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absurd. for that Theon brought it into the Elements can scarce be Book VI.
compounded of two or three intermediate ratios that are equal
not continual proportionals, the first is said to have to the last “ the ratio compounded of all the intermediate ratios,
this reason, that these intermediate ratios are interposed betwixt " the two extremes, viz. the first and last magnitudes; even as in “ the 1ɔ. Definition of the 5. 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 betwixt this compounding “ of ratios, and the duplication or triplication of them which are “ defined in the 5. 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 necesary that the “ intermediate ratios should be equal to one another.” Also Mr. Edmund Scarburgh, in his English translation of the first fix Books, page 238, 266. exprefly affirms that the 5. Definition of the 6. Book, is supposititious, and that the true Definition of Compound ratio is contained in the 1o. Definition of the 5. 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 preceeling citation. Yet these, and the rest of the Moderns, do notwithstanding
Book VI. retain this 5. Def. of the 6. B. 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. B. 6. with Prop. 5. B. 8. it will clearly appear that this Definition has been put into the Elements in place of the right one which has been taken out of them. because in Prop. 5. B. 8. it is demonstrated that the plane number of which the sides are C, D has to the plane number of which the sides are E, Z (see Hervagius's or Gregory's Edition) the ratio which is compounded of the ratios of their fides; that is, of the ratios of C to E, and D to Z. and by Def. 5. B. 6. and the explication given of it by all the Commentators, the ratio which is compounded of the ratios of C to E, and D to Z, is the ratio of the proluct made by the multiplication of the antecedents C, D, to the product of the consequents E, Z, that is the ratio of the plane number of which the sides are C, D to the plane number of which the sides are E, Z. wherefore the Proposition which is the 5. Def. of B. 6. is the rery same with the 5. Prop. of B. 8. and therefore it ought necellarily to be cancelled in one of these places; because it is absurd that the fame Proposition should stand as a Definition in one place of the Elements, and be demonstrated in another place of them. Now there is no doubt that Prop. 5. B. 8. Mhould have a place in the Elements, as the same thing is demonstrated in it concerning pline numbers, which is deinonítrated in Prop. 23. B. 6. of equiangular parallelograms; wherefore Def. 5. B. 6. ought not to be in the Elements. and from this it is evident that this Definition is not Euclid's but Theon's, or some other unskilful Geomcter's.
But no body, as far as I know, has hitherto fewn the true use of Compound ratio, or for what purpose it has been introduced into Geometry; for every Propofition in which Compound ratio is made use of, may without it be both enuntiated and demonstrated. Now the use of Compound ratio consists wholly in this, that by means of it, circumlocutions may be avoided, and thereby Propositions may be more briefiy either enuntiated or demonstrated, or both may be done; for instance, if this 23. Propofition of the 6. Book were to be enuntiated, without mentioning Compound ratio, it might be done as follows; If two parallelograms be equiangular, and if as a fide of the first to a side of the second, fo any assumed straight line be made to a second straight line; and as the other side of the firft to the other side of the second, so the second straight line be made
to a third: the first parallelogram is to the second, as the first Book VI.
rallelograms AC, CH, CF, the first AC (by the Definition of Com: pound ratio) has to the third CF, the
A ratio which is compounded of the ratio
HI of the first AC to the fccond CH, and of the ratio of CH to the third CF; B
C С but the parallelogram AC is to the parallelogram CH, as the straight line BC to CG; and the parallelogram CH
E is to CF, as the straight line CD is to CE; therefore the parallelogram AC has to CF the ratio which is compounded of ratios that are the same with the ratios of the sides, and to this Demonstration agrees the Enuntiation which is at present in the text, viz, equiangular parallelograms have to one another the ratio which is com. pounded of the ratios of the sides. for the vulgar reading “ whiclı “ is compounded of their sides” is absurd. But in this Edition we have kept the Demonstration which is in the Greek text, tho' not so short as Candalla's; because the way of finding the ratio which
Bock Vl. is compounded of the ratios of the sides; that is, of finding the
ratio of the parallelograms, is shewn in that, but not in Candalla's Demonstration; whereby beginners may learn, in like cases, how find the ratio which is compounded of two or more given ratios.
