6 



KNOWLEDGE 



[January 1,18. 



year as a grey mark, which became red in April, and con- 

 tinued visible till Jimo. The southern edge of the south 

 belt was also very red at times, mm-h more so than the 

 great spot, so that there was a tendency to become red in 

 various portions of the southern hemisphere. The round 

 dark mark near the equator is the shadow of the second 

 satellite. Natu. E. Green. 



PERIODICAL COMETS DUE IN 1896. 



15y W. T. Lynn, B.A., F.R.A.S. 



OF all the periodical comets wliose orbits are known 

 with some accuracy, two only arc due to return to 

 perihelion in ISOC. One of these has been seen 

 at seven previous returns, all consecutive ; the 

 other has hitherto been seen at only one appear- 

 ance, unless a remarkable theory which was then started 

 should be found on the forthcoming return to have been 

 founded on fact. 



The former of these two comets is that known as Faye's, 

 because it was first discovered by the veteran astronomer, 

 M. Fayc, at Paris, on the 22nd of November, in the year 

 1843. Its orbit was calculated to be a short elhpse, with 

 a period of about seven and a half years, and it duly 

 returned to perihehon in 1851, being first seen on that 

 occasion by Prof. Challis, at Cambridge, on the 28th of 

 November, 1850. It has also been observed at every 

 subsequent return, and was last in perihelion on the 20th 

 of August, 1888. Another retin-n to that position will be 

 due on the I'.lth of March next, but the comet was nearest 

 to the earth in October last, and was seen by M. Javelle at 

 Nice so early as the 2Cth of September, nearly six months 

 before perihehon passage. This comet is a very faint 

 object, and has never been visible to the naked eye. Its 

 orbit is remarkable for its very small eccentricity, which 

 amounts to only about 0-55. When in perihehon, the 

 comet never approaches the sun so nearly as the greatest 

 distance of Mars ; when in aphelion, its distance somewhat 

 exceeds that of Jupiter, and it was probably the attraction 

 of that planet which first brought it into our system. 



The other comet to which we referred was discovered by 

 Mr. Brooks, of the Smith Observatory, Geneva, N.Y., on 

 the 6th of July, 1889. Mr. S. C. Chandler, of Boston, 

 U.S., showed that it had made a very near approach to 

 Jupiter three years before, which would have the effect of 

 greatly changing its orbit. Now, a comet discovered by 

 Messier, at Paris, more than one hundred and twenty-five 

 years ago, on the 11th of June, 1770, was calculated by 

 Lexell to be moving in an elUptic orbit, with a period of 

 only five and a half years, but failed to put in an 

 appearance when afterwards due, so that it acquired 

 the name of Lexell's lost comet. This failure, however, 

 was explained as arising from violent perturbations 

 produced by the attraction of Jupiter. The comet had 

 in fact approached that planet in 1767 within a dis- 

 tance of only about one-sixtieth part of the radius of 

 Jupiter's orbit, and this circumstance it was that brought 

 the comet within view from the earth, which it approached 

 in 1770 within a distance of little more than seven times 

 that of the moon. It probably returned to perihelion in 

 1775 or 1776, but in a position unfavourable for observa- 

 tion ; and in 1779, before another return was due, it 

 approached Jupiter even closer than before, coming, indeed, 

 nearer to that planet than the distance of his fourth or 

 most distant satellite, and its orbit thus again undergoing 

 a great alteration. Now, Mr. Chandler thought, fi-om his 

 calculations, that the comet discovered by Mr. I3rooks in 

 1889 might be Lexell's comet, brought once more into 

 visibility from the earth by the attraction of Jupiter in 



1886. Dr. C. Lane Poor, however, made a re-investi- 

 gation of its motions, the result of which was not to 

 confirm this theory. Mr. lirooks's comet was moving 

 when discovered in an orl)it with a period of nearly seven 

 years, so that it will probably come into perihelion again 

 in the spring or summer of the present year. It is hoped 

 that it will be possible to obtain observations on that 

 occasion, by which astronomers will be enabled to come to 

 a definite decision with regard to the suggested identity 

 with Lexell's lost comet of 1770. The orbit, as determined 

 from those made in 1889, is even less eccentric than that 

 of Faye's comet, and the perihelion distance from the sun 

 is greater than the mean distances of some of the small 

 planets, being more than twice as great as that of the 

 earth. The eccentricity of the orbit amounts to only 04K ; 

 this is the smallest of any known comet, with the doubtful 

 exception of one denominated Tempel's first periodical 

 comet, which was discovered by tlio late M. Tempel at 

 Marseilles in 1867 and observed at returns in 1873 and 1H79, 

 but not seen subsequently. It should be added that M. 

 Schulhof has recently made some calculations which seem 

 to show that the comet discovered by Prof. Swift in August 

 last year (and not Brooks's of 1889) was identical with 

 Lexell's. The period of Swift's comet is about seven and 

 a-quarter years, but at the next return in 1902 it is likely 

 to be very unfavourably placed for observation, particularly 

 in the northern hemisphere, so that we may have long to 

 w'ait for the decision of this question. 



WAVES.-I. 



THE "WAVES OF THE OCEAN. 



By Vaughan Cornish, M.Sc. 



WHEN the lightest zephyr passes over a per- 

 fectly calm sea the glassy surface remains 

 unruffled. As Byron wrote : — 

 " Winds come wliisporiiig liglitly from the West, 

 Kir^siiif,', not rullling, the blue clci'p's SLTcno." 



A breeze which travels at rather more than half a mile an 

 hour just ruffles the water, darkening the surface, which 

 no longer reflects like a mirror ; but if the breeze drops 

 the wavelets instantly cease and the water resumes its 

 glassy look. If, on the other hand, the breeze increases 

 until it has a velocity of about two miles an hour, 

 permanent waves begin to rise, which increase in sifte as 

 the wind continues to blow. The form of the waves in a 

 rising sea is somewhat different from the smoothly sloping 

 curve of a ground swell, for whilst the waves are rising the 

 crests are exposed excessively to the force of the wind, 

 and the troughs are sheltered. The tops of the waves are 

 consequently driven forward, and break at the crest into 

 the " white horses " which chase each other across a rising 

 sea. At first the wind increases the height of the waves 

 over every part of the sea upon which it blows, but after a 

 time the waves near the windward shore attain such 

 dimensions that the power of the wind is only sufficient to 

 maintain, without increasing, them. Further to leeward 

 the waves are still increasing, and still show the " white 

 horses." However long a wind of constant strength may 

 blow, the height to which it can raise the waves in any 

 part of the sea is limited, being greater, however, as the 

 distance from the windward shore increases. This 

 increase in the size of the waves can readily be explained, 

 for at any place the sea is raised not c:nly by the force of 

 the wind there, but also by the energy transmitted from 

 the leeward waves. Off the coasts of Britain the greatest 

 waves — greatest not only in height, but more especiaUy in 

 length from crest to crest — come rolling in fi-om the broad 



