* Sir Ifaac Newton, indeed, lays it down, as one of his regula philofophandi, that the qualities of natural bodies which cannot be

increased or diminished, and agree to all bodies in which experi⚫ments can be made, are to be reckoned as the qualities of all bodies whatsoever. Thus, becaufe extenfion, divifibility, hardness, impenetrability, mobility, the vis inertia, and gravity, are found in all bodies which fall under our cognizance or infpection, we may justly conclude they belong to all bodies whatfoever, and are, therefore, to be efteemed the original and univerfal properties of all natural bodies.'

without diftinction. Every body that could come under their 443 examination, in the way of mechanical experiment, was, doubtless, poffeffed of that power; but it did not thence follow, that all matter, or the primary impenetrable folids, of which fuch bodies were fuppofed to be compounded, would be fo too: nor has it yet appeared from experiment, that the vis inertia of compound bodies is in all circumstances the fame, or always directly proportionable to their quantity of matter*. But to return to Mr. 'fGravefande. In the year 1715,

may

This rule, however, is a mere ipfe dixit. For fure there properties of whofe increase or decrease we are ignorant, that are yet the effect of a combination of elements, or fmaller bodies; and, be therefore are not the properties of thofe elements or bodies themfelves. But, fuppofing the above rule to be just, we do not know that it has been ever demonftrated, that the vis inertia of bodies, or of any certain quantity of matter, will not admit of increase and decrease. On the contrary, Sir Ifaac Newton has himfelf demonftrated, that if a certain quantity of matter were particularly modified, and put in a certain manner in motion, its velocity would alternately diminish and increase, altho' follicited by no external force whatever. Now, the vis inertia being that power with which bodies endeavour to persevere in their prefent ftate, either of motion or reft, it is plain that power muft, in the cafe fuppofed by Sir Ifaac Newton, admit of an alternate increase and decreafe. For, while the fame, or no, refiftance fhould be made to the moving body, how could it go fafter or flower at one time than at another, unless the power of preferving its prefent state of motion were altered?

The fuppofition of that great philofopher is this: if two bodies were made to revolve round one common centre, and that centre be carried forward in a right line, the whole will move failer, when the revolving bodies move toward the line of direction, than when they move from it. Undoubtedly they will: and two bodies, fo united to one common centre, may well be confidered as parts of one compound body, whofe is inertie will thereby admit of increafe and diminution, For, fuppofe the revolution of these bodies round their centre, fo quick, as not to be fenfible to experiment, would not they apparently compofe a circular body, or hoop; which

would

1715, he was appointed Secretary to the embafly on which Baron Waffenaar and Mr. Van Borfelle were fent to England by the States-General, to felicitate King George the firft on his acceffion to the throne.

would move alternately fafter and flower? and, at the fame time, vary it's form into an ellipfis, whofe longest axis would be fome times in one direction, and fometimes in another?

Now, if the vis inertia of compound bodies, does not depend on the number and magnitude of their component parts, it must arife either from the difpofition, or motion of thofe parts. If merely from their difpofition, thofe parts would refift being difplaced, in proportion to the fquare of the velocity of the body, endeavouring to pafs through them: and this they would do too, notwithflanding any other reliftance which might arife from their motion. Hence, the impreffions made by falling bodies on clay, and other fubftances of little tenacity, would always follow fome proportion, nearly as the quantity of matter in the falling body multiplied into the square of the velocity.

It appears, nevertheless, that the force, or momentum, of the falling body, fuppofing it a perfect folid, fhould be, as Sir Ifaac Newton affirmed, viz. as the mafs fimply multiplied into the velocity. In the motion of compound bodies, however; as their wis inertice depends either on the difpofition or motion of their parts, or both, fo its quantity of force muft confift of the fum of the refiftance of all the parts; and the momentum of such compound bodies, of that fum multiplied into the velocity of the whole; and not fimp ly of the quantity, or number and magnitude of those parts fo multiplied.

We fhall here add further, (as in fome degree refpecting the matter in question) that the argument of Sir Ifaac Newton, founded on the fuppofition above-mentioned, does not, in our opinion, anfwer the purpofe, for which we have feen it often quoted, viz. to demonftrate, that the quantity of motion in nature is not always the fame, but may be abfolutely increased and diminiflied.

In the falling of bodies, we know, motion is communicated by the power of gravity to fuch bodies, in the time of their defcent: but we will venture to fay, the caufe of gravity will admit of mechanical explication; fo that no motion is hereby abfolutely generated. And, as to the fuppofed proof in question; though it be certain that, when the revolving bodies tend toward the direction of their common centre, they will both move fafter forward in that direction than before, yet it is certain too that those bodies will not, during the fame time, move fo falt round their center, as when they are in the other part of their revolution: fo that the velocity which is gained in the whole, is loft in the velocity of the parts, and the momentum of the whole, is neither increased by the quicker motion, nor decreased by the flower.

On

On his arrival in London, he renewed his intimacy with feveral men of letters, whom he had known in Holland; and became acquainted with many others of the first repute. But the friendship he most affiduously cultivated, was with Sir Ifaac Newton, for whom he had a particular veneration and efteem.

During his stay in England, he was admitted Member of. the Royal Society; and, while employed in his office of Secretary, is faid to have acquired an amazing facility of thinking, and writing, on the moft profound fubjects, and of making the most abftrufe and difficult calculations, in the midft of a numerous and noify affembly, without being in the leaft disturbed or affected.

The business of the embaffy being over, Mr. 'fGravefande returned to Holland, and was chofen, about a year afterwards, Profeffor of Mathematics and Aftronomy at Leyden. At that time the Newtonian Philofophy was in its infancy, and our Profeffor had an opportunity of reaping great honour, as one of the firft who publicly taught it in the fchools abroad.

