IV.

than that of Jupiter. This feems to establish S ECT. it as a law in the system, that the nearer the feveral Planets approach to the Sun, the denfity of their matter is the greater: a constitution of things which would feem to be the most advantageous of any that could have been established; as water of the fame density with that of our Earth, would freeze under the Equator of Saturn, and boil under that of Mercury.

Such is the fyftem of Sir Ifaac Newton, a fyftem whofe parts are all more strictly connected together, than those of any other philofophical hypothefis. Allow his principle, the univerfality of gravity, and that it decreases as the fquares of the distance increase, and all the appearances, which he joins together by it, neceffarily follow. Neither is their connection merely a general and loose connection, as that of most other fyftems, in which either thefe appearances, or fome fuch like appearances, might indifferently have been expected. It is every where the most precife and particular that can be imagined, and ascertains the time, the place, the quantity, the duration of each individual phænomenon, to be exactly fuch as, by obfervation, they have been determined to be. Neither are the principles of union, which it employs, fuch as the imagination can find any difficulty in going along with. The gravity of matter is, of all its

qualities,

SECT qualities, after its inertnefs, that which is most IV. familiar to us. We never act upon it without having occafion to obferve this property. The law too, by which it is fuppofed to diminish as it recedes from its centre, is the fame which takes place in all other qualities which are propagated in rays from a centre, in light, and in every thing else of the fame kind. It is fuch, that we not only find that it does take place in all fuch qualities, but we are neceffarily determined to conceive that, from the nature of the thing, it must take place. The oppofition which was made in France, and in fome other foreign nations, to the prevalence of this fyftem, did not arife from any difficulty which mankind naturally felt in conceiving gravity as an original and primary mover in the conftitution of the univerfe. The Cartefian fyftem, which had prevailed fo generally before it, had accustomed mankind to conceive motion as never beginning, but in confequence of impulfe, and had connected the defcent of heavy bodies, near the furface of the Earth, and the other Planets, by this more general bond of union; and it was the attachment the world had conceived for this account of things, which indisposed them to that of Sir Ifaac Newton. His fyftem, however, now prevails over all oppofition, and has advanced to the acquifition of the most universal empire that was ever established in philofophy. His prin

ciples,

.IV.

ciples, it must be acknowledged, have a degree S ECT.
of firmnefs and folidity that we should in vain
look for in any other fyftem. The most scep-
tical cannot avoid feeling this. They not only
connect together most perfectly all the phæ
nomena of the Heavens, which had been
obferved before his time; but those also which
the perfevering industry and more perfect
inftruments of later Aftronomers have made
known to us have been either easily and
immediately explained by the application of
his principles, or have been explained in con-
fequence of more laborious and accurate cal-
culations from these principles, than had been
inflituted before. And even we, while we
have been endeavouring to reprefent all phi-
lofophical fyftems as mere inventions of the
imagination, to connect together the otherwise
disjointed and difcordant phænomena of na-
ture, have infenfibly been drawn in, to make
ufe of language expreffing the connecting
principles of this one, as if they were the real
chains which Nature makes ufe of to bind
together her feveral operations.
Can we

wonder then, that it should have gained the
general and complete approbation of mankind,
and that it fhould now be confidered, not as
an attempt to connect in the imagination the
phænomena of the Heavens, but as the greatest
difcovery that ever was made by man, the
discovery of an immenfe chain of the most

L

IV.

SE C T. important and fublime truths, all clofely connected together, by one capital fact, of the reality of which we have daily experience.

NOTE by the EDITORS.

The Author, at the end of this Effay, left fome Notes and Memorandums, from which it appears, that he confidered this last part of his History of Aftronomy as imperfect, and needing feveral additions. The Editors, however, chofe rather to publish than fupprefs it. It must be viewed, not as a History or Account of Sir Ifaac Newton's Aftronomy, but chiefly as an additional illuftration of those Principles in the Human Mind which Mr. Smith has pointed out to be the univerfal motives of Philofophical Researches.

THE

PRINCIPLES

WHICH LEAD AND DIRECT

PHILOSOPHICAL ENQUIRIES;

ILLUSTRATED BY THE

HISTORY OF THE ANCIENT PHYSICS.