2 Dr. Huggins [Jan. 20, 



nomena which have to be explained, namely, the essential appear- 

 ances and changes which comets present during their approach to the 

 sun, at the time that they are visible to us. 



It is not necessary here to describe in detail the more purely 

 astronomical side of the subject. It will be sufficient to say, in two 

 words, that some comets have become permanent members of our 

 system, while others probably visit us once only, never to return. It 

 depends upon a comet's velocity whether its course shall be a hyper- 

 bola, a parabola, or an ellij)se. In the latter case only can it become a 

 permanently attached member of our system. If the velocity of the 

 comet when at the earth's distance from the sun exceeds 26 miles a 

 second, the comet must go off into space, never to come back to us. If 

 the comet is moving less swiftly its path will return into itself, and 

 it will visit us periodically after longer or shorter wanderings. In 

 the case of many comets, including the brightest comet of last year, 

 their velocity is so near the parabolic limit that it is scarcely possible, 

 from observations made in the small part of their orbit near the sun, 

 to be quite sure whether they will return to us or not. A number 

 of comets, chiefly small ones, arc certainly periodic, and of some 

 comets several returns, true to the calculated time, have been 

 observed. 



The small portion of the comet's life during which we are able to 

 study it is quite unlike its ordinary humdrum existence. It con- 

 sists of the short period of extreme excitement into which it is thrown 

 by a more or less near approach to the sun — a state of things which is 

 accompanied by rapid and marvellous changes, often on a stupendous 

 scale. 



The appearances which comets put on under the sun's influence 

 differ widely from each other. A few of these forms, passing from an 

 almost invisible nebulosity up to a brilliant comet of the grand type, 

 are represented on these diagrams. In nearly all these forms three 

 essentially distinctive parts may be seen. 



1. The nucleus. With the aid of a telescope, in the heads of most 

 comets a minute bright point may be found. This apparently insig- 

 nificant speck is truly the heart and kernel of the whole thing — poten- 

 tially it is the comet. It is this small part alone which conforms 

 rigorously to the laws of gravitation, and moves strictly in its orbit. 

 If we could see a great comet during its distant wanderings when it 

 has put off the gala trappings of perihelion, it would be a very sober 

 object, and consist of little more than nucleus alone. It is only this 

 part of the comet which can have any claim to solidity, or even appre- 

 ciable weight. Though many of the telescopic comets are of extremely 

 small mass, nucleus included — so small, indeed, that they are unable 

 to perturb such small bodies as Jupiter's satellites — yet in some large 

 comets the nucleus may be a few hundred miles in diameter, and may 

 consist of solid matter. I need not say that the collision of a cometary 

 nucleus of this order with the earth would bo fraught with danger on 

 a very wide scale. 



