208 Observations on Comet II. 



that of Neptune. The period of these six comets vary from 

 about three years and a quarter — that of the comet known as 

 Encke's — to seven years and a hundred and sixty-three days, 

 that of Faye's Comet. The probable returns of other comets 

 have been foretold, but the day of fulfilment, in many cases, is 

 far away ; centuries, or even thousands of years hence. 



Is then so little known, it may be asked, about each comet 

 which appears ? Have they not then the interest which belongs 

 to the planets as known and established denizens of the solar 

 system ? Rather say they have a very special interest of another 

 kind. 



A comet — speaking in a general way — is composed of head 

 and tail. There is a large ill-defined mass of light called the 

 head, which is usually much brighter towards its centre, offering 

 the appearance of a vivid nucleus like a star or planet. " From 

 the head," I quote Sir John Herschel, "and in a direction 

 opposite to that in which the sun is situated from the comet, 

 appear to diverge two streams of light, which grow broader 

 and more diffused at a distance from the head," and these com- 

 monly uniting into one mass of filmy light, and extending to an 

 immense distance, form the comet's tail. This is not a matter 

 of seeming. It is not merely that the brightness fades away 

 from the region of the nucleus to that of the tail ; the nucleus 

 forms the tail, and subsequently retains a control over it, and 

 this, says Mr. Bond, in his valuable account of Donates Comet, 

 1 ' is one of the most curious phenomena presented in nature." 



When a comet approaches that part of its parabolic or 

 elliptical path which brings it nearest to the sun, some extra- 

 ordinary phenomena begin to be observed in the region sur- 

 rounding its nucleus. The nucleus (sometimes becoming sud- 

 denly brighter than before) throws out a jet of light towards 

 the sun, which jet, though bright at its point of emanation from 

 the nucleus, fades rapidly away, and becomes diffused as it 

 expands into the " coma," or head. Another and another jet 

 succeeds ; and each, while expanding in the coma, curves back- 

 wards, as if impelled by a force of great intensity directed from 

 the sun. And thus the comet's tail is formed. These strange 

 phenomena, though observed in the cases of the great comet of 

 1811 and of some others, were first noted with minute attention 

 by the illustrious Bessel in the case of Halley's Comet in 1835. 

 The very night on which these wonders first manifested them- 

 selves was also signalized by the commencement of the tail of 

 that comet. It is impossible (says Professor Grant) to doubt 

 that this appendage derived its origin from the nebulous matter 

 which had been in the first instance raised from the head by a 

 force directed to the sun, and was subsequently impelled by a 

 powerful force in the opposite direction. 



