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EUCLID'S DATA.

PREFACE.

Euclid's Data is the first in order of the books written by the ancient geometers to facilitate and promote the method of resolution or analysis. In the general, a thing is said to be given which is either actually exhibited, or can be found out, that is, which is either known by hypothesis, or can be demonstrated to be known; and the Propositions in the Book of Euclid's Data show what things can be found out or known from those that by hypothesis are already known; so that in the analysis or investigation of a problem, from the things that are laid down to be known or given, by the help of these propositions other things are demonstrated to be given, and from these, other things are again shown to be given, and so on, until that which was proposed to be found out in the problem is demonstrated to be given; and when this is done, the problem is solved, and its composition is made and derived from the compositions of the Data which were made use of in the analysis. And thus the Data of Euclid are of the most general and necessary use in the solution of problems of every kind.

Euclid is reckoned to be author of the Book of the Data, both by the ancient and modern geometers; and there seems to be no doubt of his having written a book on this subject, but which, in the course of so many ages, has been much vitiated by unskilful editors in several places, both in the order of the propositions, and in the definitions and demonstrations themselves. To correct the errors which are now found in it, and bring it nearer to the accuracy with which it was, no doubt, at first written by Euclid, is the design of this edition, that so it may be rendered more useful to geometers, at least to beginners who desire to learn the investigatory method of the ancients. And for their sakes, the compositions of most of the Data are subjoined to their demonstrations, that the compositions of problems solved by help of the Data may be the more easily made.

Marinus, the philosopher's, preface, which, in the Greek edition, is prefixed to the Data, is here left out, as being of no use to understand them. At the end of it, he says, that Euclid has not used the synthetical, but the analytical method in delivering them; in which he is quite mistaken; for in the analysis of a theorem, the thing to be demonstrated is assumed in the analysis ; but in the demonstrations of the Data, the thing to be demonstrated, which is, that something or other is given, is never once assumed in the demonstration, from which it is manifest, that every one of them is demonstrated synthetically; though indeed, if a proposition of the Data be turned into a problem, for example, the 84th or 85th in the former editions, which here are the 85th and 86th, the demonstration of the proposition becomes the analysis of the problem.

WHEREIN this edition differs from the Greek, and the reasons of the alterations from it, will be shown in the notes at the end of the Data.

DEFINITIONS.

1.

Spaces, lines, and angles, are said to be given in magnitude, when equals to them can be found.

II. . A ratio is said to be given, when a ratio of a given magnitude

to a given magnitude which is the same ratio with it can be found.

III. Rectilineal figures are said to be given in species, which have each of their angles given, and the ratios of their sides given.

IV. Points, lines, and spaces, are said to be given in position, which

have always the same situation, and which are either actually exhibited or can be found.

A. An angle is said to be given in position, which is contained by

straight lines given in position.

V.

A circle is said to be given in magnitude, when a straight line from its centre to the circumference is given in magnitude.

VI. A circle is said to be given in position and magnitude, the centre

of which is given in position, and a straight line from it to the circumference is given in magnitude.

VII. Segments of circles are said to be given in magnitude, when the angles in them, and their bases, are given in magnitude.

VIII. Segments of circles are said to be given in position and magni

tude, when the angles in them are given in magnitude, and their bases are given both in position and magnitude.

IX. A magnitude is said to be greater than another by a given

magnitude when this given magnitude being taken from it, the remainder is equal to the other magnitude.

X.
A magnitude is said to be less than another by a given mag-

nitude, when this given magnitude being added to it, the

whole is equal to the other magnitude. 1.

PROPOSITION I. See N. The ratio of given magnitudes to one another is

given.

Let A, B be two given magnitudes, the ratio of A to B is given.

Because A is a given magnitude, there may a 1. def. a be found one equal to it; let this be C: And dat. because B is given, one equal to it may be

found; let it be D; and since A is equal to b 7. 5. C, and B to D; therefore 6 A is to B, as

C to D; and consequently the ratio of A to
B is given, because the ratio of the given
magnitudes C, D, which is the same with it,
has been found.

A B C D

PROP. II.

See N.

If a given magnitude has a given ratio to another magnitude, and if to the two magnitudes by which the given ratio is exhibited, and the given magnitude, a fourth proportional can be found ;" the other magnitude is given.

Let the given magnitude A have a given ratio to the magnitude B; if a fourth proportional can be found to the three magnitudes above named, B is given in magnitude.

Because A is given, a magnitude may be found equal to it a ; let this be C: And because the ratio of A to B is given, a ratio which is the same with it may be found; let this be the ratio of the given magnitude E A B C D to the given magnitude F: To the magni EF tudes E, F, C, find a fourth proportional D, which, by the hypothesis, can be done. Wherefore, because A is to B, as E to F; and as E to F, so is C to D; A is to B, as C to

a 1. der,

which is he ratio of Act this be cole may be

1

b 11.5.

• The figures in the margin show the number of the proposition in the other editions.

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