations on the night of the 6th September 1865:- &quot;A stormy cloud was observed about 11 in the N.N.W., and lightning was distinctly visible in the dark cumulous mass. Around this mass extended glories of a phosphorescent whiteness, which melted away into the darkness of the starry sky. Eound the cloud was a single and unin terrupted corona, and outside this, two fainter coronas broken by rifts which corresponded with each other. After the cloud had sunk below the horizon the glories were still visible. The light could not have been due to the moon or any foreign cause. The rays showed great mobility, and a sort of vibration intermediate between that of the aurora and the 'brush discharge' of the electric machine.&quot; He goes on to say that&mdash;

A still more curious fact is mentioned by Sabine, who, during his magnetic survey, anchored some days at Loch Scavaig in Skye. This loch is surrounded by high and bare mountains, one of which was nearly always enveloped in a cloud, resulting from the vapours which almost constant west winds brought from the Atlantic. This cloud at nights was permanently self-luminous, and Sabine frequently saw rays similar to those of the aurora. He entirely repudiates the idea that the rays could be due to auroras beyond the mountain, and is sure that these phenomena, whatever their nature, were produced in the cloud itself.

Silbermann asserts that auroræ are preceded by the same general phenomena as thunderstorms, and concludes that everything had happened as if the auroras of 1859 and 1869 had been storm-clouds, which, instead of bursting in thunder, had been drawn into the upper parts of the atmosphere, and their vapour being crystallised in tiny prisms by the intense cold, the electricity had become luminous in flowing over these icy particles. This view is very strongly supported by the observation of Professor Piazzi Smyth that the monthly frequency of aurora varies inversely with that of thunderstorms. The following are his numbers of relative frequency, the means of all observa tions of the Scottish Meteorological Society prior to 1871:&mdash;

It must, however, be remembered that the observed frequency of auroræ is much affected in Scotland by the continuous twilight during the summer months. If there be this connection between thunder-clouds and auroras, it is not improbable that the &quot; dark segment&quot; is sometimes a real cloud or mist, situated at a height where the density of the air is too great for luminous discharge; and in several cases Silbermann has seen auroral rays rise from small clouds, which gradually melted entirely away, or left a small non-luminous nucleus when their electricity was discharged.

If, as would certainly be the case in a mist, any portion 