be, as is taken notice of in the corollary; and another co- Boox 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 bis 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 1-5 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 inproperly 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 hook being thus corrected, I must readily agree to what the learned Dr. Barrow says*, " That there is no * Page 336. 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 rea. son, since Theon's time, till the present. DEF. II. and V. of B. VI. Book VI. The ad 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. DEF. II. · 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, bas 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. Bonx VI. It 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 Euclid'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 urryeometrical, Book VI. by substituting the 5th definition of the 6th book in place of the right definition, which without doubt Eudoxus or book, as appears from Hervagius's edition : But Zambertus Book VI. and Commandine, in their Latin translations, subjoin the are same to these definitions. Neither Campanus, nor, as it seems, the Arabic manuscripts from which he made his translation, have this definition. Clavius, in his observations upon it, rightly judges that the definition of compound ratios might have been made after the same manner in which the definitions of duplicate and triplicate ratio are given, viz. “ That as in several magnitudes that are conti*nual proportionals, Euclid named the ratio of the first to « the third, the duplicate ratio of the first to the second; “ and the ratio of the first to the fourth, the triplicate ratio “ of the first to the second ; that is, the ratio compounded of two or three intermediate ratios that are equal to one another, and so on; so, in like manner, if there be several “ magnitudes of the same kind, following one another, “ which are not continual proportionals, the first is said to “ have to the last the ratio compounded of all the interme“diate ratios,—-only for this reason, that these interme« diate ratios are interposed betwixt the two extremes, viz. " the first and last magnitudes; even as, in the 10th defi“nition of the 5th book, the ratio of the first to the third “ was called the duplicate ratio, merely upon account of “ two ratios being interposed betwixt the extremes, that are equal to one another : so that there is no difference be“ tivixt the compounding of ratios, and the duplication or triplication of them which are defined in the 5th book, « but that in the duplication, triplication, &c. of ratios, all " the interposed ratios are equal to one another; whereas, “ in the compounding of ratios, it is not necessary that the “ intermediate ratios should be equal to one another." Also Mr. Edmund Scarburgh, in his English translation of the first six books, pages 238, 266, expressly affirms, that the 5th definition of the 6th book is supposititious, and that the true defioition of compound ratio is contained in the 10th definition of the 5th book, viz. the definition of duplicate ratio, or to be understood from it, to wit, in the same manner as Clavius has explained it in the preceding citation. Yet these, and the rest of the moderns, do notwithstanding retain this 5th def, of the 6th book, and illustrate and explain it by long commentaries, when they ought rather to have taken it quite away from the Elements, For, by comparing def. 5, book 6, with prop. 5, book 8, it will clearly appear that this definition has been put into Y |