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Book III. the point of infection, where the two straight lines meet. And
in the like sense two straight lines are said to be inflected from two points to a third point, when they make an angle at this point; as may be seen in the description given by Pappus Ales. andrinus of Appollonius's books de Locis planis, in the preface to his 7th book: we have made the expression fuller from the 90th
prop. of the Data.
There are two cases of this proposition, the second of which viz. when the angles are in a segment not greater than a semicircle, is wanting in the Greek: and of this a more simple demonstration is given than that which is in Commandine, as being derived only from the first case, without the help of triangles.
PROP. XXIII, and XXIV. B. III.
In proposition 24 it is demonstrated, that the segment AFB must coincide with the segment CFD, (see Commandine's figure), and that it cannot fall otherwise, as CGD), so as to cut the other circle in a third point G, from this, that, if it did, a circle could cut another in more points than two: but this ought to have been proved to be impossible in the 23d prop. as well as that one of the segments cannot fall within the other : this part then is left out in the 24th, and put in its proper place, the 23d proposition.
PROP. XXV. B. III.
This proposition is divided into three cases, of which two have the same construction and demonstration ; therefore it is now divided only into two cases.
PROP. XXXIII. B. III.
This also in the Greek is divided into three cases, of which two, viz. one in which the given angle is acute, and the other in which it is obtuse, have exactly the same construction and demonstration ; on which account, the demonstration of the last case is left out as quite superfluous, and the addition of some unskilful editor; besides the demonstration of the case when the angle given is a right angle, is done a roundabout way, and is therefore changed to a more simple one, as was done by Clavius.
PROP. XXXV. B. III.
As the 25th and 33d propositions are divided into more cases, so this 35th is divided into fewer cases than are necessary. Nor can it be supposed that Euclid omitted them because they are easy; as he has given the case, which by far, is the easiest of them all, viz. that in which both the straight lines pass through the centre: and in the following proposition he separately demonstrates the case in which the straight line passes through the centre, and that in which it does not pass through the centre: so that it seems Theon, or some other, has thought them too long to insert: but cases that require different demonstrations, should not be left out in the Elements, as was before taken notice of: these cases are in the translation from the Arabic, and are now put into the text.
PROP. XXXVII B. III.
At the end of this, the words, “in the same manner it may “ be demonstrated, if the centre be in AC,” are left out as the addition of some ignorant editor.
DEFINITIONS OF BOOK IV.
WHEN a point is in a straight line, or any other line, this Book IV. point is by the Greek geometers said aztec faci, to be upon, or in that line, and when a straight line or circle meets a circle any way, the one is said aftsfer to meet the other : but when a straight line or circle meets a circle so as not to cut it, it is said 10a778f, to touch the circle ; and these two terms are never promiscuously used by them : therefore, in the 5th definition of book 4, the compound &PARTITAI must be read, instead of the simple arthT.: and in the 1st, 2d, 3d, and 6th definitions in Commandine's translation, “ tangit," must be read instead of “ contingit:" and in the 2d and 3d definitions of book 3, the same change must be made : but in the Greek text of propositions 11th, 12th, 13th, 18th, 19th, book 3, the compound verb is to be put for the simple.
PROP. IV. B. IV.
In this, as also in the 8th and 13th proposition of this book, it is demonstrated indirectly, that the circle touches a straight line : whereas in the 17th, 33d, and 37th propositions of book 3, the same thing is directly demonstrated: and this way we
Book IV. have chosen to use in the propositions of this book, as it is
PROP. V. B. IV.
The demonstration of this has been spoiled by some unskil. ful hand : for he does not demonstrate, as is necessary, that the two straight lines which bisect the sides of the triangle at right angles must meet one another; and, without any reason, he divides the proposition into three cases; whereas, one and the same construction and demonstration serves for them all, as Campanus has observed ; which useless repetitions are now left out: the Greek text also in the corollary is manifestly si. tiated, where mention is made of a given angle, though there neither is, nor can be any thing in the proposition relating to a given angle.
PROP. XV. and XVI. B. IV.
In the corollary of the first of these, the words equilateral and equiangular are wanting in the Greek : and in prop. 16, instead of the circle of ABCD, ought to be read the circumference ABCD : where mention is made of its containing fifteen equal parts.
DEF, III. B. V.
