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abandoned by tbe bead at an antecedent period — a period more 
distant in proportion as tbe section itself is further removed 
from tbe nucleus. Eacb of these sections has followed, in 
space, an orbit absolutely different from that of tbe bead of 
the comet; and tbe tail, in its entirety, is nothing but tbe 
envelope of tbe positions occupied at a given moment by tbe 
series of puffs of cometary matter successively emitted and 
driven off on tbe preceding days, without there being between 
them any other connexion than tbe velocity of translation which 
they possessed in common at their points of departure. 
Calculation applies perfectly to these singular but by no 
means mysterious phenomena. Bessel furnished their formula, 
which enables us to determine by the curvature of the tail the 
intensity of the force that produced it. Quite recently, M. Bre- 
dichin, director of the Observatory at Moscow, has obtained 
from it most interesting results. 
As to this force which M. Flammarion denies, although in 
every comet we see its effects marked in the heavens in gigantic 
features, it is certain that matters go on as if the Sun was 
endowed with two actions — one attractive, belonging to its 
mass, the other repulsive, due to its electric (Olbers), magneti- 
polar (Bessel), or calorific (Faye) state. We may dispute its 
essence, or its physical nature, but not its mechanical characters, 
as I have defined them, because these characters result from 
the observed facts, namely, — 
1. This repulsive force is not proportionate to the masses, 
like attraction, but to the surfaces. Hence it produces the 
more marked effects in proportion as the matters subjected to it 
are less dense. 
2. This force is not exerted through all matter, like attrac- 
tion ; it is on the contrary weakened, or even arrested by the 
interposition of the smallest screen. 
3. It is not propagated instantaneously, like attraction, but 
gradually, like light and heat. It results from this that its 
action upon a point in motion is not exerted in the same direc- 
tion as attraction, even though the two forces emanate from 
the same body. 
4. Lastly, this force varies inversely to the square of the 
distance, like the intensity of light and heat. This is the sole 
point of resemblance between the two forces which the Sun 
exerts simultaneously upon all bodies, one which is connected 
with its mass, and therefore invariable, the other with its phy- 
sical condition and consequently transitory. 
This latter force necessarily affects the planets and their 
satellites as well as the comets. The first of the four characters 
that I have just indicated will explain how its action upon the 
planets, which are of incomparably greater density, has hitherto 
