Boox XI. given of this Proposition, like to that which Euclid gives in Prop. 22, Book 6, as Clavius has done. PROP. XXXVIII. B. XI. When it is required to draw a perpendicular from a point in one plane which is at right angles to another plane, unto this last plane, it is done by drawing a perpendicular from the point to the common section of the planes; for this perpendicular will be perpendicular to the plane, by def. 4. of this Book: And it would be foolish in this - 17. 12. case to do it by the 11th Prop. of the same: But Euclida, in other Apollonius, and other geometers, when they have occasion . PROP. XXXIX. B. XI. B. XII. Book XII. The learned Mr. Moore, Professor of Greek in the Uni versity of Glasgow, observed to me, that it plainly appears from Archimedes's Epistle to Dositheus, prefixed to his books of the Sphere and Cylinder, which epistle he has restored from ancient manuscripts, that Eudoxus was the author of the chief propositions in this 12th book. PROP. II. B. XII. At the beginning of this it is said, “ if it be not so, the square of BD shall be to the square of FH, as the circle “ ABCD is to some space either less than the circle “ EFGH, or greater than it.”. And the like is to be found near to the end of this proposition, as also in Prop. 5, 11, 12, 18 of this Book : Concerning which it is to be observed, that in the demonstration of theorems, it is suffi- Book XII. cient, in this and the like cases, that a thing made use of in the reasoning can possibly exist, provided this be evident, though it cannot be exhibited or found by a geometrical construction : So, in this place, it is assumed, that there may be a fourth proportional to these three magnitudes, viz. the squares of BD, FH, and the circle ABCD; because it is evident that there is some square equal to the circle ABCD, though it cannot be found geometrically; and to the three rectilineal figures, viz. the squares of BD, FH, and the square which is equal to the circle ABCD, there is a fourth square proportional; because to the three straight lines which are their sides, there is a fourth straight line proportional, and this fourth square, or a space equal o 12. 6. to it, is the space which in this proposition is denoted by the letter S: And the like is to be understood in the other places above cited: And it is probable that this has been shown by Euclid, but left out by some editor; for the lemma which some unskilful hand has added to this proposition explains nothing of it. PROP. III. B. XII. In the Greek text and the translations, it is said, “ and “ because the two straight lines BA, AC, which meet one “ another,” &c. Here the angles BAC, KHL, are demonstrated to be equal to one another, by 10th Prop. B. 11, which had been done before : Because the triangle EAG was proved to be similar to the triangle KHL: This repetition is left out, and the triangles BAC, KHL, are proved to be similar in a shorter way by Prop. 21. B. 6. PROP. IV. B. XII. A Few things in this are more fully explained, than in the Greek text. PROP. V. B. XII. In this, near to the end, are the words, wis žu. Ti pooley εδειχθη, as was before shown;" and the same are found again in the end of Prop. 18. of this Book ; but the demonstration referred to, except it be in the useless lemma annexed to the 2d Prop. is no where in these Elements, and has been perhaps left out by some editor, who has forgot to cancel those words also. Book XII. PROP. VI. B. XII. COR. PROP. VIII. B. XII. The demonstration of this is imperfect, because it is not shown, that the triangular pyramids into which those upon multangular bases are divided, are similar to one another, as ought necessarily to have been done, and is done in the like case in Prop. 12. of this Book : The full demonstration of the corollary is as follows: Upon the polygonal bases ABCDE, FGHKL, let there be similar and similarly situated pyramids which have the points M, N, for their vertices : The pyramid ABCDEM has to the pyramid FGHKLN the triplicate ratio of that which the side AB has to the homologous side FG. Let the polygons be divided into the triangles ABE, 20. 6. EBC, ECD; FGL, LGH, LHK, which are similar, each bu Def . 11. to each : And because the pyramids are similar, thereforeb the triangle EAM is similar to the triangle LFN, and the * 4. 6. triangle ABM to FGN: Wherefore CME is to EA, as NL to LF; and as AE to EB, so is FL to LG, because the M. 1 B be ex æquali, as EM to MB, so is LN to NG : Wherefore the Book XII. triangles EMB, LNG, having their sides proportionals, ared equiangular, and similar to one another : Therefore * 5. 