﻿Prof. Tyndall on Cometary Theory. 243 



Nothing could more perfectly illustrate that ' spiritual texture ' 

 which Sir John Herschel ascribes to a comet than these actinic 

 clouds. Indeed the experiments prove that matter of almost 

 infinite tenuity is competent to shed forth light far more intense 

 than that of the tails of comets. The weight of the matter which 

 sent this body of light to the eye would probably have to be 

 multiplied by millions to bring it up to the weight of the air in 

 which it hung. 



" And now will you bear with me for five minutes while I en- 

 deavour to apply these results to cometary theory ? I am encou- 

 raged to do so by a remark of BessePs, who said that had any 

 theory preceded his observations on Halley's comet, by fixing 

 his attention either upon its verification or its confutation, it 

 would have enabled him to return from his observations with a 

 greater store of knowledge than he had actually derived from 

 them. If time permitted, I should like to lead you by an easy 

 gradient up to the view that I wish to submit to you ; but time 

 does not permit of this, and therefore the speculation must suffer 

 from the baldness arising from the absence of such preparation. 



" You are doubtless aware of the tremendous difficulties which 

 beset cometary theory. The comet examined by Newton in 

 1680 shot out a tail sixty millions of miles in length in two 

 days. The comet of 1843, if I remember aright, shot out in a 

 single day a tail which covered 100 degrees of the heavens. This 

 enormous reach of cloudy matter is supposed to be generated in 

 the head of the comet and driven backwards by some mysterious 

 force of repulsion exerted by the sun. Bessel devised a kind of 

 magnetic polarity and repulsion to account for it. 'It is clear/ 

 says Sir John Herschel, ( that if we have to deal here with matter 

 such as we conceive it, viz. possessing inertia, at all, it must be 

 under the dominion of forces incomparably more energetic than 

 gravitation, and quite of a different nature/ And in another 

 place he states the difficulties of the subject in the following re- 

 markable words : — 



" ' There is beyond question some profound secret and mystery 

 of nature concerned in the phenomenon of their tails. Perhaps 

 it is not too much to hope that future observation, borrowing 

 every aid from rational speculation, grounded on the progress 

 of physical science generally (especially those branches of it 

 which relate to the gethereal or imponderable elements), may ere 

 long enable us to penetrate this mystery, and to declare whether 

 it is really matter, in the ordinary acceptation of the term, which 

 is projected from their heads with such extravagant velocity, and 

 if not impelled, at least directed in its course by a reference to 

 the sun as its point of avoidance. In no respect is this ques- 

 tion as to the materiality of the tail more forcibly pressed on us 



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