204, and Langier, 'Comptes Rendus
des Seances de l'Acad.', 1843, t. xvi., 1006.
p 100
Although comets have a smaller mass than any other cosmical bodies -- being,
according to our present knowledge, probably not equal to 1/5000th part of
the Earth's mass -- yet they occupy the largest space, as their tails in
several instances extend over many millions of miles. The cone of luminous
vapor which radiates from them has been found, in some cases (as in 1680 and
1811), to equal the length of the Earth's distance from the Sun, forming a
line that intersects both the orbits of Venus and Mercury. It is even
probable that the vapor of the tails of comets mingled with our atmosphere
in the years 1819 and 1823.
Comets exhibit such diversities of form, which appear rather to appertain to
the individual than the class, that a description of one of these "wandering
light-clouds," as they were already called by Xenophanes and Theon of
Alexandria, contemporaries of Pappus, can only be applied with caution to
another. The faintest telescopic comets are generally devoid of visible
tails, and resemble Herschel's nebulous stars. They appear like circular
nebulae of faintly-glimmering vapor, with the light concentrted toward the
middle. This is the most simple type; but it can not, however, be regarded
as rudimentary, since it might equally be the type of an older cosmical
body, exhausted by exhalation.
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