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litude of ratios, is of the same kind, and can serve for no Book V.

purpose in mathematics, but only to give beginners some " general, tho' gross and confused notion of analogy : But the " whole of the doctrine of ratios, and the whole of mathema. " tics, depend upon the accurate mathematical definitions which “ follow this : To these we ought principally to attend, as the " doctrine of ratios is more perfectly explained by them; this " third, and others like it ; may be entirely spared without any " lofs to geometry : As we see in the 7th book of the elements, " where the proportion of numbers to one another is defined, " and created of, yet without giving any definition of the ratio " of numbers; tho' such a definition was as necessary and use. "ful to be given in that book, as in this: But indeed there is " scarce any need of it in either of them : Though I think that " a thing of so general and abstracted a nature, and thereby the " more difficult to be conceived, and explained, cannot be more “ commodiovfly defined, than as the author has done: Upon " which account I thought fic to explain it at large, and defend “ it against the captious objections of those who attack it." To this citation from Dr Barrow I have nothing to add, except that I fully believe the 3d and 8th definitions are not Euclid's, but added by fome unikilful editor.

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It was necessary to add te word "continual” before “pro.

portionals” in this definition ; and thus it is cited in the 33d prop. of book u.

After this definition ought to have followed the definition of compound ratio, as this was the proper place for it ; duplicate and triplicate ratio being species of compound ratio. But Theon has made it the 5th def. of B. 6. where he gives an absurd and entirely useless definition of compound ratio : For this reason we have placed another definition of it betwixt the 11th and 12th of this book, which, no doubt, Euclid gave; for he cites it expressly in prop. 23. B. 6. and which Clavius, Herigon, and Barrow have like wife given, but they retain also Thcon's, which they ought to have left out of the elements.

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This, and the rest of the definitions following, contain the explication of some terms which are used in the 5th and following books; which, except a few, are easily enough understood from

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Book V. the propofitions of this book where they are first mentioned,

They seem to have been added by Theon, or some other. How. ever it be, they are explained something more distinctly for the fake of learners.

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In the construction preceding the demonstration of this, the words à &Tuxe, any whatever, are twice wanting in the Greek, as also in the Latin translations; and are now added, as being wholly neceffary. Ibid. in the demonstration ; in the Greek, and in the Latin

2 translation of Commandine, and in that of Mr Henry Briggs, which was published at London in 1620, together with the Greek text of the first fix books, which translation in this place is followed by Dr Gregory in his edition of Euclid, there is this sentence following, viz. “ and of A and C have been taken e“ quimultiples K, L; and of B and D, any equimultiples “ whatever (TUX) M, N;" which is not true, the words

ang whatever," ought to be left out : And it is strange that neither Mr Briggs, who did right to leave out these words in one place of prop. 13. of this book, nor De Gregory, who changed them into the word “ some" in three places, and left them out in a fourth of that same prop. 13. did not also leave them out in this place of prop. 4. and in the second of the two places where they occur in prop. 17. of this book, in neither of which they can stand consistent with truth: And in none of all these places, even in those which they corrected in their Latin translation, have they cancelled the words a stuxe in the Greek text, as they ought to have done.

The fame words á stuxe are found in four places of prop. 11. of this book, in the first and last of which, they are neces. fary, but in the second and third, though they are true, they are quite superfluous; as they likewise are in the second of the two places in which they are found in the 12th prop. and in the like places of prop. 22. 23. of this book : But are wanting in the last place of prop. 23. as alfo in prop. 25. Book 11.

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This corollary has been unskilfully annexed to this propofition, and has been made instead of the legitimate demonstration, which, without doubt, Theon, or some other editor, has taken away, not from this, but from its proper place in

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this book : The author of it designed to demonstrate, that if four Book V. magnitudes E, G, F, H be proportionals, they are also proportionals inversely; that is, G is to E, as H to F; which is true, but the demonstration of it does not in the least depend upon this 4th prop. or its demonstration : For, when he says, “be“ cause it is demonstrated that if K be greater than M, L is

greater than N,” &c. This indeed is Thewn in the demon. ftration of the 4th prop, but not from this that E, G, F, H are proponionals; for this last is the conclusion of the proposition. Wherefore these words, “because it is demonstrated," &c. are wholly foreign to his design : And he should have proved, that if K be greater than M, L is greater than N, from this, that E, G, F, H are proportionals, and from the 5th def. of this book, which he has not; but is done in proposition B, which we have given, in its proper place, instead of this corollary; and another corollary is placed after the 4th prop. which is often of use ; and is necessary to the demonstration of prop. 18. of this book,

PROP. V. B. V.

