ll.V) 



MKTEOROLOGY. 



[HALOS ACRORJL 



of glass rendered dull by smoke, these halo* arc fre- 

 i|Uuiitly rendered vuiblo (Fig. 59). 



FJ..W. 



Although halos. and also the phenomena next to be 

 described, are referable to the action of atmospheric 

 moisture on luminous rays, yet it is evident that the 

 condition of that moisture will vary according to tem- 

 perature ; in other words, the aerial moisture which 

 would be mere cloud- vesicles at temperatures above the 

 freezing joint, would, if depressed below 32 F., be con- 

 verted into enow, or apiculie of ice. The alteration 

 which tliese are capable of effecting on luminous rays 

 ln-ini: far greater than mere uncongcaled vesicular water 

 can effect, the resulting optical phenomena are far more 

 brilliant and remarkable. Hence, in northern latitudes, 

 the phenomena of halo* and parhelia as arcs of liiilit 

 appearing near the sun, and sometimes intersecting each 

 other, are called are brilliant and impressive beyond 

 anything which corresponding phenomena occurring in 

 this region would lead us to conceive. These luminous 

 arcs, sometimes intersecting each other, are as often 

 occasioned by the moon as by the sun. As the phe- 

 nomena in question, when referable to the latter cause, 

 are demonstrated parhelia, so, when dependent on the 

 former cause, they are termed paraselena. 



Kri-qiiently parhelia and paraselenes consist, not only of 

 the intersecting arcs just mentioned, but of circular 

 luminous meteors, to which the term.fnocfc suns are espe- 

 cially applicable. Associated with parhelia, and some- 

 time* included under the same name, is a luminous band, 

 passing horizontally through the sun, and not unfrc- 

 quently making a circuit of the whole heavens. Where 

 this luminous band and the inner parhelion cross, a 

 mock sun usually appears, as represented in the following 

 diagram (Fig CO). 



AURORA BORF.ALIS. It has already been indicated, in 

 the section on Electricity, that various circumstances 

 materially operate to produce the condition termed elec- 

 trical By far the best-studied of atmospherical el> 

 cal phenomena are thunder-storms ; but it is an error to 

 suppose that the atmosphere contains, at the time of a 

 thunder-storm, its maximum of electricity ; the experi- 

 in. Hi-, of Faraday have sufficiently made out this point. 



Iteserving the consideration of thunder-storms for the 

 present, we shall introduce here the subject of electrical 

 phenomena by a description of the aurora borealis a 

 phenomenon sometimes said to be magnetic, inasmix li 

 as the magnetic needle is strongly affected during it* pro- 

 <, but which, nevertheless, seems more naturally to 

 belng to electricity. 



The term aurora bortalit, or northern light, is applied 



when the phenomenon presently to be described occurs 



n the north ; and tho term aurora auitralit is applied 



when it occurs in the south. l!ut tho former has boon 

 far south as 45 of southern latitude, and tho 

 lattor has, more than once, boon visible 

 in liritain. Nevertheless, tho 1 nutiful 

 phenomena of northern and southern 

 lights are most prevalent towards the 

 north and south poles respectively. 



Northern and southern lights, when 

 in their greatest ]><" feet ion, mn-i-t of a 

 well-defined arc of white light, and 

 luminous i" coloured light flow- 



ing therefrom. The arc is not permanent, 

 as in the rainbow, but bends and twists 

 in all directions like a ribbon agitated 

 Bt by the wind. The intensity of the 

 aurora varies within extensive limits : 

 when faint, the light is only recog- 

 iiisaMe at night by careful examination ; 

 but when highly developed, the aurora 

 borealis, or australis, can be seen during 



sunshine. 



These phenomena may occur at any 

 season, but they are most prevalent in 

 the months of March, September, and 

 October, or about the period of the 

 equinoxes. The aurora borealis and aus- 

 tralis are sometimes said to be magnetic 

 Rtorms ; a more reasonable foundation 

 is required for the remark than is supplied 



by the turl ulont agitations of the magnetic iieedl- with 

 rig. co. 



Fig. 01. 



which they are attended ; but wo have already seen that 

 i he functions of magnetism and electricity are so nearly 



connected, that it is impos- 

 sible in some cases to dis- 

 tininiish between tho two. 



There is a very common 

 electrical experiment which 

 furnishes an artificial phe- 

 nomenon very nearly re- 

 sembling the northern and 

 southern light, in all re- 

 spects except in the shape 

 of the illuminated body, 

 which is a luminous arc. 



The experiment consists 

 in exhausting, by means of 

 an air-pump, all tho air out 

 of a glass tube furnished 

 with a metallic point at 

 each end, looking inter- 

 nally, and placed in the elec- 

 tric current, as represented 

 in Fig. 61. When a stream 

 of electricity, of adequate 

 intensity, is passed through 

 the apparatus from -f- to , 

 the wliolo interior of the 