In the year 1721, the Landgrave of Heffe Caffel invited him to his court, in order to confult him about certain mechanical engines he had thoughts of erecting. Here Mt. 'Gravefande faw the famous Wheel of Orfyreus, but without being able to decide whether it was a Perpetual Motion

or not.

This confeffion, which he made to Sir Ifaac Newton and others, being made public, drew on him the clamours of almoft all thofe mathematicians, who had already conceived the impoffibility of a perpetual motion to be demonstrable. Their demonftrations, however, had not convinced our profeffor, who treated them only as pretenfions to a certainty,. unattainable in mechanics.

Indeed, what Mr. 'fGravefande was an eye-witnefs of, as to that machine, was fufficient to furprize the moft profound reafoner: and his teftimony in favour of the inventor, redounds much to the credit of the latter; at least, it effectually puts to filence the reproaches made him on account of the depofition of his fervant, who fwore that the herfelf, ftanding in another room, turned the machine; the impoffibility of which, is fufficiently attefted by our judicious profellor *.

The

It is furprizing that, during forty years paft, no one should hit pon, and publiin," the expedient by which the wheel of Crfyrcus

continued

6

The most confiderable of all Mr. 'Gravefande's publications, is his introduction to the Newtonian philosophy, or a treatife on the elements of phyfics, confirmed by experiments. This performance, being only a more perfect copy of his public lectures, was first printed in the year 1720; and hath fince gone through many editions, with confiderable improvements. He obliged the public, alfo, with a small treatife on the elements of Algebra, calculated for the use of young ftudents; and, on being promoted to the chair of philofophy, in 1734, publifhed, foon after, a course of logic and metaphyfics, which, for method and perfpicuity, is perhaps inferior to none. This laft work, gave much offence to the advocates for man's free-agency, on account of what the author had advanced, in the eleventh chapter of his metaphyfics, regarding human liberty: nor did it lefs affect the zeal of many ignorant divines; who, making no

*

continued it's motion. The ableft mechanics, in general, fit down contented with the fuppofition of it's being an impofture: but, from the evidence of facts, attefted by fuch as we have abundant reafon to think, could not be dupes to any artifice practiced in that inftance, it appears, that the principle of it's motion was some how contained in the wheel; although we do not pretend to conceive, in what manner it pollibly could be fo.

It is true, the maid-fervant of Orfyreus depofed, that she, or her companion, kept the machine conftantly in motion: but the most accurate fcrutiny into the conftruction of the external part of the machine, proved this to be abfolutely impoffible; the axes having no manner of communication with any other other room, or distant object.

That the fecret was loft, and the inventor rendered contemptible, is, however, certain: but this was probably more owing to the extreme oddity of the man, than to any deceit in the machine itself. In the first place, it feems, he was whimfical and preverse to the laft degree; and, in the next, highly conceited, and profoundly ignorant. He broke his machine to piece, merely because Mr. 'Gravefande made that minute examination, which is the greatest testimony in it's favour. And, when accufed by his fervant of having employed her to turn his wheel, and exacting from her a terrible oath to oblige her to fecrefy, he refufed to exculpate hintfelf by making another of the fame kind; but fupported the obloquy, under which he was fallen, with obftinacy.

It is not impoffible, however, but the depofition of his fervant might have been brought about by perfons, who wanted to penetrate his fecret; and that, knowing this, and defpairing of obtaining his demand of 20,000l. as a recompente for the fecret, he refolved to give them no farther opportunity of stealing it, by expofing another machine to fuch curious enquirers,

fect

447

diftinction between a moral and a mechanical neceffity, unadvifedly accufed him of favouring the doctrines of Hobbes and Spinofa. No one, however, could harbour fentiments more contrary to fatalism than Mr. 'Gravefande, or be more ready, on all occafions, to avow principles diametrically oppofite.

Befides the pieces of his own compofition, published by this learned man, the public are obliged to him for feveral correct editions of the valuable works of others: and, had not death prevented his putting a moft excellent defign in execution, might have been much more fo, for a system of .Morality which he intended to have published.

As a Citizen, we shall find few men of letters that have done more service to their country than himself: having hardly, quitted the college before his known abilities in calculation recommending him to the notice of the Minifters of the Republic, he was confulted on all thofe occafions wherein his talents were requifite to affift them, in raising money for the ufe of the State. As a Decypherer alfo, he was frequently ferviceable in the detection of the fecret correfpondence of their enemies: while in his capacity of Profeffor of the Mechanic Arts, perhaps, no one was ever more fuccessful

applying the powers of Nature to the purposes of occonomical

improvement.

If we add to this, that Mr. 'fGravefande was equally amiable in his private, as refpectable in his public character, we fhall close this sketch of his History with an eulogium, to which it were to be wifhed many others of equal genius and abilities had as juft pretenfions.

K=n=K

MONTHLY CATALOGUE.

For NOVEMBER, 1759.
MISCELLANEOUS.

Art. 1. An Anfwer to a Letter to a late noble Commander of the
British Forces. In which the Candour is proved to be affected,
-the Facts untrue, the Arguments delufive, and the Defign ini-
quitous. 8vo. IS. Owen.

ΤΗ

HIS boafting title-page is, to the difcerning Reader, a fufficient intimation of the merits of the pamphlet. Writers who ridiculously undertake to prove too much, are feldom able to make good a fingle propofition. Thefe Empirics in Letters, are like Quacks in Phyfic; they make a fenfelefs parade of their talents, which they