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Quantity, will dilate itself much more strongly by the Heat, and hinder the Fifh, unless he conftantly exerts all his Strength, from being able to defcend again.

The fame Inconvenience would happen if there were lefs Air in the Bladder, and only fo much, that the Fish might eafily fupport itself at Top of the Water in Summer; for upon the return of Winter, or upon a Fifh's defcending lower, and meeting with more Cold and a greater Pref fure of the Column of Water upon it, and by both these Means, the Bladder being contracted without any Concurrence on the part of the Fish, much Strength must be used to raise it up again; infomuch, that with the change of the Seafons, the Fifh wou'd oftentimes have too much Air in the Summer, and too little in Winter: So likewife the Fishes paffing into Water of different Gravities, would be many times furnished with too much or too little Air in their Bladders; and in order to avoid all these troublesome Alterations, and to pafs conveniently from one place to another, they would be obliged to remain always in a Wa ter of about the fame Weight, and as much as poffible in the like Depth and Temper, as to Heat and Cold.

To prevent all thefe Inconveniences, the readieft manner feems to be, that the Fithes fhould be endowed with the Faculty of encreafing or leffening the quantity of Air in their Bladder,according as occafion required, which likewife we fee happen by the Wisdom of the Creator; forafmuch as their Bladders have a Communication with their Stomach by the Means of a very fmall and narrow Tube; fo that they can diminish the Air charging from the Bladder thro' the Mouth, and increase it, by drawing it in again; about which Borelli, Prop. CXI. Part I. has this Obfervation;

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That the Bladder is empty when the Fish being in Vacuo, difcharges a great many Air-bubbles by its Mouth; and the fwailowing in of Air may perhaps be the reafon, why we often fee the Fishes moving their Mouths in the upper Part of the WaSter near the Air.

SECT. XXX. Convictions from the foregoing Obfex

vations.

Now if a deplorable Atheist has taken the Pains to read this, and understands it, let him tell us, whether it can any ways feem probable to him, that fo many Laws of the Water, of the Air, and of the Motion of the Muscles in Fifhes, are fo accurately obferved by meer Chance? or could blind Nature, ignorant in itfélf of all its Effects, produce fuch a Difference, as on the one hand to furnish the Fishes with fuch a Bladder, and Birds on the other hand, tho' they likewife move in a fluid Matter, or in the Air, with quite a different Method of Progreffion; fince fuch a Bladder by which a Bird were to be raifed up, muft be lighter than the Air, and for that reafon empty of it. Now they who ever proposed to raise a heavy Body

the Aif, with a Globe out of which the Air is exhaufted, know firft, That the Shell of it muft be made pretty thick, leaft, being thin, it should be unable to refift the Preffure of the external Air upon any Accident; and befides, tho' all this were not obferved, yet it must be of fo difproportionate a Magnitude, that no Bird, being incumber'd with it, could be able to fly: Not to take Notice, that the Greatnefs of a hollow Braf Globe (that being empty of Air without lifting up any heavy Body, it might afcend alone, and of itfelf) is computed by Mr. Leibnitz, in the Philofophical Tranfactions of Berlin, publifhed in the Year

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1710, p. 127. to be fuch, that the half Diameter thereof would require to be above twenty thoufand times longer than the Thickness of the Metal of which the Cruft of the faid Globe muft be compofed; fo that the faid Globe being an Inch thick (tho' that perhaps would not be sufficient to refift the Preffure of the external Air) the whole Magnitude of this hollow Globe would take up fome thousands of Feet. I have expatiated here fomething the more, to convince Sceptical Philofophers, that are any ways verfed in Modern Experiments, that the Structure of Fifhes is entirely oppofite to what is proper for the flying of Birds; and that it is undeniable, that in order to make Fish and Birds move upwards and downwards, (each of 'em in their different Fluids) different Means must neceffarily be applyed; which being performed in both, in a manner fo fuitable to all these Circumstances, I leave it again to their own Judgment, whether this does not plainly fhew the Wisdom and the good Pleafure of a Great Creator, ili je` klasut di

SECT. XXXI. Fish Swim with their Tails.

Now, if we obferve farther in fo many Fishes, that in order to their Progreffive Motion in Water by Swimming, they do not make use of their Fins as Oars to row with, nor after the fame man, ner as the Birds do their Wings in the Air, but by the help of their Tails, much after the same manner as a Boat moves when they put an Oar out at the Stern, and Paddle with it backwards and forwards.

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Is there no Wifdom to be difcover'd in this (fince Fishes ftand in need of no external Motions for raifing and finking their Bodies, as we have fhewn before) that their Inftruments are fo formed, that no time fhould be loft in their advancing for

wards;

wards? And that having made a Motion with their Tail, by which they are protruded, they have no occafion to draw it back again, in order to difpofe the fame, to repeat the faid Protrufive Motion: This the Birds are forced to do with their Wings, that they may strike upon the Air every time perpendicularly, in order to fupport themselves therein; but the Fishes by putting their Tail in its former Place and Difpofition, exert the fame Force on the other fide, which contributes as much to their Progreffion, as the first Stroke had done? Is it now by Chance, that these Tails, like the paddling Oars, are broad at Bottom, that they may act with greater Force upon the Water; and that they are composed of a strong Membranous Matter, which is however flexible; that the Muscles of the Back are of fuch a StruЯure,as to move the Tail with a fufficient Strength; even fo far, that the Violence which the larger kind of Fishes, fuch as Whales, exert therewith, is fo terrible, that one can hardly read the Accounts thereof without being amaz'd?

SECT. XXXII, The Ufe of the Fins.

BUT forafmuch as in all Bodies that float in Water, the heaviest Part always tends downwards, according to the Laws of Hydrostaticks, would it not likewife follow from hence, that fince the Backs of Fishes, quite contrary to thofe of Birds, are the heaviest Part of their Body, they must always turn their Bellies upwards in the Water, as it is commonly obferved to happen in dead and floating Fish, fince their Bladder cannot be then compreffed, but the Air being dilated therein, makes the Fifh float and turn its Belly upwards, the Back being not only heavier, but the Belly al

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fo lighter by fuch Expansion of the Bladder than when the Fishes were alive.

Can it then be imagined that the Wisdom of the Creator did not foresee this in the Forming of Fishes, to which he has given two Fins under their Belly, by which they fupport themselves upon the Water, and by giving them the Faculty of Swimming whilft alive with their Bellies downwards! of which we may find an accurate Examination in Prop. CXIII. of Borelli, who having cut off all the Fins under the Belly of a Fifh, and in that Condition thrown it into the Water again, found it continually staggering on one fide or t'other, without being able to fupport itself in the natural and common Pofition of Fishes.

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But befides this, to the end that the Fifhes might be provided of every thing that is neceffary for them towards Swimming, it seemed to be ftill wanting, that they fhould be able conveniently to stop that Progrefs which they had acquired by their Tails, and to be able to turn to the Right or to the Left in their Course, neither of which could be done by the Tail but with great Trouble. For this Purpose we find the Fifhes provided with two Fins on the Sides, by which, when they extend 'em both together against the Water, their Motion may be ftopt; and if they ftretch out one and keep the other clofe, they may turn to that fide whence the Fin is difplayed; juft as we see happen in a Boat which turns to that fide where one Oar is thruft out in the Water to ftop its Progrefs.

SECT. XXXIII. Creatures that live in the Air fee confufedly in the Water.

IN Cafe this does not yet fuffice to convince a Sceptick that there is a GoD propofing to himself

a wife

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