386 ANNUAL OF SCIENTIFIC DISCOVERY. 



the nucleus 6" = 4660 miles; September 8th and 9th, diameter 3" = 1980 

 miles. On the 23d of September, the head of the comet, to the naked eye, 

 appeared brighter than a star of the first magnitude, and during the remain- 

 ing period of its visibility it went through a series of periodic changes, ac- 

 quiring more light just before an eruption, and suddenly diminishing after- 

 wards. 



This comet, says Mr. Bond, although surpassed by many others in size, 

 has not often been equalled in the intensity of the light of the nucleus. The 

 diameter of the surrounding nebulosity, on the other hand, was unusually 

 small, never much exceeding 100,000 miles; while that of the great comet of 

 1811 was ten times larger, its envelop attaining an elevation of more than 

 300,000 miles above the central body. 



The various observations on Donati's comet leave no room to doubt that 

 it is periodical, and has a time of revolution of about 2000 years, the max- 

 imum period calculated being 2415 years, and the minimum 18-5-1. Its hourly 

 velocity at perihelion was calculated at 127,000, and its aphelion velocity at 

 480 miles. 



In illustration of these times and distances, the London Times uses the fol- 

 lowing comparisons : 



Let any one take a half-sheet of note paper, and, marking a circle with a 

 sixpence, in one corner of it, describe therein our solar system, drawing the 

 orbits of the earth and inferior planets as small as he can by the aid of a 

 magnifying glass. If the circumference of the sixpence stands for the orbit 

 of Neptune, then an oval filling the page will fairly represent the orbit of 

 our comet ; and if the paper be laid on the pavement under the west door of 

 St. Paul's, the length of that edifice will inadequately represent the distance 

 of the nearest fixed star. That the comet should take more than 2,000 years 

 to travel round the page of note paper we have supposed, is explained by 

 its great diminution of speed as it recedes from the sun. At its perihelion, 

 it travelled 127,000 miles an hour, or more than twice as fast as the earth, 

 whose motion is about one thousand miles a minute. At its aphelion, how- 

 ever, or its greatest distance from the sun, the comet is a very slow body, 

 sailing along, as if doubtful whether to return, at the rate of 480 miles an 

 hour. This is only eight times the speed of a railway express. At this pace, 

 even if the comet could wholly shake off the attraction of the sun which 

 it certainly could not and were it to travel onward in a straight line, the 

 lapse of a million years would find it still travelling half way between our 

 sun and the nearest fixed star. Cornets, then, can hardly be imagined visit- 

 ors from our system to any other, or from any other to our own. 



Donati's comet passed within nine millions of miles of Venus, and if it 

 had any density at all comparable to our own, would have effected the orbit 

 of that planet in an appreciable degree. Were that found to be the case, it 

 wouid be the first fact of the kind in the history of these singular bodies. 



Mr. Bond, in connection with his article in the Mathematical Journal/be- 

 fore referred to, also presents the following sketch of the more distinctive 

 phenomena presented in the motions and physical aspect of comets gener- 

 ally : 



The first characteristic of these singular bodies is that of their being 

 mainly, perhaps in most instances, entirely composed of an ill-defined gas- 

 eous or nebulous substance, endowed with properties so extraordinary that 

 it can scarcely be classed with matter in the ordinary acceptation of the 

 term. Of its extreme attenuation and lightness there can be no question. 