6. the pyramids which have the triangles EAB, LFG, for their bases, and the points M, N for their vertices, are similar to one another, for their solid angles are equal, and 11 Def . 11. the solids themselves are contained by the same number of < B. 11. similar planes: In the same manner the pyramid EBCM may be shown to be similar to the pyramid LGHN, and the pyramid ECDM to LHKN: And because the pyramids EABM, LFGN, are similar, and have triangular bases, the pyramid EABM has to LFGN the triplicate ra- 13. 12. tio of that which EB has to the homologous side LG. And, in the same manner, the pyramid EBCM has to the pyramid LGHN the triplicate ratio of that which EB has to LG: Therefore as the pyramid EABM is to the pyramid LFGN, so is the pyramid EBCM to the pyramid LGHN: In like manner, as the pyramid EBCM is to LGHN, so is the pyramid ECDM to the pyramid LHKN: And as one of the antecedents is to one of the consequents, so are all the antecedents to all the consequents: Therefore as the pyramid EABM to the pyramid LFGN, so is the whole pyramid ABCDEM to the whole pyramid FGH "KLN : And the pyramid EABM has to the pyramid LFGN the triplicate ratio of that which AB has to FG; therefore the whole pyramid has to the whole pyramid the triplicate ratio of that which AB has to the homologous side FG. Q. E. D. PROP. XI. and XII. B. XII. The order of the letters of the alphabet is not observed in these two propositions, according to Euclid's manner, and is now restored : By which means, the first part of Prop. 12. may be demonstrated in the same words with the first part of Prop. 11; on this account the demonstration of that first part is left out, and assumed from Prop. 11. PROP. XIII. B. XII. In this proposition, the common section of a plane parallel to the bases of a cylinder, with the cylinder itself, is supposed to be a circle, and it was thought proper briefly to demonstrate it; from whence it is sufficiently manifest, that this plane divides the cylinder into two others : And the same thing is understood to be supplied in Prop. 14. Book XII. PROP. XV. B. XII. “ And complete the cylinders AX, EO.” Both the enunciation and exposition of the proposition represent the cylinders as well as the cones, as already described: Wherefore the reading ought rather to be," and let the cones be “ALC, ENG; and the cylinders AX, EO." The first case in the second part of the demonstration is wanting; and something also in the second case of that part, before the repetition of the construction is mentioned; which are now added. PROP. XVII. B. XII. In the enunciation of this proposition, the Greek words εις την μειζονα σφαιραν στερεον πολυεδρον εγγραψαι μη ψαυον της ελασσονος σφαιρας κατα την επιφανειαν are thus translated by Commandine and others, “ in majori solidum polyhe“ drum describere quod minoris sphæræ superficiem non “tangat;" that is, to describe in the greater sphere a so“ lid polyhedron which shall not meet the superficies of “ the lesser sphere :" Whereby they refer the words xata την επιφανειαν to these next to them της ελασσονος σφαιρας: But they ought by no means to be thus translated : for the solid polyhedron doth not only meet the superficies of the lesser sphere, but pervades the whole of that sphere: Therefore the aforesaid words are to be referred to TO OTEÇEOY TO Avedcov, and ought thus to be translated, viz. to describe in the greater sphere a solid polyhedron whose superficies shall not meet the lesser sphere; as the meaning of the proposition necessarily requires. The demonstration of the proposition is spoiled and mutilated: For some easy things are very explicitly demonstrated, while others not so obvious are not sufficiently explained; for example, when it is affirmed, that the square of KB is greater than the double of the square of BZ, in the first demonstration, and that the angle BZK is obtuse, in the second : Both which ought to have been demonstrated: Besides, in the first demonstration, it is said, “ draw KS2 from the point K, perpendicular to BD;" whereas it ought to have been said, “join KV,” and it should have been demonstrated, that KV is perpendicular to BD: For it is evident from the figure in Hervagius's and Gregory's editions, and from the words of the demon stration, that the Greek editor did not perceive that the perpendicular drawn from the point K to the straight line |