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them feem less eafy to be accounted for: fuch are joyns, joynt, joyntly, enjoyned; fet, for fit; fetting, for fitting; complane, difpare, waive.
Upon the whole, were we to judge of this piece by the Horatian standard, we fhould be obliged to conclude, that altho' it every where abounds in the Utilé, as may be seen by this small specimen of it; yet it hath very little in it of the Dulcé.
Aftronomy explained upon Sir Ifaac Newton's Principles, and made easy to those who have not ftudied Mathematics. By James Ferguson. Sold by the Author, at the Globe, oppofite Cecil-ftreet in the Strand. 4to. 15s.
HERE is fcarce any study that seems better calculated to enlarge the mind, to raise it above mean and vulgar prejudices, and to fill it with fentiments of the most profound reverence towards the ORIGINAL PARENT MIND, the Father of the Univerfe, the Ever-flowing Fountain of Good, than the study of Aftronomy. Whoever, therefore, employs his pen in conveying fome general knowlege of this useful branch of science, to thofe who are unacquainted with mathematical calculations, and who have neither leifure nor capacity to tread the dry and intricate paths of Geometry, certainly deferves the thanks of the public. Among the vaft numbers, indeed, of those who are engaged in literary purfuits, there are but few who are qualified to treat fubjects of this kind in an eafy and familiar manner; to ftrip them of that stiff and uncouth drefs in which they have generally made their appearance, and to bring them down to the level of vulgar capacities.
The Author of the work now under confideration appears to be very well qualified for the task he has undertaken; his ideas feem to be clear and diftinct; his language is eafy and perfpicuous; and his illuftrations are ingenious and pertinent. He has divided his performance into fixteen chapters, the first of which contains a brief defcription of the Solar System, the truth of which is demonftrated in the fecond; and the appearances refulting from the earth's motion defcribed. In treating of the earth's motion, he endeavours to illuftrate it in the following manner.
'Let us imagine,' fays he, a prodigious large room of a round form, all hung with pictures of men, women, birds,
beafts, and fifhes; the floor covered with water deep enough to carry a boat with a person fitting ftill in it; and that there is a great taper burning in the midst of the room, the flame "being of equal height with the perfon's head from the water. If a diver under the boat, unknown to, and unperceiv ed by, this perfon, fhould turn it gently and equally round and round, as on an axis, giving it at the fame time a flow progreffive motion round the taper, the fame way, but so as to turn it three hundred and fixty-five times round its axis ' while it went once round the taper; to the person in the ⚫ boat the whole room and taper would feem to go round the 6 contrary way every time the boat turned round; the flame 'would appear to change its place gradually among the pictures, fo as to make a tour round the room among them in · every revolution of the boat round the taper. And in that time the observer would be turned so much fooner towards any particular picture than to the taper, in each turning of the boat, that the whole room and pictures would seem to go once more round him than the taper did. The application is obvious, if we imagine the pictured room to repre<fent the visible heavens fet all round with stars ranged in dif⚫ferent conftellations; the taper the fun, and the boat the "earth.'
In the third chapter he refutes the Ptolomaic fyftem; and explains briefly the motions and phafes of Mercury and Venus. In the fourth he treats of the phyfical causes of the motions of the planets, of the excentricities of their orbits, the times in which the action of gravity would bring them to the fun, and of the ideal problem of Archimedes for moving the earth.
The quick motions of the moons of Jupiter and Saturn round their primaries, demonftrate, he tells us, that these two planets have stronger attractive powers than the earth. For, fays he, the stronger that one body attracts another, the greater must be the projectile force, and confequently the quicker must be the motion of that other body, to keep it from falling to its primary or central planet. Jupiter's fecond moon is one hundred and twenty-four thousand miles farther from Jupiter than our moon is from us; and yet this • fecond moon goes more than eight times round Jupiter whilft our moon goes only once round the earth. What a ⚫ prodigious attractive power muft the fun then have, to draw all the planets and fatellites of the fyftem towards him; and ⚫ what an amazing strength must it have required at first, to put all thefe planets and moons in motion! Amazing to
"us, because impoffible to be effected by the ftrength of all the people in an unlimited number of worlds, as will ap pear by the following article; but it is nothing to the Almighty, whofe Planetarium takes in the whole universe.
It is reported of Archimedes, (falfely I believe) that he faid he could move the earth, if he had any place at a diftance from it to fix a prop for his lever. Now, fuppofe a man could prefs upon the end of a lever the force of two hundred pounds, and that the weight of the earth be 399,784,700,118,074,464,789,750; if we imagine the earth to be placed at one end of the lever, at the distance of fix thoufand miles from the prop or center of motion, then muft the perfon or power be applied to the other end of the lever, at the diftance of 11,993,541,003,542,233,943, 692,500 miles from the earth to fuftain it; which is 15,569, 745,951,035,731 times the mean distance of Saturn from the earth. And, to raise the earth but one mile, the power muft move through the space of 1,998,923,500,590,322, 323,948 miles: confequently, if Archimedes, or the pow er, could move as fwift as a cannon-ball, i. e. four hundred and eighty miles every hour, he would require 44,963, 540,000,000 years to raise the earth one inch,'
Our Author proceeds now to offer fome reflections upon gravity.
