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a straight line, or in the same direction, be plain, when two straight Book 1. lines are said to be in a straight line, it does not appear what ought to be understood by these words, when a straight line and a curve, or two curve lines, are said to be in the same direction; at least it cannot be explained in this place; which makes it probable that this Definition, and that of the angle of a segment, and what is said of the angle of a semicircle, and the angles of segments, in the 16. and 31. Propositions of Book 3. are the additions of some less skilful Editor. on which account, especially since they are quite useless, these Definitions are distinguished from the rest by inverted double commas.
DE F. XVII. B. I. The words “ which also divides the circle into two equal parts”. are added at the end of this Definition in all the copies, but are now left out as not belonging to the Definition, being only a Corollary from it. Proclus demonstrates it by conceiving one of the parts into which the diameter divides the circle, to be applied to the other, for it is plain they must coincide, else the straight lines from the center to the circumference would not be all equal. the same thing is easily deduced from the 31. Prop. of Book 3. and the 24. of the same; from the first of which it follows that semicircles are similar segments of a circle, and from the other, that they are equal to one another.
DEF. XXXIII. B. I. This Definition has one condition more than is necessary; bes Cause every quadrilateral figure which has its opposite sides equal to one another, has likewise its opposite angles equal; and on the contrary.
Let ABCD be a quadrilateral figure, of which the opposite sides AB, CD are equal to one another; as
A also AD and BC. join BD; the two sides AD, DB are cqual to the two CB, BD, and the bafe AB is equal to the base CD; therefore by Prop. 8.
C of Book 1. the angle ADB is equal to the angle CBD; and by Prop. 4. B. 1. the angle BAD is equal to the angle DCB, and ABD to BDC; and therefore also the angle ADC is equal to the angle ABC,
Book I. And if the angle BAD be equal to the opposite angle BCD, and mthe angle ABC to ADC; the opposite sides are equal. Because by
Prop. 32. B. 1. all the angles of the quadrilateral figure ABCD are together equal to four right angles, and
D the two angles BAD, ADC are together equal to the two angles BCD, ABC. wherefore BAD, ADC are the half of all the four angles; that is,
с BAD and ADC are equal to two right angles. and therefore AB, CD are parallels by Prop. 28. B. 1. in the fame manner AD, BC are parallels. therefore ABCD is a parallelogram, and its opposite fides are equal by 34. Prop. B. 1.
PROP. VII. B. I. There are two cases of this Proposition, one of which is not in the Greek text, but is as necessary as the other. and that the case left out has been formerly in the text appears plainly from this, that the second part of Prop. 5. which is neceffary to the Demonstration of this case, can be of no use at all in the Elements, or any where else, but in this Demonstration; because the second part of Prop. 5. clearly follows from the first part, and Prop. 13. B. I. this part must therefore have been added to Prop. 5. upon account of fome Proposition betwixt the 5. and 13. but none of these stand in need of it except the 7. Proposition, on account of which it has been added. besides the translation from the Arabic has this case explicitely demonstrated. and Proclus acknowledges that the second part of Prop. 5. was added upon account of Prop: 7. but gives a ridiculous rcafon for it," that it might afford an answer to objections made “ against the 7." as if the case of the 7. which is left out, were, as he exprelly makes it, an objection against the proposition itfelf. Whoever is curious may read what Proclus says of this in his commentary on the 5. and 7. Propofitions; for it is not worth while to relate his trifles at full length.
It was thought proper to change the enuntiation of this 7. Prop. fo as to preserve the very fame meaning; the literal translation from the Greek being extremely harsh, and difficult to be understood by beginners.
PRO P. XI. B. I.
PROP. XX. and XXI. B. I. Proclus in his Commentary relates that the Epicureans derided this Proposition, as being manifest even to asses, and needing no Demonstration, and his answer is, that tho' the truth of it be manifest to our senses, yet it is science which must give the reason why two sides of a triangle are greater than the third. but the right anfwer to this objection against this and the 21, and some other plain Propofitions, is, that the number of Axioms ought not to be encreased without necessity, as it must be if these Propositions be not demonstrated. Monf. Clairault in the Preface to his Elements of Geometry published in French at Paris Ann. 1741, says that Euclid has been at the pains to prove that the two sides of a triangle which is included within another are together less than the two fides of the triangle which includes it; but he has forgot to add this condition, viz. that the triangles must be upon the fame base; because unless this be added, the sides of the included triangle may be greater than the sides of the triangle which includes it, in any ratio which is less than that of two to one, as Pappus Alexandrinus has demonstrated in Prop. 3. B. 3. of his Mathematical Collections.
PROP. XXII. B. I. Some Authors blame Euclid. because he does not demontrate that the two circles made use of in the construction of this Problein must cut one another. but this is very plain from the determination he has given, viz. that any two of the straight lines DF, FG, GH must be greater than the third. for who is so dull, tho' only beginning to learn the Elements, as not to perceive that the circle described from the center F, at the distance FD, must meet FH betwixt F and
H H, because FD is less than FH; and that, for the like reason, the circle described from the center G, at the distance GH or GM must meet DG betwixt D and G; and T3
Book I. that these circles must meet one another, because FD and GH are
together greater than FG? and this
F G H
PROP. XXIV. B. I. To this is added “ of the two sides DE, DF, let DE be that “ which is not greater than the other ;” that is, take that side of the two DE, DF which is not greater than the other, in order to make with it the angle EDG equal to D BAC. because without this restriction, there might be three different cases of the Proposition, as Campanus and others make.
Mr. Thomas Simpson in p. 262. of the second edition of his Elements of Geometry printed Ann. 1760. obferves in his E
G Notes, that it ought to have been shown that the point F fails below the line EG; this probably Euclid omitted, as it is very easy to perceive that DG bcing equal to DF, the point G is in the circumference of a circle described from the center D at the distance DF, and must be in that part of it which is above the straight line EF, because DG falls above DF, the angle EDG being greater than the angle EDF.
PROP. XXIX. B. I.
great deal to do both to antient and modern Geometers, it seems Look I. not to be properly placed among the Axioms, as, indeed, it is not in self-evident; but it may be demonstrated thus.
DEFINITION I. The distance of a point from a straight line, is the perpendi. cular drawn to it from the point,
DEF. 2. One straight line is said to go nearer to, or further from another straight line, when the distance of the points of the first from the other straight line become less or greater than they were; and two straight lines are said to keep the same distance from one another, when the distance of the points of one of them from the other is always the same.
A X I 0 M. A straight line cannot first come nearer to another straight line, and then go further from it, be A
B fore it cuts it; and, in like man
с ner, a straight line cannot go fur.D
E ther from another straight line, and
H then come nearer to it; nor can a straight line keep the same distance from another straight line, and then come nearer to it, or go further from it; or a straight line keeps always the same direction.
For example, the straight line ABC cannot first come nearer to the straight line DE, as from the
See the 6point A to the point B, and then, A
с from the point B to the point C, go D
E further from the fame DE, and, in
PROP. I. If two equal straight lines AC, BD be each at right angles to the same straight line AB; if the points C, D be joined by the straight linc CD, the straight line EF drawn from any point E in AB unto CD, at right angles to AB, shall be equal to AC, or BD.
If EF be nor equal to AC, one of them must be greater than the other; let AC be the greater ; then because FE is less than CA, the