﻿244 Prof. Tyndall on Comet ary Theory, 



for consideration than in that of the enormous sweep which it 

 makes round the sun in pei'ihelio, in the manner of a straight 

 and rigid rod, in defiance of the law of gravitation, nay, even of 

 the received laws of motion, extending (as we have seen in the 

 comets of 1680 and 1843) from near the sun's surface to the 

 earth's orbit, yet whirled round unbroken — in the latter case 

 through an angle of 180° in little more than two hours. It 

 seems utterly incredible that in such a case it is one and the 

 same material object which is thus brandished. [I would espe- 

 cially invite the reader's attention to these words in reference to 

 the following theory. — J. T.] If there could be conceived such 

 a thing as a negative shadow, a momentary impression made 

 upon the luminiferous sether behind the comet, this would re- 

 present in some degree the conception such a phenomenon irre- 

 sistibly calls up.' 



" I now ask for permission to lay before you a speculation which 

 seems to do away with all these difficulties, and which, whether 

 it represents a physical verity or not, ties together the pheno- 

 mena exhibited by comets in a remarkably satisfactory way. 



" 1 . The theory is, that a comet is composed of vapour decom- 

 posable by the solar light, the visible head and tail being an 

 actinic cloud resulting from such decomposition ; the texture of 

 actinic clouds is demonstrably that of a comet. 



" 2. The tail, according to this theory, is not projected matter, 

 but matter precipitated on the solar beams traversing the come- 

 tary atmosphere. It can be proved by experiment that this 

 precipitation may occur either with comparative slowness along 

 the beam, or that it may be practically momentary throughout 

 the entire length of the beam. The amazing rapidity of the 

 development of the tail would be thus accounted for without in- 

 voking the incredible motion of translation hitherto assumed. 



" 3. As the comet wheels round its perihelion, the tail is not 

 composed throughout of the same matter, but of new matter 

 precipitated on the solar beams, which cross the cometary atmo- 

 sphere in new directions. The enormous whirling of the tail 

 is thus accounted for without invoking a motion of translation. 



" 4. The tail is always turned from the sun for this reason : — 

 Two antagonistic powers are brought to bear upon the cometary 

 vapour, — the one an actinic power tending to produce preci- 

 pitation, the other a calorific power tending to effect vaporiza- 

 tion. Where the former prevails, we have the cometary cloud ; 

 where the latter prevails, we have the transparent cometary va- 

 pour. As a matter of fact, the sun emits the two agents here 

 invoked. There is nothing whatever hypothetical in the assump- 

 tion of their existence. That precipitation should occur behind 

 the head of the comet, or in the space occupied by the head's 



