LXVIII. And why should we be surprized that with respect to these
movements, which as we may say, nature executes behind the curtain,
human understanding has made so little, or next to no advances? what we
ought more to wonder at, is, that it happens the same with respect to all
local motion, which lies so plain and open to our observation.
LXIX. The motion with which heavy bodies descend, is the most frequent
and common to our view. And what do we know of this? Of its properties
very little; of its causes nothing. We know it acquires some degree
of acceleration in its passage, because we see it; but what is the
proportion of the increased acceleration, is matter of great debate,
both among the philosophers, and mathematicians. We know that it is a
motion of descent, but we do not know whether its course is directed
to the centre, or the axis of the earth. The cause of this motion
remains so hidden, that the philosophers to this day, have given no
opinion concerning it, that I will not venture to pronounce an absurd
one. The Aristotelites, by saying the cause of this motion proceeds
from an innate disposition to move in every thing, say nothing, unless
they point out the particular virtue or faculty, which excites motion
in heavy bodies, for what we have just mentioned, is the reason they
generally assign for all species of movements. They should not be
allowed to dispute or controvert what they themselves have advanced;
and if they should attempt to give a more rigorous definition of their
opinion, they would fall into a still greater absurdity; which made the
learned Father Saguens say, _Quis non palpat crassitiem hujus chimericæ
opinionis?_ The Cartesians to account for this phenomenon, recur to the
vertical motion of the subtil matter, which separating itself from the
earth, and pursuing the angle or direction of tangents to the circle,
impels heavy bodies to descend. But this reasoning has been confuted by
most efficacious mathematical arguments. Gassendo invented an effluvia
composed of terraqueous corpuscles, which mount into the air, and
penetrate the pores of heavy bodies, which they generally encounter,
and after turning their first course into a descending one, impel them
downwards. Nothing has so much convinced me of the great difficulty of
this question, as seeing a man of the subtil ingenuity of Gassendo,
recur for the solution of it, to a fiction destitute of all probability;
and which is exposed to invincible objections. Father Maignan, and his
followers also, make use of the terraqueous effluvia for the solution of
this difficulty; they do not allow that they act by impulse, but that by
their sympathetic and magnetic virtue, when they come in contact with
heavy bodies, they dispose them to descend.
LXX. It is very probable, and perhaps more than probable, that the ascent
of light bodies, is caused by the descent of heavy ones; because the
heavy body has power, in consequence of the impetus of its descent, to
occupy the lower station, where meeting with the light body, it obliges
him to leave that situation, and to mount upwards; for many argue with
great colour of reason, that there is no such thing as absolute lightness
to be conceived in any body whatever, nor is such a quality necessary,
a respective or comparative lightness, being sufficient to answer every
purpose. Thus we say a body is light, not because it is void of gravity,
but because it is less heavy than the one with which we compare it. In
the same manner we say the air is light, not because it is not ponderous,
but because it is less so, than earth, water, and all the other bodies
that surround us; and that no other levity but the respective, is
necessary to cause bodies which we call light to ascend, may be clearly
seen in the case of oil, which notwithstanding that it is heavy, if
you pour a quantity of water into the vessel where it is, the water on
account of its superior gravity, will occupy the inferior station, and
oblige the oil which was at the bottom to ascend. The same thing happens
with regard to air. If you dig a ditch to any depth in dry ground, the
air will descend and occupy the whole of it; and there is no other way
to dispossess the air from the bottom, and to make it rise upwards, but
conveying water into the ditch, or by throwing into it some body that is
heavier than the air.
LXXI. It is not owing to the principles of physics, but to experience,
that we understand the little we know of this matter; and with respect to
this little, there still remains great difficulties for philosophers to
contemplate; and the greatest of all, is ascertaining the cause of the
ascent of vapours to the region of the air. It is certain, that vapours
are nothing else but water, resolved into exceedingly minute particles.
The water however being heavier than air, how can the water arise to the
height occupied by the clouds? Every particle of water notwithstanding it
weighs very little, is much heavier than a particle of air of equal size,
and the greater or less gravity of liquids, for the purpose of their
impelling one another, is computed in the gross, and not according to the
proportion of globules of equal size; and we know that a pound of water,
will cause a quarter of an hundred of oil to rise in a vessel.
LXXII. Some philosophers, who were aware of this serious difficulty,
upon reflexion, conjectured that some portion of ethereal matter or
pure air, might adhere to each particle of vapour; and in consequence
of this conjunction of the two, the whole might become lighter than
an equal quantity of the inferior gross air of our atmosphere; and on
this account, might mount on the top of it; for although iron is much
heavier than water, if we fasten a small portion of iron to a bit of deal
board, it will swim, because the quantity of iron and board together, is
lighter than an equal bulk of water. Francis Bayle, adopts this opinion
of a portion of the ethereal matter adhering to the vapour; and on the
other hand, Father Pardies a French jesuit supposes, that a particle of
extended vapour, in the form of an air bubble, contains in its cavity the
ethereal matter. All this is far from certain; but I shall not detain
myself in controverting, either the one or the other of these modes of
reasoning. Others again imagine, that various igneous particles, which
arise from the earth, after separating from the water or some other
liquid those small particles which we call vapour, with their continual
impulse and agitation, force them upwards. But neither does this
system appear to me very defensible, any more, than that of the vulgar
philosophers, who say, the sun by its activity attracts the vapours;
for if this was so, the vapours would not stop till they reached the
sun, or at least till they were obstructed in the heaven of the moon, by
the moon itself, or some other solid body; for the attractive power is
stronger, the nearer the thing attracted advances to that which attracts
it; and the first would never cease to move towards the other, till it
came in contact with it, if it was not interrupted in its course by some
obstruction; and besides, the attractive virtue is a certain something,
which nobody can comprehend, and is therefore in a manner wholly banished
from philosophy.
LXXIII. Who would not be surprised, that physics should not have helped
us to attain a knowledge of so common a phenomenon, as that of the ascent
of vapours? but so far has it been from attaining it to any degree
that comes near to truth or certainty, that hitherto, we have not been
able to discover any thing upon the point, that is satisfactory to the
understanding. And with respect to all other species of motion, we are in
the same predicament.