From what has been said it may be observed, that in any magniindes whatever of the same kind A, B, C, D &c. the ratio compound of the ratios of the first to the second, of the fecond to the third, and so on to the last, is only a name or expression by which the ratio which the frit A has to the last D is signified, and by which at the same time the ratios of all the magnitudes A to B, B to C, C to D from the first to the last, to one another, whether they be the fame, or be not the same, are indicated; as in magnitides which are continual proportionals A, B, C, D &c, the Dup'icate ratio of the fi ft to the second is only a name, or expression hy which the ra:io of the first A to the third C is signified, and by Blich, at the fame time, is Mewn that there are two ratios of the macritudos frcm the first to the last, viz, of the first A to the lecord B, and cf the fecond to the third or lant C, which are the same with cne another; and the Triplicate ratio of the first to the second is a name or expreTon by which the ratio of the first A to che fourth D is fignifed, and by which, at the same time, is shown that ti ere are three ratios of the magnitudes from the first to the last, viz. of the first A to the second B, and of to the third C, and of C to tlie fourth or last D, which are all the fame with one another; and so in the case of any other Multiplicare ratios. And that this is the right cxplication of tle meaning of these ratios is plain from the Definitions of Duplicate and Triplicate ratio in which Eucli makes use of the word niyetou, is said to be, or is called; which word, le no doubt made use of also in the Definition of Compound 12tio wich Theon, or some other, has expunged from the Elements; for the very same word is still retained in the wrong Definition of Compound ratio which is now the 5. of the 6. Book. but in the citation of these Definitions it is sometimes retained, as in the Demonftration of Prop. 19. B. 6. “ the first is faid to have, you
azercy, to the third the Duplicate ratio" &c. which is wrong trantated by Commandine and others " has" instead of " is said to " hare;" and fometimes it is left out, as in the Demonstration of Prop. 33. of the 11. Book, in which we find “the first has, eye, " to the third the Triplic'te ratio;” but without doubt yet," has," in this place fignifies the same as i xerr aéyé 704, is said to have. fo
kikewife in Prop. 23. B. 6. we find this citation “ but the ratio of Bock !. “ K to M is compounded, ouyx6704, of the ratio of K to L, and “ the ratio of L to M,” which is a shorter way of expresing the fame thing, which, according to the Definition, ought to have been expressed by ovyx bintoy aéyetay, is said to be compounded.
From these Remarks, together with the Propofitions subjoined to the 5. Book, all that is found concerning Compound ratio cither in the antient or modern Geometers may be understood an: explained.
PROP. XXIV. B. VI. It seems that some un kilful Editor has made up this De:rostration as we now have it, out of two others; one of which may be made from the 2. Prop. and the other from the 4. of this Book. for after he has from the 2. of this Book, and Composition and Permutation, demonstrated that the sides about the angle consmon to the two parallelograms are proportionals, le might have immediately concluded that the sides about the other equal angles were proportionals, viz. from Prop. 34. B. I, and Prop. 7. B. 5. this he does not, but proceeds to thew that the triangles and para:lelograms are equiangular, and in a tedious way, by lielp of Fro;. 4. of this Book, and the 2 2. of B. 5. deduces the fame conclusion. from which it is plain that this ill compoled Demonfuation is no Euclid's. these superfluous things are now left out, and a mois simple Demonstration is given from the 4. Prop. of this Beak, the fame which is in the Translation from the Arabic, by hcp of the 2. Prop. and Composition; but in this the Author neglects l'ermu. tation, and does not lew the parallelograms to be equiangular, as is proper to do for the sake of beginners.
PRO P. XXV. B. VI. It is very evident that the Demonstration which Euclid had given of this Proposition, has been vitiated by fome un kilud haad. her after this Editor had demonstrated that " as the rectilincal figure
!! “ ABC is to the rectilineal KGH, so is the parallelogram BE to the
parallelogram EF,” nothing more should have been added but this, " and the rectilineal figure ABC is equal to the paralle!ogramı “ BE, therefore the rectilineal KGH is equal to the parallcloruin "EF,” viz. from Prop. 14. B. 5. but betwixt these two fentenes he has inerted this, " wherefore, by Permatation, as the rectifical X 3