The fun and planets mutually attract each other: the power by which they do fo, we call Gravity. But whether this power be mechanical or not, is very much disputed. We are certain that the planets difturb one another's motions by it, and that it decreafes according to the fquares of the distances of the fun and planets; as light, which is ⚫ known to be material, likewife does. Hence Gravity fhould feem to arife from the agency of fome subtile matter ifluing from the fun and planets, and acting like all mechanical caufes by contact. But, on the other hand, when we confider, that the degree of it is exactly in proportion to the quantities of matter in thofe bodies, without any regard to their bulks or quantity of furface, acting as freely on their internal as external parts, it feems to furpafs the power of mechanifm; and to be either the immediate agency of the Deity, or effected by a law originally eftablished and imprefied on all matter by him. But fome affirm, that matter being altogether inert, cannot be impreffed with any law, even by Almighty Power; and that the Deity must therefore be conftantly impelling the planets towards the fun, and moving them with the fame irregularities and difturb
ances which Gravity would caufe, if it could be fupposed to exift. But, if a perfon may venture to publish his own thoughts, (and why should not one as well as another?) it feems to me no greater abfurdity, to fuppofe the Deity capable of fuperadding a law, or what law he pleafes, to matter, than to fuppofe him capable of giving it exiftence at first. The manner of both is equally inconceivable to but neither of them imply a contradiction in our ideas; and what implies no contradiction, is within the power of Omnipotence. Do we not fee that a human creature can prepare a bar of fteel, fo as to make it attract needles and filings of iron; and that he can put a stop to, and again ⚫ call forth that power or virtue as often as he pleases? To fay that the workman infufes any new power into the bar, is faying too much; fince the needle and filings to which he has done nothing, re-attract the bar. And from this it ap6 pears, that the power was originally impreft on the matter of which the bar, needle, and filings are compofed; but -does not feem to act until the bar be properly prepared by "the artificer: fomewhat like a rope coiled up in a fhip, ⚫ which will never draw a boat, or any other thing, towards her, unless one end be tied to her and the other end to that which is wanted to be hauled up; and then it is no matter 'which end of the rope the failors pull at, for it will be 6 equally ftretched throughout, and the fhip and boat will • move towards one another. To fay that the Almighty has • infused no fuch virtue or power into the materials which compofe the bar, but that he waits till the operator be pleafed to prepare it by due pofition and friction; and then, when the needle or filings are brought pretty near the bar, the Deity preffes them towards it, and withdraws his hand whenever the workman, either for ufe, fancy, or whim, does what appears to him to destroy the action of the bar, • feems quite ridiculous and trifling; as it fuppofes God to do what would be below the dignity of any rational man to be employed about.
That the projectile force was at firft given by the Deity, is evident. For, fince matter can never put itself into motion, and all bodies may be moved in any direction whatfo< ever; and yet all the planets, both primary and fecondary, ❝ move from weft to eaft, in planes nearly coincident; whilst ⚫ the comets mové in all directions, and in planes fo different ⚫ from one another, thefe motions can be owing to no me⚫chanical caufe or neceffity, but to the free choice and power ⚫ of an intelligent Being. • Whatever
Whatever Gravity be, it is plain that it acts every mo ment of time for fhould its action cease, the projectile 'force would that very moment carry off the planets in ftraight lines from those parts of their orbits where Gravity left them. But, being once put into motion, there is no occafion for any new projectile force, unless they meet with ⚫ fome refiftance in their orbits; nor for any mending hand, • unless they disturb one another by their mutual attractions.
It is found, that there are difturbances among the planets motions, arifing from their mutual attractions, when in the fame quarter of the heavens: and that our years are not always precifely of the fame length. Befides, there is reafort ‹ to believe that the moon is fomewhat nearer the earth now ⚫ than fhe was formerly; her periodical month being shorter ⚫ than it was in former ages. For, our Aftronomical Tables, which, in the prefent age, fhew the times of folar and lu nar eclipfes to great precifion, do not answer fo well for · very ancient eclipfes. Hence it appears, that the moon does ❝ not move in a medium void of all refiftance; and therefore •her projectile force being a little weakened, whilft there is nothing to diminish her Gravity, the must be gradually approaching nearer the earth, defcribing lefs circles round it in every revolution, and finishing her period fooner, although her abfolute motion, with regard to space, be not fo quick as formerly. Hence, fhe must come to the earth at laft; unless that Being, which gave her a fufficient projectile force at first, adds a little more to it in due time. And, as all the planets move in spaces full of æther and light, which are material fubftances, they too must meet with fome refiftance. And, therefore, if their gravities ⚫ are neither diminished, nor their projectile forces increased, they must neceffarily approach nearer and nearer the fun; and at length fall upon and unite with him.
Here we have a ftrong philofophical argument against the eternity of the world. For, had it exifted from "eternity, and been left by the Deity to be governed by the combined actions of the above forces or powers, generally called laws, it had been at an end long ago. And if it be left to them; it must come to an end. But we may be certain, that it will last as long as intended by its Author, who ought no more to be found fault with for framing fo perishable a work; ⚫ than for not making our bodies immortal.'
In the fifth chapter our Author treats of light; its propor tional quantities on the different planets; its refractions in wa ter and air: the Atmof here, its weight and properties: the