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demonstrate the following proposition, which is in page 115 Book V of his book, viz.
"Let A, B, C, D, be four magnitudes of which the two ❝ first are of one kind, and also the two others either of "the same kind with the two first, or of some other the "same kind with one another. I say 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 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 mul"tiple 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 A"happen, [i.e. "if such e- B "quality is to
"be found] it
"from what is
"before de- D
"monstrated, that A is to B, as C to D; but if such simulta-
"Therefore the axiom [i. e. the proposition before set "down] remains demonstrated," &c.
Not in the least; but it still remains undemonstrated: For what he says may happen, may, in innumerable cases, never happen; and therefore his demonstration does not hold: For example, if A be the side, and B the diameter of a square; and C the side, and D the diameter of another square; there can in no case be any multiple of A equal to any of B; nor any one of C equal to one of D, as is well known; and yet it can never happen that when any multiple of A is greater than a multiple of B, the multiple of C can be less than the multiple of D, nor when the multiple of A is less than that of B, the multiple of C can be greater than that of D, viz. taking equimultiples of A and C, and equimultiples of B and D: For A, B, C, D, are proportionals, and so if the multiple of A be greater, &c. than that of B, so must that of C be greater, &c. than that of D; by 5th Def. b. 5.
The same objection holds good against the demonstration which some give of the first prop. of the 6th book, which we have made against this of the 18th prop. because it de pends upon the same insufficient foundation with the other.
PROP. XIX. B. V.
A COROLLARY is added to this, which is as frequently used as the proposition itself. The corollary which is subjoined to it in the Greek, plainly shows that the 5th book has been vitiated by editors who were not geometers: For the conversion of ratios does not depend upon this 19th, and the demonstration which several of the commentators on Euclid give of conversion is not legitimate, as Clavius has rightly observed, who has given a good demonstration of it which we have put in proposition E; but he makes it a corollary from the 19th, and begins it with the words, "Hence it easily follows," though it does not at all follow from it.
PROP. XX. XXI. XXII. XXIII. XXIV. B. V.
THE demonstrations of the 20th and 21st propositions, are shorter than those Euclid gives of easier propositions, either in the preceding or following books. Wherefore it was proper to make them more explicit, and the 22d and 23d propositions are, as they ought to be, extended to any number of magnitudes: And, in like manner, may the 24th
be, as is taken notice of in the corollary; and another co- Book V. rollary is added, as useful as the proposition, and the words "any whatever" are applied near the end of prop. 23, which are wanting in the Greek text, and the translations from it. In a paper writ by Philippus Naudæus, and published after his death, in the History of the Royal Academy of Sciences of Berlin, anno 1745, page 50, the 23d prop. of the 5th book is censured as being obscurely enunciated, and, because of this, prolixly demonstrated: The enunciation there given is not Euclid's but Tacquet's, as he acknowledges, which, though not so well expressed, is, upon the matter, the same with that which is now in the Elements. Nor is there any thing obscure in it, though the author of the paper has set down the proportionals in a disadvantageous order, by which it appears to be obscure: But 10 doubt Euclid enunciated this 23d, as well as the 22d, so as to extend it to any number of magnitudes, which, taken two and two, are proportionals, and not of six only; and to this general case the enunciation which Naudæus gives, cannot be well applied.
The demonstration which is given of this 23d, in that paper, is quite wrong; because, if the proportional magnitudes be plane or solid figures, there can be no rectangle, (which he improperly calls a product), conceived to be made by any two of them: And if it should be said, that in this case straight lines are to be taken which are proportional to the figures, the demonstration would this way become much longer than Euclid's: But, even though his demonstration had been right, who does not see that it could not be made use of in the 5th book?
PROP. F, G, H, K. B. V.
THESE propositions are annexed to the 5th book, because they are frequently made use of by both ancient and modern geometers: And in many cases, compound ratios cannot be brought into demonstration, without making use of them.
Whoever desires to see the doctrine of ratios delivered in this 5th book solidly defended, and the arguments brought against it by And. Tacquet, Alph. Borellus, and others, fully refuted, may read Dr. Barrow's mathematical lectures, viz. the 7th and 8th of the year 1666.
The 5th book being thus corrected, I must readily agree to what the learned Dr. Barrow says, "That there is no
Book V. "thing in the whole body of the Elements of a more subtile "invention, nothing more solidly established, and more "accurately handled, than the doctrine of proportionals.” And there is some ground to hope, that geometers will think that this could not have been said with as good reason, since Theon's time, till the present.
DEF. II. and V. of B. VI.
BOOK VI. THE 2d definition does not seem to be Euclid's but some unskilful editor's: For there is no mention made by Euclid nor, as far as I know, by any other geometer, of reciprocal figures: It is obscurely expressed, which made it proper to render it more distinct: It would be better to put the following definition in place of it, viz.
Two magnitudes are said to be reciprocally proportional to two others, when one of the first is to one of the other magnitudes, as the remaining one of the last two is to the remaining one of the first.
But the 5th definition, which, since Theon's time, has been kept in the Elements, to the great detriment of learners, is now justly thrown out of them, for the reasons given in the notes on the 23d prop. of this book.
PROP. I. and II. B. VI.
To the first of these a corollary is added, which is often used: And the enunciation of the second is made more general.
PROP. III. B. VI.
A SECOND case of this, as useful as the first, is given in prop. A; viz. the case in which the exterior angle of a triangle is bisected by a straight line: The demonstration of it is very like to that of the first case, and upon this account may, probably, have been left out, as also the enunciation, by some unskilful editor. At least it is certain, that Pappus makes use of this case, as an elementary proposition, without a demonstration of it, in Prop. 39, of his 7th Book of Mathematical Collections.
PROP. VI. B. VI.
To this a case is added which occurs not unfrequently in demonstration.
> PROP. VIII. B. VI.
→ Ir seems plain that some editor has changed the demonstration that Euclid gave of this proposition: For, after he has demonstrated that the triangles are equiangular to one another, he particularly shows that their sides about the equal angles are proportionals, as if this had not been done in the demonstration of the 4th prop. of this book: This superfluous part is not found in the translation from the Arabic, and is now left out.
PROP. IX. B. VI.
THIS is demonstrated in a particular case, viz. that in which the third part of a straight line is required to be cut off; which is not at all like Luclid's manner: Besides, the author of the demonstration, from four magnitudes being proportionals, concludes that the third of them is the same multiple of the fourth, which the first is of the second; now, this is no where demonstrated in the 5th book, as we now have it; but the editor assumes it from the confused notion which the vulgar have of proportionals: On this account it was necessary to give a general and legitimate demonstration of this proposition.
PROP. XVIII. B. VI.
THE demonstration of this seems to be vitiated. For the proposition is demonstrated only in the case of quadrilateral figures, without mentioning how it may be extended to figures of five or more sides: Besides, from two triangles being equiangular, it is inferred, that a side of the one is to the homologous side of the other, as another side of the first is to the side homologous to it of the other, without permutation of the proportionals: which is contrary to Euclid's manner, as is clear from the next proposition: And the same fault occurs again in the conclusion, where the sides about the equal angles are not shown to be proportionals, by reason of again neglecting permutation. On these accounts, a demonstration is given in Euclid's manner, like to that he makes use of in the 20th prop. of this book; and it is extended to five-sided figures, by which it may be seen how to extend it to figures of any number of sides.
PROP. XXIII. B. VI.
NOTHING is usually reckoned more difficult in the elements of geometry by learners, than the doctrine of compound ratio, which Theon has rendered absurd and ungeometrical,