METEOROLOGY. 



1151 



tube becomes illuminated with flashes of light, very 

 similar in appearance to the flashes of the aurora. 



The phenomenon of the aurora borealis was noticed by 

 Aristotle and Pliny, although neither philosopher could 

 have seen it to advantage. Gassendi first originated the 

 term aurora borealis, to indicate the phenomena of this 

 kind observed by him on September 12th, 1621. These 

 phenomena appear to be subject to some laws of secular 

 variation not yet understood. That they have appeared 

 in certain years, and certain groups of years more than 

 others, is certain. According to De Mairan, twenty-six 

 occurred between A.D. 583 and 1354; thirty-four between 

 1446 and 1560 ; sixty-nine between 1561 and 1592 ; 

 seventy between 1593 and 1633 ; thirty-four between 

 1634 and 1684 ; two hundred and nineteen between 

 1685 and 1721 ; nine hundred and sixty-one between 

 1722 and 1745; and twenty-eight between 1746 and 

 1751. 



After 1790, auroras became uufi orient, but since 

 1825 they have been on the increase. A very remark- 

 able aurora borealis occurred in the 

 autumn of 1847 : it was conspicuous 

 not only in England, but even so far 

 south as Italy and Spain (See Fig. 

 62). 



ll'iyht of the Aurora. As a f>roof 

 of the doubt which exists concerning 

 the height of the aurora, they have 

 been variously estimated from 3,000 

 or 4,000 feet to several miles. The 

 lality is, that the conditions 

 on which the aurora depends vary in 

 the altitude of their operation ; but 

 the truth is, that, notwithstanding tin: 

 elect rici: in , I) y his artificial experiment**, 

 can imitate the light of the aurora bo- 

 realis and aiutralis notwithstanding 

 revalence of the phenomena iu 

 question near the magnetic poles, seems 

 to point to magnetic agency as the 

 cause our real knowledge concerning 

 the aurora borealis and australis is 

 very slight. 



It may bo as well here to present 

 the reader with a summary of the 

 various opinions which have prevailed 

 at different times relative to the 

 phenomenon in question. Many early writers referred 

 the appearances presented to mere optical causes, 

 considering them to be due to the reflection of the 

 sun's light thrown upwards by a mirror of gnow 

 and ice, and a subsequent reflection downwards 

 by atmospheric agencies. De Mairan, the observer who, 

 jK-rhaps more than any one else, is entitled to be con- 

 sidered the chronicler ;<>ir excellence of the phenomena 

 of the aurora, attributed them to the penetration of our 

 planet at certain jtcri'xls into the solar atmosphere. On 

 this supposition it will be remarked, that the solar at- 

 mosphere must be assumed to extend to the orbit of our 

 planet, an hypothesis totally irreconcilable with the 

 .js of optics and astronomy. The celebrated 



Fig. 63. 



have offered a most crude and improbable theory ex- 

 planatory of the aurora. Adopting the molecular theory 

 of light, he assumed that the solar rays, striking against 

 the particles of our atmosphere, actually carried particles 

 of the latter up into the heavens to a height of more 

 than four thousand miles the height at which Euler be- 

 lieved auroras to exist. Some philosophers, of whom 

 Volta may be regarded the Coryphaeus, adopted a 

 chemical theory of auroras, referring them to the igni- 

 tion of hydrogen gas spontaneously generated on the 

 earth, and rising, by its light specific gravity, to the 

 higher atmospheric regions. It was assumed by these 

 philosophers tliat the evolution in question took place in 

 the tropical regions chiefly, and that it was wafted by 

 the upper current of air treated of iu connection with 

 the trade wind to the north and south poles respec- 

 tively. 



Halley was the first, we believe, who suggested that 

 the phenomena of aurora borealis and australis might bo 

 due to the passage of magnetism from one magnetic 



! -, the jiliPoinpher who could dra! so R-itisfnctorily 

 with the abstractions of number and quantity, seems to 



pole to the other ; and the theory of Halley is so far re- 

 tained. th.it the aurora is assumed to be in some w.iy 

 connected with electricity and magnetism, but in what 

 manner is beyond the competence of observers to decide. 

 ON- THK PIIKNOMI-.XA OF THUNDER AND LIGHTMND. 

 Perhaps no meteorologic phenomena are now so well 

 understood as these ; though, before electrical science 

 had been studied by the philosophers of the last cen- 

 tury, and the crowning experiment of Franklin per- 

 formed, the phenomena of thunder and lightning were 

 so mysterious, that even philosophers were content to 

 refer them to the operation of an occult came. 



The intimate study of electrical science opens a field 

 of somewhat abstruse matters for consideration ; the 

 field is far too wide and too abstruse 

 to be dealt with satisfactorily here. 

 In the section on Electricity it has 

 been treated in detail ; and to this we 

 must refer the reader who desires to 

 know more on this subject than strict- 

 ly belongs to the necessities of what 

 we may term practical meteorology. 



With this explanation we shall not 

 hesitate to adopt the tormelectricjluid, 

 although the reader has already been 

 I made aware that no such fluid is 

 at all likely to exist. Let us now 

 contemplate the phenomenon of that 

 electrical excitation, the solution 

 of which is lightning, under the 

 simplest conditions that the phenomenon can assume. 

 Let A and B (Fig. 63) represent two clouds, which, 



