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HISTORY OF PHYSICAL ASTRONOMY.

atmosphere through which it is viewed. Thus, at Paris , the tail of thecomet of 1680 was 62° long; at Constantinople it was 90°. The tail ofthe comet of 1769 was 43° long at London on the 9th of September. Onthe same day it was 55° long at Paris ; at the Isle of Bourbon it was 60°;at Teneriffe it was 75°. On the Utli of September it was 90° at sea; atthe Isle of Bourbon it was 97°*. The tail of the comet of 1843 did notmeasure more than 40° in England or France ; in tropical countries it wasfound to extend over an arc of 65°.

The absolute lengths of the tails of some comets are still more calcu-lated to excite surprise than their apparent lengths. The tail of the greatcomet of 1680 was 96 millions of miles long. The tail of the comet of1769 had an absolute length of 38 millions of miles. According toHerschel, the tail of the comet of 1811 must have been more than 100millions of miles long on the 15tli of October. The tail of the comet of1843 attained a length of 150 millions of miles.

The apparent breadth of the tail of a comet seldom exceeds a fewdegrees. Each of the six tails of the comet of 1744 was 4° broad. Thetail of the comet of 1811 was found by Herschel, on the 12tli of October,to have a maximum breadth of 6° 45'. Small as these measures mayappear to be, they indicate absolute dimensions of very considerable mag-nitude. Thus Herschel found that, on the 12th of October, the greatestannular section of the tail of the comet of 1811 had an absolute diameterof 15 millions of miles j-.

It is obvious, from the foregoing results, that the volumes of the tails ofcomets must be enormous. Nor is there any difficulty in accounting forsuch vast emanations from the comparatively small dimensions of thehead, for such is the excessive tenuity of the substance composing the tailof a comet, that a very small quantity of matter would suffice for its pro-duction. Newton, with the view of illustrating this point, calculated thatif a globe of common atmospheric air, one inch in diameter, was expandedso as to have an equal degree of rarity with the air situated at an elevationabove the earth’s surface equal to the earth’s semi-diameter, it would fillthe whole planetary regions as far as the sphere of Saturn , and would evenextend a great deal farther j.

The phenomena exhibited by one of the more conspicuous comets duringthe period of its apparition afford unequivocal indications of the powerfulinfluence exercised by the sun upon the physical constitution of thesebodies. When a comet first becomes visible previous to its passage of theperihelion, it usually presents the appearance of a pale nebulous body witha point more or less bright in the centre, but without any trace of coma ortail. As it continues to approach the sun the nebulosity becomes moreapparent, and the head exhibits an increased degree of brightness, on theside which is turned towards the sun, subject, however, to variations of anexceedingly irregular character. The tail at the same time comes intoview, and gradually increases in length. After the passage of the peri-helion, the same succession of changes occurs in a reverse order, the cometfinally assuming the appearance presented by it when it first becamevisible, and soon afterwards vanishing altogether from observation. Thephenomena witnessed on the occasion of the last apparition of Halley’scomet, both before and after the passage of the perihelion, are highly

• Cometographie, tome ii., p. 194. + Phil. Trans., 1812, p. 124.

f Princip., lib. iii., prop. 41.