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and join AU. if then the circumference BE be bifected, and its Book XII. half again bifected, and so on, there will at length be left a circumference less than the circumference which is fubtended by a ftraight line equal to GU infcribed in the circle BCDE. let this be the circumference KB. therefore the straight line KB is lefs than GU. and because the angle BZK is obtuse, as was proved in the preceding, therefore BK is greater than BZ. but GU is greater than BK; much more then is GU greater than BZ, and the fquare of GU than the fquare of BZ. and AU is equal to AB; therefore the square of AU, that is the fquares of AG, GU are equal to the fquare of AB, that is to the squares of AZ, ZB; but the square of BZ is less than the square of GU; therefore the fquare of AZ is greater than the square of AG, and the straight line AZ consequently greater than the straight line AG.

COR. And if in the leffer sphere there be described a folid polyhedron by drawing straight lines betwixt the points in which the ftraight lines from the center of the sphere drawn to all the angles of the folid polyhedron in the greater sphere meet the fuperficies of the leffer; in the fame order in which are joined the points in which the fame lines from the center meet the fuperficies of the greater sphere; the folid polyhedron in the sphere BCDE has to this other solid polyhedron the triplicate ratio of that which the diameter of the sphere BCDE has to the diameter of the other sphere. for if these two solids be divided into the fame number of pyramids, and in the same order; the pyramids shall be similar to one another, each to each. because they have the folid angles at their common vertex, the center of the sphere, the fame in each pyramid, and their other folid angles at the bafes equal to one another, each to each, because a. B. II. they are contained by three plane angles equal each to each; and the pyramids are contained by the fame number of fimilar planes; and are therefore fimilar to one another, each to each. but fimilar b ramids have to one another the triplicate ratio of their homologous fides. therefore the pyramid of which the base is the quadrilateral KBOS, and vertex A, has to the pyramid in the other sphere of the fame order, the triplicate ratio of their homologous fides; that is, of that ratio which AB from the center of the greater sphere has to the ftraight line from the fame center to the superficies of the leffer sphere. and in like manner each pyramid in the greater sphere has to each of the fame order in the leffer, the triplicate ratio of that which AB has to the femidiameter of the leffer sphere. and as one

C

py

b. 11. Def.

II.

c. Cor.8.12.

Book XII. antecedent is to its confequent, fo are all the antecedents to all the confequents. Wherefore the whole folid polyhedron in the greater sphere has to the whole folid polyhedron in the other, the triplicate ratio of that which AB the femidiameter of the first has to the femidiameter of the other; that is, which the diameter BD of the greater has to the diameter of the other sphere.

SPHE

PROP. XVIII. THEOR.

PHERES have to one another the triplicate ratio of that which their diameters have.

Let ABC, DEF be two fpheres of which the diameters are BC, EF. the fphere ABC has to the sphere DEF the triplicate ratio of that which BC has to EF.

For if it has not, the sphere ABC fhall have to a sphere either lefs or greater than DEF, the triplicate ratio of that which BC has to EF. First, let it have that ratio to a lefs, viz. to the sphere GHK; and let the sphere DEF have the fame center with GHK; 3. 17. 12. and in the greater sphere DEF describe a a solid polyhedron the fuperficies of which does not meet the leffer sphere GHK; and in

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12.

the sphere ABC defcribe another fimilar to that in the fphere DEF. therefore the folid polyhedron in the sphere ABC has to b. Cor. 17. the folid polyhedron in the sphere DEF, the triplicate ratio of that which BC has to EF. but the sphere ABC has to the 'sphere GHK, the triplicate ratio of that which BC has to EF; therefore as the sphere ABC to the sphere GHK, so is the folid polyhedron in the sphere ABC to the folid polyhedron in the sphere DEF. but the fphere ABC is greater than the folid polyhedron in it

c. 14. 5.

therefore also the sphere GHK is greater than the folid polyhe- Book XII. dron in the sphere DEF. but it is also lefs, because it is contained god within it, which is impoffible. therefore the sphere ABC has not to any sphere less than DEF, the triplicate ratio of that which BC has to EF. In the fame manner it may be demonstrated that the sphere DEF has not to any sphere less than ABC, the triplicate ratio of that which EF has to BC. Nor can the sphere ABC have to any fphere greater than DEF, the triplicate ratio of that which BC has to EF. for if it can, let it have that ratio to a greater sphere LMN. therefore, by inverfion, the sphere LMN has to the sphere ABC, the triplicate ratio of that which the diameter EF has to the diameter BC. but as the fphere LMN to ABC, fo is the sphere DEF to some sphere, which must be less than the sphere ABC, because the sphere LMN is greater than the sphere DEF. therefore the sphere DEF has to a sphere less than ABC the triplicate ratio of that which EF has to BC; which was fhewn to be impoffible. therefore the sphere ABC has not to any sphere greater than DEF the triplicate ratio of that which BC has to EF. and it was demonstrated that neither has it that ratio to any sphere less than DEF. Therefore the sphere ABC has to the sphere DEF, the triplicate ratio of that which BC has to EF. Q. E. D.

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CRITICAL AND GEOMETRICAL;

CONTAINING,

AN ACCOUNT OF THOSE THINGS IN WHICH THIS EDITION DIFFERS FROM THE GREEK TEXT; AND THE REASONS

OF THE ALTERATIONS WHICH HAVE BEEN MADE.

AS ALSO

OBSERVATIONS ON SOME OF THE PROPOSITIONS.

BY

ROBERT SIMSON, M. D.

EMERITUS PROFESSOR OF MATHEMATICS

IN THE UNIVERSITY OF GLASGOW.

GLASGOW:

PRINTED BY J. & M. ROBERTSON,

SALTMARKET.

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