CONTENTS. 



CHAP. VI. 



Proved in the preceding Chapters, that Mind is the only Caufe of Motion m the Hea- 

 vens Minti very properly typified by i^;V^— Proved alfo that the Motion of the Celef- 



tial Bodies is not compounded h\xi /mple — The Purpofe of this Chapter to fliow that 

 Sir Ifaac's Syftem of Aftronomy can be fupported without arbitrary or impofftble Sup- 

 fo/itions— Prejudice removed that may arife from the Author's not being learned in 

 Geometry or Mechanics — No inferior Science demonftratcs its own Principles — Thefe 

 to be found only in Metaphy/tcs, or the Firji Philofaphy— Euclid's Geometry an Ex- 

 ample of this — The fame is true of Sir Tfaac Newton's Aftronomy — The Laws of the 

 Planetary Motion difcovered by Sir Ifaac Newton — Before him Aftronomy no Science, 

 but only a Colledion of Fails— h curious FaiSt difcovered by Kepler, concerning the 

 Proportions betwixt the Periods and Dijlances of the Planets— T^\% he knew only as a 

 Fa£i ; but Sir Ifaac has made a Science of it — No Science of any Thing, if an eflential 

 Property be not known, from which all its other Properties can be deduced — The 

 Nature and Eflence of every Motion confifts of two Things, the Motive Force and 

 the Direaion — The Dire^ion of the Planetary Motion is a Fadl that is known— No- 

 thing can be difcovered of the Planetary Morion, or of any Thing in Nature, except 

 from Fails — The Bufinefs of Aftronomy is not to inquire into the Caufe of the Pla- 

 netary Motion, but to calculr.te that Motion, and to difcover i's Laws — This the No- 

 tion which Sir Ifaac himfelf had of this Science — Diftinflion betwixt Metaphyfics, 

 Phyfics, and Ajironomy— Forces or Powers are latent Things, to be difcovered only by 

 their EffeSis — The EfFedl of a Moving Force is Vehcity — That not abfolute, but re- 

 lative to two Things, Time and Space— \i the Motion be equable in a Straight Line, 

 and the Time of it be known, the Law of the Motion is obvious — But if the Motion 

 be in a Curve and not equable, though the Time be knoiun, the Lawoflhe AJotion not 

 eafy to be difcovered ~T\\e Force muft be eftimated by a Motion in a Straight Line, one 

 or rr.orc — But how apply Motion in a Straight Line to a Curve? — This impoflible to be 

 done, if no Straight Line could be difcovered neceflarily refulting from the Nature of 

 the Motion — But fuch a Straight Line has been obferved in the Defcent of the Planet 

 from its Tangent — This Line obferved with refped to the Moon's Defcent from her 

 Tantjent— The Length of this Line known — The fame as if the Moon had dcfcended 

 direftly from the Tangential Point — The Law of this Motion of Defcent would be 

 known, if the Motion was equable — but the Motion is unequable — For difcovering the 

 Lawr of \\\\% unequable Motion, recourfe muft be had to a fimilar Motion on Earth, the 

 Motion of the Projciiile — Ttie Law of the Defcent of the Projefled Body afcertained, 

 firft by Galileo's Difcoveries, and then by Sir Ifaac's — Sir Ifaac firft difcovered that 

 the D'-fcent of falling Bodies was both begun and accelerated in the Ratio of the Dif- 

 tatice from the Centre inverfely— This could not have been difcovered by Fait and 



Obfervation 



