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tiples, as the doubles of AE, EB, are to be taken, as is done in this edition, where all the cases are at once demonstrated : and from this it is plain that Theon, or some other unskilful editor, has vitiated this proposition.
PROP. IX. B. V.
Of this there is given a more explicit demonstration than that which I found in the Elements.
PROP. X. B. V.
It was necessary to give another demonstration of this proposition, because that which is in the Greek and Latin, or other editions, is not legitimate : for the words greater, the same or equal, lesser, have a quite different meaning when applied to magnitudes and ratios, as is plain froin the 5th and 7th definitions of Book 5. By the help of these let us examine the demonstration of the 10th Prop. which proceeds thus : “ Let A have to C a greater ratio than B to C: I say “ that A is greater than B: for if it be not greater it is “ either equal or less. But A cannot be equal to B, 6 because then each of them would have the same ratio “ to C; but they have not. Therefore A is not equal “ to B." The force of which reasoning is this: if A had to C the same ratio that B has to Č, then if any equimultiples whatever of A and B be taken, and any multiple whatever of C; if the multiple of A be greater than the multiple of C, then by the 5th def. of Book 5. the multiple of B is also greater than that of C; but from the hypothesis, that A has a greater ratio to C, than B has to C, there must, by the 7th def. of Book 5. be certain equimultiples of A and B, and some multiple of C such, that the multiple of A is greater than the multiple of C, but the multiple of B is not greater than the same multiple of C, and this proposition directly contradicts the preceding; wherefore A is not equal to B. The demonstration of the 10th Prop. goes on thus : “ But neither is A less 6 than B; because then A would have a less ratio to C " than B has to it: but it has not a less ratio, therefore “ A is not less than B,” &c. Here it is said, that “ A 66 would have a less ratio to C than B has to C," or, which is the same thing, that B would have a greater ratio to C than A to C; that is, by 7th def. Book 5.
there must be some equimultiples of B and A, and some multiple of C, such that the multiple of B is greater than the multiple of C, but the multiple of A is not greater than it: and it ought to have been proved, that this can never happen if the ratio of A to C be greater than the ratio of B to C; that is, it should have been proved, that, in this case, the multiple of A is always greater than the multiple of c, whenever the multiple of B is greater than the multiple of C; for when this is demonstrated, it will be evident that B cannot have a greater ratio to C, than A has toC, or, which is the same thing, that A cannot have a less ratio to C than B has to C. But this is not at all proved in the 10th proposition : but if the 10th were once demonstrated, it would immediately follow from it, but cannot without it be easily demonstrated, as he that tries to do it will find. Wherefore the 10th proposition is not sufficiently demonstrated. And it seems that he who has given the demonstration of the 10th proposition as we now have it, instead of that which Eudoxus or Euclid had given, has been deceived in applying what is manifest, when understood of magnitudes, unto ratios, viz. that a magnitude cannot be both greater and less than another. That those things which are equal to the same are equal to one another, is a most evident axiom when understood of magnitudes; yet Euclid does not make use of it to infer, that those ratios, which are the same to the same ratio, are the same to one another, but explicitly demonstrates this in Prop. 11. of Book 5. The demonstration we have given of the 10th prop. is no doubt the same with that of Eudoxus or Euclid, as it is immediately and directly derived from the defini
A B с tion of a greater ratio, viz. the 7th of the 5th.
D F E F The above-mentioned proposition, viz. If A have to C a greater ratio than B to C; and if of A and B there be taken certain equimultiples, and some multiple of C; then if the multiple of B be greater than the multiple of C, the multiple of A is also greater than the same, is thus demonstrated.
Let D, E, be equimultiples of A, B, and F a multiple of C, such, that E the multiple of B is greater that F; D the multiple of A is also greater than F.
Because A has a greater ratio to Č, than B to C, A is
greater than B, by the 10th Prop. B. 5. therefore D the multiple of A is greater than E the same multiple of B: and E is greater than F: much more therefore D is greater than F.
PROP. XIII. B. V.
In Commandine's, Briggs's, and Gregory's translations at the beginning of this demonstration, it is said, « and the multiple of C is greater than the multiple of “D; but the multiple of Ě is not greater than the
multiple of F:" which words are a literal translation from the Greek: but the sense evidently tequires that it be read, “ so that the multiple of C be
greater than the multiple of D; but the multiple of “ E be not greater than the multiple of F.” And thus this place was restored to the true reading in the first editions of Commandine's Euclid, printed in 8vo. at Oxford: but in the later editions, at least in that of 1747, the error of the Greek text was kept in.
