IV.

Magnitudes are said to have a ratio to one another, when the less can be multiplied so as to exceed the other.

V.

The first of four magnitudes is said to have the same ratio to the second, which the third has to the fourth, when any equimultiples whatsoever of the first and third being taken, and any equimultiples whatsoever of the second and fourth; if the multiple of the first be less than that of the second, the multiple of the third is also less than that of the fourth; or, if the multiple of the first be equal to that of the second, the multiple of the third is also equal to that of the fourth; or, if the multiple of the first be greater than that of the second, the multiple of the third is also greater than that of the fourth. VI.

Magnitudes which have the same ratio are called proportionals. N.B. 'When four magnitudes are pro

portionals, it is usually expressed by saying, the first is to the se'cond, as the third to the fourth.' VII. When of the equimultiples of four magnitudes (taken as in the fifth definition), the multiple of the first is greater than that of the second, but the multiple of the third is not greater than the multiple of the fourth; then the first is said to have to the second a greater ratio than the third magnitude has to the fourth; and, on the contrary, the third is said to have to the fourth a less ratio than the first has to the second.

VIII. Analogy, or proportion, is the simi⚫litude of ratios.' IX. Proportion consists in three terms at least.

X.

When three magnitudes are proportionals, the first is said to have to

When there are any number of magnitudes of the same kind, the first is said to have to the last of them the ratio compounded of the ratio which the first has to the second, and of the ratio which the second has to the third, and of the ratio which the third has to the fourth, and so on unto the last magnitude. For example, if A, B, C, D, be four magnitudes of the same kind, the first A is said to have to the last D the ratio compounded of the ratio of A to B, and of the ratio of B to C, and of the ratio of C to D; or, the ratio of A to D is said to be compounded of the ratios of A to B, B to C, and C to D.

And if A has to B the same ratio

which E has to F; and B to C, the same ratio that G has to H; and C to D, the same that K has to L; then, by this definition, A is said to have to D the ratio compounded of ratios which are the same with the ratios of E to F, G to H, and K to L: And the same thing is to be understood when it is more briefly expressed, by saying, A has to D the ratio compounded of the ratios of E to F, G to H, and K to L.

In like manner, the same things being supposed, if M has to N the same ratio which A has to D; then, for shortness sake, M is said to have to N, the ratio compounded of the ratios of E to F, G to H, and K to L.

XII. In proportionals, the antecedent terms are called homologous to one another, as also the consequents to one another.

'Geometers make use of the follow

ing technical words to signify 'certain ways of changing either 'the order or magnitude of pro'portionals, so as that they continue 'still to be proportionals."

XIII. Permutando, or alternando, by permutation, or alternately. This word is used when there are four proportionals, and it is inferred, that the first has the same ratio to the third, which the second has to the fourth: or that the first is to the third, as the second to the fourth: As is shewn in the 16th prop. of this 5th book.

XIV.

Invertendo, by inversion: When there are four proportionals, and it is inferred, that the second is to the first, as the fourth to the third. Prop. B. Book 5.

XV. Componendo, by composition; when there are four proportionals, and it is inferred, that the first, together with the second, is to the second, as the third, together with the fourth, is to the fourth. 18th Prop. Book 5.

XVI. Dividendo, by division; when there are four proportionals, and it is inferred, that the excess of the first above the second, is to the second, as the excess of the third above the fourth, is to the fourth. 17th Prop Book 5.

XVII. Convertendo, by conversion; when there are four proportionals, and it is inferred, that the first is to its excess above the second, as the third to its excess above the fourth. Prop. E Book 5.

XVIII.

Ex æquali (se. distantia), or ex æquo, from equality of distance; when there is any number of magnitudes more than two, and as many others, so that they are proportionals when taken two and two of each rank, and it is inferred, that the first is to the last of the first rank of magnitudes, as the first is to the last of the others: Of this 'there are the two following kinds, which arise from the different order in which the magnitudes are taken two and two.'

XIX.

Ex æquali, from equality. This term is used simply by itself, when the first magnitude is to the second of the first rank, as the first to the second of the other rank; and as the second is to the third of the first rank, so is the second to the third of the other; and so on in order, and the inference is as mentioned in the preceding definition; whence this is called ordinate proportion. It is demonstrated in 24d Prop. Book 5.

XX.

Ex æquali, in proportione perturbata, seu inordinata, from equality, in perturbate or disorderly proportion. This term is used when the first magnitude is to the second of the first rank, as the last but one is to the last of the second rank; and as the second is to the third of the first rank, so is the last but two to the last but one of the second rank; and as the third is to the fourth of the first rank, so is the third from the last to the last but two of the second rank; and so on in a cross order: And the inference is as in the 18th definition. It is demonstrated in the 23d Prop. of Book 5.

to F, therefore AG and CH together are equal to (Ax. 2. 5.) E and F together: For the same reason, because GB is equal to E, and HD to F; GB and HD together are equal to E and F together. Wherefore, as many magnitudes as are in AB equal to E, so many are there in AB, CD, together equal to E and F together. Therefore, whatsoever multiple AB is of E, the same multiple is AB and CD together of E and F together.

Therefore, if any magnitudes, how many soever, be equimultiples of as many, each of each, whatsoever multiple any one of them is of its part, the same multiple shall all the first magnitudes be of all the other: For

the same demonstration holds in any 'number of magnitudes, which was ' here applied to two.' Q. E. D.

Let AB the first, be the same multiple of C the second, that DE the

third is of F the fourth; and BG tl. fifth, the same multiple of Cth

fifth, the same multiple of C the second, that DH

G C

third, together with the sixth, is of F the fourth.

Because AB is the same multiple of C, that DE is of F; there are as many magnitudes in AB equal to C, as there are in DE equal to F: In like manner, as many as there are in BG equal to C, so many are there in EH equal to F: As many, then, as are in the whole AG equal to C, so many are there in the whole DH equal to F; therefore AG is the same

therefore of the magnitudes EK, KF, shall be equal to the number of the others GL, LH: And because A is the same multiple of B, that C is of D, and that EK is equal to A, and GL to C; therefore EK is the same multiple of B, that GL is of D: For the same reason, KF is the same multiple of B, that LH is of D; and so, if there be more parts in EF, GH equal to A, C: Because, therefore, the first EK is the same multiple of the second B, which the third GL is of the fourth D, and that the fifth KF is the same multiple of the second B, which the sixth LH is of the fourth D; EF the first, together with the fifth, is the same multiple (2. 5.) of the second B, which GH the third, together with the sixth, is of the fourth D. If, therefore, the first, &c. &c. Q. E. D.