Io the construction which precedes the demonstration of this proposition, it is required that EB may be the same multiple of CG, that AE is of CF; that is, that EB be divided into as many equal parts, as there are parts in AE equal to CF : From which it is evident, that this construction is not Euclid's ; for he does not thew the way of dividing straight lines, and far less other magnitudes, into any number of equal parts, until the oth propofition of B. 6.; and he never requires any thing to be done in the construction, of which he had not before given the method of doing : For this reason, we have changed the construction to one, A which, without doubt, is Euclid's, in which no

G thing is required but to add a magnitude to itself a

E certain number of times ; and this is to be found C. in the translation from the Arabic, though the enunciation of the proposition and the demonstra

F tion are there very much spoiled. Jacobus Peletarius, who was the first, as far as I know, who took B

D notice of this error, gives also the right construction in his edition of Euclid, after he had given the other which he blames : He says, he would not leave it out, because it was fine, and might sharpen one's genius to invent others like it; whereas

there

Book V. there is not the least difference between the two demonstrations,

except a fingle word in the construction, which very probably

has been owing to an unskiltul Librarian. Clavius likewise gives both the ways; but neither he nor Peletarius takes notice of the reason why the one is preferable to the other.

PRO P. VI. B. V.: There are two cases of this proposition, of which only the first and simplest is demonstrated in the Greek : And it is probable Theon thought it was sufficient to give this one, since he was to make use of neither of them in his mutilated edition of the 5th book; and he might as well have left out the other, as also the 5th propofition, for the same reason : The demonstration of the other case is now added, because both of them, as also the 5th proposition, are neceflary to the demonstration of the 18th proposition of this book. The translation from the Arabic gives both cases briefly.

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PRO P. A. B. V. This proposition is frequently used by geometers, and it is necessary in the 25th prop. of this book, 31st of the 6th, and 34th of the uth and 15th of the 12th book : It seems to have been taken out of the elements by Theon, because it appeared evident enough to him, and others who substitute the confused and indiftinct idea the vulgar have of proportionals, in place of that accurate idea which is to be got from the 5th def. of this book, Nor can there be any doubt that Eudoxus or Euclid gave it a place in the elements, when we see the 7th and 9th of the fame book demonstrated, tho' they are quite as easy and evident as this. Alphonsus Borellus takes occafion from this propofition to censure the 5th definition of this book very severely, but most unjustly : In p. 126. of his Euclid restored, printed at Pisa in 1658, he says, “ Nor can even this least de“gree of knowledge be obtained from the foresaid property," viz, that which is contained in sth def. 5. “ That, if four magnitudes be proportionals, the third must necessarily be greater than the fourth, when the first is greater than the “ second ; as Clavius acknowledges in the 16th prop. of the

. “ 5th book of the elements." But though Clavius makes no such acknowledgement expressly, he has given Borellus a handle to fay this of him ; because when Clavius, in the above-cited place, censures Commandine, and that very justly, for demonstrating this proposition by help of the 16th of the 5th ; yet he himself gives no demonftration of it, but thinks it plain

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from the nature of proportionals, as he writes in the end of the Book V. 14th and 16th prop. B. 5. of his edition, and is followed by Herigon in Schol. 1. prop. 14. B. 5. as if there was any nature of proportionals antecedent to that which is to be derived and understood from the definition of them : And indeed, though it is very easy to give a riglit demonstration of it, no body, as far as I know, has given one, except the learned Dr Barrow, who, in answer to Borellus's objection, demonstrates it indirectly, but very briefly and clearly, from the 5th definition, in the 322d page of his Lect. Mathem. from which definition it may also be easily demonstrated disectly : On which account we have placed it next to the propositions concerning equimultiples.

PROP. B. BOOK V.

This also is easily deduced from the 5th def. B. 5. and therefore is placed next to the other; for it was very ignorantly made a corollary from the 4th prop. of this book. See the note on that corollary.

PRO P. C. B. V.

This is frequently made use of by geometers, and is necessary to the 5th and 6th propofitions of the roth book. Clavius, in his notes fubjoined to the 8th def. of book 5. demonstrates it only in numbers, by help of some of the propositions of the 7th book, in order to demonstrate the property contained in the sth definition of the 5th book, when applied to numbers, from the property of proportionals contained in the 20th def. of the 7th book : And most of the commentators judge it difficult to prove that four magnitudes which are proportionals according to the 20th def. of 7th book, are also proportionals according to the 5th def. of sih book. But this is cafily made out, as follows.

First, If A, B, C, D be four magnitudes, such that A is the fame multiple, or the fame part of B B, which is of D; A, B, C, D

H

12 are proportionals: This is demonftrated in propofition C. Secondiy, if AB contain the same

L parts of CD that EF does of GH; in this case likewise AB is to CD, as

ACE GM

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EF to GH.

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