« ForrigeFortsett »
Book XI. plane, and conclude that it will fall upon the common section of
the planes, because this is the very same thing as if they had made use of the construction above-mentioned, and then concluded that the straight line must be perpendicular to the plane; but is expressed in fewer words. fome Editor not perceiving this, thought it was necessary to add this Proposition, which can never be of any use, to the 11. Book. and its being near to the end among Propositions with which it has no connexion, is a mark of its having been added to the Text.
PROP. XXXIX. B. XI. In this it is supposed that the straight lines which bisect the sides of the opposite planes, are in one plane, which ought to have been demonstrated; as is now done.
HE learned Mr. Moor, Professor of Greek in the University
Archimedes Epistle to Dofitheus prefixed to his Books of the Sphere and Cylinder, which Epistle he has restored from antient Manuscripts, that Eudoxus was the Author of the chief Propositions in this twelfth 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 sufficient, in this and the like cases, that a thing made use of in the reasoning can possibly exist, providing this be evident, tho'it cannot be exhibited or found by a Geometrical construction. so in this place it is affumed 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, tho' 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 Book XII. to the three straight lines which are their sides there is a fourth straight line proportionala, and this fourth square, or a space equal a. 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 shewn 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 be“ cause 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 10. 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.
PROP. V. B. XII. In this, near to the end, are the words ais Trocter ed ex An, “ as “ was before shewn,” and the same are found again in the end of Prop. 18. of this Book; but the Demonstration referred to, except it be the useless Lemma annexed to the 2. Prop. is no where in these Elements, and has been perhaps left out by some Editor who has forgot to cancel those words also.
PROP. VI. B. XII. A shorter Demonstration is given of this; and that which is in the Greek Text may be made shorter by a step than it is. for the Author of it makes use of the 22. Prop. of B. 5. twice, whereas once would have served his purpose; because that Proposition extends to any number of magnitudes which are proportionals taken two and two, as well as to three which are proportional to other three.
COR. PROP. VIII. B. XII. The Demonstration of this is imperfect, because it is not shewn that the triangular pyramids into which those upon multangular bafes 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 his Book. the full Demonstration of the Corollary is as follows.
Upon the polygonal bases ABCDE, FGHKL, let there be fimilar 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 fide AB has to the homologous fide FG.
Let the polygons be divided into the triangles ABE, EBC, ECD; 2. 20. 6. FGL, LGH, LHK, which are similar a each to each. and because b. 11. Def. the pyramids are fimilar, therefore the triangle EAM is fimilar to
the triangle LFN, and the triangle ABM to FGN. wherefore C ME is to EA, as NL to LP, and as AE to EB, fo is FL to LG, because
d. 5. 6.
the triangles E AB, LFG are similar; therefore, ex aequali, as ME to EB, so is NL to LG. in like manner it may be shewn that EB is to BM, as LG to GN; therefore, again, ex aequali, as EM to MB, fo is LN to NG. wherefore the triangles EMB, LNG having their fides proportionals are d equiangular, and similar to one another. therefore 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 e equal, and the solids themselves are contained by the same number of similar planes. in the same manner the pyramid EBCM may be shewn to be similar
e. B. 11.
to the pyramid LGHN, and the pyramid ECDM to LHKN. and Book XII. because the pyramids E ABM, LFGN are similar, and have trian-m gular bases, the pyramid EABM has f to LFGN the triplicate ratio f. 8. 12. of that which EB has to the homologous fide 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, fo is the pyramid ECDM to the pyramid LHKN. and as one of the antecedents is to one of the consequents, fo 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 FGHKLN. 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 fide 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 demonftrated in the same words with the first part of Prop. II. on this account the Demonstration of that first
is left affumed from Prop. II.
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.
PROP. XV. B. XII.
; " And complete the cylinders AX, EO.” both the Enuntiation and Exposition of the Proposition represent the cylinders as well as the cones as already described. wherefore the reading ought rather
Book XII. to be " and let the cones be ALC, ENG; and the cylinders AX,
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 Enuntiation of this Proposition the Greek words, e's The μείζονα σφαίραν τερεών πολύεδρον εγγράψαι, μη ψαώον της ελασονος σφαίρας κατα την επιφάνειαν, are thus tranflated by Commandine and others, “ in majori folidum polyhedrum describere quod minoris “ sphaerae superficiem non tangat;" that is, “ to describe in the
greater sphere a solid polyhedron which shall not meet the super« ficies of the lefser sphere.” whereby they refer the words xard την επιφάνειαν to thefe next to them της ελάσονος σφαίρας. but they ought by no means to be thus translated, for the folid polyhedron doth not only meet the superficies of the lefser fphere, but pervades the whole of that sphere. therefore the foresaid words are to be referred to Tè sepsov moaved pov, and ought thus to be translated, viz. to describe in the greater sphere a solid polyhedron whose superficies shall not meet the leffer 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 obtufe, 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 Demonstration, that the Greek Editor did not perceive that the perpendicular drawn from the point K to the straight line BD must necessarily fall upon the point V, for in the figure it is made to fall
upon the point 2 a different point from V, which is likewise supposed in the Demonstration. Commandine seems to have been aware of this; for in his figure he marks one and the fame point with the two let