ASTRONOMY AND METEOROLOGY. 385 



fusion presented within the inmost envelop can only be accounted for as 

 the result of sudden and violent disruptions from the central body, projecting 

 immense volumes of its luminous substance towards the sun, which by somo 

 unknown law, is in turn repelled by that body, and driven off to the distant 

 regions of space, forming the vast train of light so characteristic of these 

 mysterious bodies. 



Prof. Mitchell, of the Cincinnati Observatory, thus describes the appear- 

 ance of the comet in the great refractor of that institution : 



" On the evening of the 25th of September the central portion, or nucleus, 

 was examined with powers varying from one hundred to five hundred, 

 without presenting any evidence of a well-defined planetary disk. It was a 

 brilliant glow of light, darting and flashing forward in the direction of the 

 motion toward the sun, and leaving the region behind in comparative ob- 

 scurity. But the most wonderful physical feature presented was a portion 

 of a nearly circular nebulous ring, with its vertex directed toward the sun, the 

 bright nucleus being in the centre, while the imperfect ring swept more than 

 half round the luminous centre. This nebulous ring resembled those which 

 sometimes escape from a steam-pipe, but did not exhibit the appearance 

 which ought to be presented by a hollow hemispherical envelop of nebulous 

 matter. 



" There was an evident concentration of light in the central portions of the 

 ring, while, in the case of a hollow envelop, the brightest portion should be 

 at the outer edge. By micrometrical measurement, the distance from the 

 central point to the circumference of the ring was found to be about nine 

 thousand miles. This would give a diameter of eighteen thousand miles, in 

 case the ring was entire. Similar measurements, made on the evening of the 

 26th of September, indicated a decided increase in the radius of the ring, 

 which was now not less than twelve thousand miles in length. On the same 

 evening I noticed the fact that the luminous envelop did not blend itself 

 into the head portion of the tail, but appeared somewhat to penetrate into 

 this nebulous mass, especially on the upper part, presenting the appearance 

 of about 200 of a spiral. The tail on the 25th was decidedly brighter and 

 better defined on the upper than on the lower portion, while on the evening 

 of the 26th there was a much nearer approach to equality in brightness, es- 

 pecially near the head of the comet. Through the telescope, and near the 

 head, the tail presented the appearance of a hollow nebulous envelop, under 

 the form of a paraboloid of revolution, the edges being brightest and well 

 defined, while there was a manifest fading away of light towards the central 

 region. Through the vast depth of nebulous matter composing this won- 

 derful appendage, the faintest telescopic stars shone with undiminished 

 brightness." 



Donati's comet attained its least distance from the sun 55,000,000 of miles 



on the morning of the 30th of September. Its least distance from the earth 



52,000,000 of miles occurred on the 12th of October. On the 10th of 

 October, its tail stretched over 60 of the heavens, and was 51,000,000 of 

 miles in length, and 10,000,000 in breadth, at its extremity. The lon- 

 ger diameter of its orbit was estimated as 184 times that of the earth, or 

 35,000,000,000 miles, a space, however, considerably less than one-thousandth 

 of the distance of the nearest fixed star. For the nucleus, Mr. Bond gives the 

 following measurements : July 19th, 5" 5000 miles ; August 19th, equal to 

 a star of the 7th magnitude; August 20th, head of the comet visible to tho 

 unassisted eye as a star of the sixth magnitude; August 30th, diameter of 



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