Fairclough. — On the Constitution of Comets. 481 



that there is a resisting medium in all space, comets do now prove far more 

 clearly, by the astounding velocity that their attenuated substances attain, 

 how nearly absolutely empty space must be to admit of their motion. 



However great the tenuity of the substance of comets, there is a point 

 in each one which rigidly obeys the law of gravitation. This point is the 

 nucleus, or centre of the head. A considerable number of short-period 

 comets have the time of their perihelion passage fixed almost as accurately 

 as the time of an eclipse. Halley's comet, observed in 1682, was predicted 

 to return in 77 years. Computists calculated the retarding influences of 

 the known planets, and allowed 30 days for possible error in the time fixed 

 by them for the perihelion sweep. Neither Uranus nor Neptune were then 

 known, yet the comet was in perihelio in 1756, within 29 days of the time 

 fixed. For its return in 1835 — the planet Neptune being still unknown — 

 the perihelion passage was fixed by Rosenberger between the 11th and the 

 16th of November. It took place on the 15th. 



It is evident, therefore, that however small the mass of the huge 

 volume of a comet, it yields the same obedience to law as the densest 

 planet. 



But a question naturally rises in our minds as to how the great body of a 

 comet is held together, when its own power of gravity is known to be so small. 

 If the difference of the distances of the centre and the surface of the minute 

 and dense earth from the sun suffices to raise a considerable tide, we might 

 naturally expect to find a comet, with its tremendous diameter, its small 

 power of cohesion, and its proximity to the sun, rent into several sections, 

 to be thrown into somewhat different orbits. The fact remains, however, 

 that no such disruption takes place in the majority of cases ; though there 

 are several records of comets parting into two or more fragments. 



But a curious phenomenon is observed upon the approach of a comet to 

 the sun ; it is that the nucleus appears to shrink in a wonderful degree. 

 M. Struve, in observing Encke's comet in 1828, found that on December 

 24th it only occupied jg-JoTT P ar ^ 0I> the space it had occupied on the 28th 

 of October. When it begins to recede from the sun, the comet begins also 

 to recover its former volume. In this situation Halley's comet was observed 

 by Sir J. Herschel, in 1835, to increase forty-fold in apparent size in a 

 single week. 



Several explanations of this have been given. A recent one is that of 

 M. Valz, that the shrinking is due to the pressure of the sun's atmosphere. 

 This theory, however, assumes an enormously extended atmosphere for the 

 sun which can scarcely be granted ; it also seems to require the comet to be 

 enclosed in an envelope, to prevent it from mingling, like vapour from an 

 engine, with the supposed atmosphere ; it also supposes the comet to move 

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