Farrctoucn.—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 14,155 part of 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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