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 sake, 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 shewn in the notes at the end of the Data.

esanigad of janal 28 470 me

To bodtem motrat Suomust of slash ody enqmos stad indi not be

tons odr Tor of baniojde DEFINITIONS.' Mom to waoit For to enolfieon

9tont of ad vam silts I I.

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

A ratio is said to be given, when a ratio of a given magnitude to a given magnitude which is the same I ratio with it can be found.

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

IV.

Points, lines, and spaces, are said to be given in posi tion, which have always the same situation, and which. are either actually exhibited, or can be found. ofgmazs zo) meldongA.

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.

$1.

See N.

*1 Def.

Dat.

*7.5.

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 magnitude, 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 magnitude, when this given magnitude being added to it, the whole is equal to the other magnitude.

PROPOSITION I.

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
found one equal to it; let this be C:
and because B is given, one equal to
it may be found; let it be D: and
since A is equal to C, and B to D:
therefore* 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.

* be

A B C D

The figures in the margin shew the number of propositions in the other editions.

If a given magnitude have a given ratio to another magni- See N. tude, "and if unto 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.

A B C D

E F

*

1 Def.

Because A is given, a magnitude may be found equal to it *; 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 to the given magnitude F: unto the magnitudes 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 D. But A is equal to C: therefore * B, *14. 5. is equal to D. The magnitude B is therefore given* *1 Def. because a magnitude D equal to it has been found. The limitation within the inverted commas is not in the Greek text, but is now necessarily added; and the same must be understood in all the propositions of the book which depend upon this second proposition, where it is not expressly mentioned. See the note upon it.

PROP. III.

If any given magnitudes be added together, their sum shall be given.

Let any given magnitudes AB, BC, be added together, their sum AC is given.

Because AB is given, a magnitude equal to it may

be found; let this be DE: and because BC is given, one equal to it may be found; let this be EF: wherefore because AB is equal to DE, and BC equal to

*11.5.

*

1 Def.

A

B

C

E

F

D

EF; the whole AC is equal to the whole DF; AC is therefore given, because DF has been found which is equal to it.

PROP. IV.

If a given magnitude be taken from a given magnitude; the remaining magnitude shall be given.

A

From the given magnitude AB, let the given magnitude AC betaken; the remaining magnitude CB is given. Because AB is given, a magnitude equal to it may * be found; let this be DF: and because AC is given, one equal to it may be found; let this be DE: wherefore, because AB is equal to DF, and AC to DE; the remainder CB is equal to the remainder FE. given*, because FE which is equal to

D

PROP. V.

CB is therefore

it

has been found.

"If of three magnitudes, the first together with the second be given, and also the second together with the third; either the first is equal to the third, or one of them is greater than the other by a given magnitude.

Let AB, BC, CD, be three magnitudes, of which AB together with BC, that is, AC, is given; and also BC together with CD, that is, BD, is given. Either AB is equal to CD, or one of them is greater than the other by a given magnitude.

Because AC, BD, are each of them given, they are either equal to one another,

or not equal. First, let them

be equal, and because AC A B

is equal to BD, take away

the common part BC; therefore the remainder AB is equal to the remainder CD.

Therefore, CE is given,

But if they be unequal, let AC be greater than BD, and make CE equal to BD. because BD is given. And the whole AC is given; therefore* AE the remainder is given. And because

AE B C D