MANY of the modern mathematicians reject this definition: the very learned Dr. Barrow has explained it at large at the end of his third lecture of the year 1666, in which also he answers the objections made against it as well as the subject would allow: and at the end gives his opinion upon the whole, as follows :
“ I shall only add, that the author had, perhaps, no other “ design in making this definition, than that he might more “ fully explain and embellish his subject) to give a general " and summary idea of ratio to boginners, by premising • this metaphysical definition, to the more accurate defini6 tions of ratios that are the same to one another, or one of “ which is greater, or less than the other: I call it a meta“physical, for it is not properly a mathematical definition, “ since nothing in mathematics depends on it, or is deduced, “ nor, as I judge, can be deduced from it: and the defini“tion of analogy, which follows, viz. Analogy is the simi
“ litude of ratios, is of the same kind, and can serve for no Book V. “ purpose in mathematics, but only to give beginners some “ general, though groșs and confused notion of analogy : but « the whole of the doctrine of ratios, and the whole of mathe“ matics, depend upon the accurate mathematical definitions “ which follow this: to these we ought principally to attend, as " the doctrine of ratios is more perfectly explained by them; " this third, and others like it, may be entirely spared without
any loss to geometry; as we see in the 7th book of the Ele
ments, where the proportion of numbers to one another is " defined, and treated of, yet without giving any definition of " the ratio of numbers; though such a definition was as neces
sary and useful to be given in that book, as in this : but in“ deed shere is scarce any need of it in either of them: though “ I think that a thing of so general and abstracted a nature, and " thereby the more difficult to be conceived and explained, can.
not be more commodiously defined than as the author has “ done: upon which account I thought fit to explain it at large, “ and defend it against the captious objections of those who “ attack it.” To this citation from Dr. Barrow I have nothing to add, except that I fully believe the 3d and 8th definitions are not Euclid's, but added by some unskilful editor.
DEF. XI. B. V.
It was necessary to add the word "continual” before “pro“ portionals” in this definition ; and thus it is cited in the 33d prop. of book 11.
After this definition ought to have followed the definition of compound ratio, as this was the proper place for it ; duplicate and triplicate ratio being species of compound ratio. But Theon has made it the 5th def. of book 6, where he gives an absurd and entirely useless definition of compound ratio : for this reason we have placed another definition of it betwixt the 11th and 12th of this book, which, no doubt, Euclid gave; for he cites it expressly in prop. 23, book 6, and which Clavius, Herigon, and Barrow have likewise given, but they retain also Theon's, which they ought to have left out of the Elements.
DEF. XIII. B. V.
This, and the rest of the definitions following, contain the explication of some terms which are used in the 5th and following books; which, except a few, are easily enough understood from
Book V. the propositions of this book where they are first mentioned :
they seem to have been added by Theon, or some other. However it be, they are explained something more distinctly for the sake of learners.
PROP. IV. B. V.
In the construction preceding the demonstration of this, the words a $70XC, any whatever, are twice wanting in the Greek, as also in the Latin translations; and are now added, as being wholly necessary.
Ibid. in the demonstration; in the Greek, and in the Latin translation of Commandine, and in that of Mr. Henry Briggs, which was published at London in 1620, together with the Greek text of the first six books, which translation in this place is followed by Dr. Gregory in his edition of Euclid, there is this sentence following, viz. “ and of A and C have been taken “ equimultiples K, L; and of B and D, any equimultiples “ whatever (ci stuxs) M, N;" which is not true, the words
any whatever,” ought to be left out: and it is strange that neither Mr. Briggs, who did right to leave out these words in one place of prop. 13 of this book, nor Dr. Gregory who changed them into the word “some" in three places, and left them out in a fourth of that same prop. 13, did not also leave them out in this place of prop. 4, and in the second of the two places where they occur in prop. 17, of this book, in neither of which they can stand consistent with truth : and in none of all these places, even in those which they corrected in their Latin translation, have they cancelled the words á stuxe in the Greek text, as they ought to have done.
The same words á stuys are found in four places of prop. II, of this book, in the first and last of which they are necessary, but in the second and third, though they are true, they are quite superfluous; as they likewise are in the second of the two places in which they are found in the 12th prop. and in the like places of prop. 22, 23, of this book; but are wanting in the last place of prop. 23, as also in prop. 25, book 11.
COR. IV. PROP. B. V.
This corollary has been unskilfully annexed to this proposition, and has been made instead of the legitimate demonstration, which, without doubt, Theon, or some other editor, has taken away, not from this, but from its proper place in