There is a corollary added to Prop. 13, as it is necessary to the 20th and 21st Prop. of this Book, and is as useful as the proposition.
PROP. XIV. B. V.
The two cases of this, which are not in the Greek, are added; the demonstration of them not being exactly the same with that of the first case.
PROP. XVII. B. V.
The order of the words in a clause of this is changed to one more natural: as was also done in Prop. 11.
PROP. XVIII. B. V.
The demonstration of this is none of Euclid's, nor is it legitimate; for it depends upon this hypothesis,
that to any three magnitudes, two of which, at least, are of the same kind, there may be a fourth proportional : which, if not proved, the demonstration now in the text is of no force : but this is assumed without any proof; nor can it, as far as I am able to discern, be demonstrated by the propositions preceding this: so far is it from deserving to be reckoned an axiom, as Clavius, after other commentators, would have it, at the end of the definitions of the 5th Book. Euclid does not demonstrate it, nor does he show how to find the fourth proportional, before the 12th Prop. of the 6th Book : and he never assumes any thing in the demonstration of a proposition which he had not before demonstrated; at least, he assumes nothing the existence of which is not evidently possible; for a certain conclusion can never be deduced by the means of an uncertain proposition : upon this account we have given a legitimate demonstration of this proposition instead of that in the Greek and other editions, which very probably Theon, at least some other, has put in the place of Euclid's because he thought it too prolix: and as the 17th Prop., of which this 18th is the converse, is demonstrated by help of the first and second propositions of this book; so, in the demonstration now given of the 18th, the 5th Prop. and both cases of the 6th are necessary, and these two propositions are the converses of the 1st and 2d. Now the 5th and 6th do not enter into the demonstration of any proposition in this book as we now have it: nor can they be of use in any proposition of the Elements, except in this 18th, and this is a manifest proof, that Euclid made use of them in his demonstration of it, and that the demonstration now given, which is exactly the converse 'of that of the 17th, as it ought to be, differs nothing from that of Eudoxus or Euclid : for the 5th and 6th have undoubtedly been put into the 5th book for the sake of some propositions in it, as all the other propositions about equimultiples have been.
Hieronymus Saccherius, in his book named “Euclides « ab omni nævo vindicatus," printed at Milan, ann. 1733, in 4to, acknowledges this blemish in the demonstration of the 18th; and that he may remove it, and render the demonstration we now have of it legitimate, he endeavours to demonstrate the following proposition, which is in page 115 of his book, viz. :
“ Let A, B, C, D, be four magnitudes of which the 6 two first are of one kind, and also the two others “ either of the same kind with the two first, or of some 66 other the same kind with one another. I
the ratio “ of the third C to the fourth D, is either equal to, or “ greater, or less, than the ratio of the first A to the 66 second B.”
And after two propositions premised as lemmas, he proceeds thus :
“ Either among all the possible equimultiples of the “ first A, and of the third C, and, at the same time,
among all the possible equimultiples of the second B, " and of the fourth D, there can be found some one
multiple EF of the first A, and one IK of the second “ B, that are equal to one another; and also in the
same case) some one multiple GH of the third C “ equal to LM the multiple of the fourth D, or such
equality is no where to be found. If the first case
happen [i. e. if such equality be to be found), it is 6 manifest from what is before demonstrated, that A is " to B, as C to D; but if such simultaneous equality be “ not to be found upon both sides, it will be found ei“ther upon one side, as upon the side of A [and B];
or it will be found upon neither side: if the first “i bappen; therefore (from Euclid's definition of greater 6 and lesser ratio foregoing) A has to B a greater or less “ ratio than C to D; according as GH the multiple of “ the third C is less, or greater, than LM the multiple 6 of the fourth D: but if the second case happen; 66 therefore
upon the one side, as upon the side of A “the first and B the second, it may happen that the
multiple EF (viz. of the first] may be less than IK « the multiple of the second, while, on the contrary, “ upon the other side (viz. of C and D], the multiple 6 GH [of the third cj is greater than the other mul“ tiple LM (of the fourth D]: and then (from the same