T.Harvey
E LE MĘ NIS

THE

Ε Ν Τ

The ERRORS by which Theon, or others, have long ago

vitiated these Books, are CORRECTED,

And fome of EUCLID'S DÉMONSTRATIONS are Reftored.

ALSO

THE BOOK OF

EUCLID'S DA TA,

In like Manner CORRECTED.

By ROBERT SIMSON, M. D..
Emeritus Professor of Mathematics in the University of Glasgow.

To this TWELFTH EDITION are also annexed

ELEMENTS OF PLAIN AND SPHERICAL TRIGONOMETRY.

LONDON:

Printed for F. WINGRAVE, in the Strand, Successor to Mr. NOURSE.

1804.

PREF A CE.

THE opinions of the moderns concerning the author of the

Elements of Geometry, which go under Euclid's name, are very different and contrary to one another. Peter Ramus ascribes the Propositions, as well as their Demonstrations, to Theon; others think the Propofitions to be Euclid's, but that the Demonstrations are Theon's; and others maintain, that all the Propofitions and their Demonstrations are Euclid's own. John Buteo and Sir Henry Savile are the authors of greatest note who affert this laft; and the greater part of geometers have ever since been of this opinion, as they thought it the most probable. Sir Henry Savile, after the several arguments he brings to prove it, makes this conclusion (Page 13. Prælect.) “ That, excepting a very few interpolations, explications, and ço additions, Theon altered nothing in Euclid.” But, by often considering and comparing together the Definitions and Demonstrations as they are in the Greek editions we now have, I found that Theon, or whoever was the editor of the present Greek text, by adding some things, suppressing others, and mixing his own with Euclid's Demonstrations, had changed more things to the worse than is commonly supposed, and those not of small moment, especially in the fifth and eleventh Books of the Elements, which this editor has greatly vitiated; for instance, by substituting a shorter, but insufficient Demonftration of the 18th Prop. of the 5th Book, in place of the legitimate one which Euclid had given; and by taking out of this Book, besides other things, the good definition which Eudoxus or Euclid had given of compound ratio, and giving an absurd one in place of it in the 5th Definition of the 6th Book, which neither Euclid, Archimedes, Apollonius, nor any geometer before Theon's time, ever made use of, and of which there is not to be found the least appearance in any of their writings; and, as this Definition did much embarrass beginners, and is quite useless, it is now thrown out of the Elements, and another, which, without doubt, Euclid had given, is put in its proper place among the Definitions of the

5th