of which the one shall be greater than the circumference of that circle, and the other lefs. In the fame manner, the quadrature of the circle is performed only by approximation, or by finding two rectangles nearly equal to one another, one of them greater, and another lefs than the space contained within the circle. In the Second Book of the Supplement, which treats of the interfection of Planes, I have departed as little as poffible from Euclid's method of confidering the fame fubject in his eleventh Book. The demonftration of the fourth propofition is from Legendre's Elements of Geometry; that of the fixth is new, as far as I know; as is alfo the folution of the problem in the nineteenth propofition; a problem which, though in itself extremely fimple, has been omitted by Euclid, and hardly ever treated of, in an elementary form, by any geometer. With respect to the Geometry of Solids, in the Third Book, I have departed from EUCLID altogether, with a view of rendering it both shorter and more comprehenfive. This, however, is not attempted by introducing a mode of reafoning lefs rigorous than that of the Greek geometer; for this would be to pay too dear even for the time that might thereby be faved; but it is done chiefly by laying afide a certain rule, which, though though it be not effential to the accuracy of demonftration, EUCLID has thought it proper, as much as poffible, to obferve. The rule referred to, is one which influences the arrangement of his propofitions through the whole of the Elements, viz. That in the de-. monftration of a theorem, he never fuppofes any' thing to be done, as any line to be drawn, or any figure to be conftructed, the manner of doing which he has not previoufly explained. Now, the only use of this rule is to prevent the admiffion of impoffible or contradictory fuppofitions, which, no doubt, might lead into error; and it is a rule well calculated to anfwer that end, as it does not allow the existence of any thing to be fuppofed, unless the thing itself be actually exhibited. But it is not always neceffary to make ufe of this defence; for the exiftence of many things is obviously poffible, and very far from implying a contradiction, where the method of actually exhibiting them may be altogether unknown. Thus, it is plain, that on any given figure as a base, a folid may be conftituted, or conceived to exift, equal to a given folid, (because a folid, whatever be its base, as its height may be indefinitely varied, is capable of all degrees of magnitude, from nothing upwards), and yet it may in many cafes be a problem of extreme difficulty to affign the height of fuch a folid, and actually actually to exhibit it. Now, this very fuppofi-, tion, that on a given bafe a folid of a given magnitude may be conftituted, is one of thofe, by the introduction of which, the Geometry of Solids is much shortened, while all the real accuracy of the demonstrations is preferved; and therefore, to follow, as EUCLID has done, the rule that excludes this, and fuch like hypothefes, is to create artificial difficulties, and to embarrass geometrical investigation with more obftacles than the nature of things has thrown in its way. It is a rule, too, which cannot always be followed, and from which even EUCLID himself has been forced to depart, in more than one instance. In the Book, therefore, on the Properties of Solids, which I now offer to the public, I have not fought to fubject the demonftrations to the law juft mentioned, and have never hesitated to admit the existence of fuch folids, or fuch lines as are evidently poffible, though the manner of actually defcribing them may not have been explained. In this way, I have been enabled to offer that very refined artifice in geometrical reafoning, to which we give the name of the Method of Exhauftions, under a much fimpler form than it appears in the 12th of EUCLID; and the fpirit of the method may, I think, be beft learned when it is thus difengaged from every thing not effential. That it may be the better understood, and because the the demonftrations which require exhauftions are, no doubt, the most difficult in the Elements, they are all conducted as nearly as poffible in the fame way, in the cafes of the different folids, from the pyramid to the fphere. The comparison of this laft folid with the cylinder, concludes the laft Book of the Supplement, and is a propofition that may not improperly be confidered as terminating the elementary part of Geometry. The Book of the Data has been annexed to feveral editions of Euclid's Elements, and particularly to Dr SIMSON's, but in this it is omitted altogether. It is omitted, however, not from any opinion of its being in itself useless, but because it does not belong to this place, and is not often read by beginners. It contains the rudiments of what is properly called the Geometrical Analyfis, and has itself an analytical form; and, for these reasons, I would willingly referve it, or rather a compend of it, for a feparate work, intended as an introduction to the ftudy of that analyfis. In explaining the elements of Plane and Spherical Trigonometry, there is not much new that can be attempted, or that will be expected by the intelligent reader. Except, perhaps, some new demonstrations, and fome changes in the arrangement, these two treatifes have, accordingly, no novelty to boast of. The Plane Trigonometry is fo divided, divided, that the part of it that is barely fufficient. for the refolution of Triangles, may be eafily taught by itself. The method of conftructing the Trigonometrical Tables is explained, and a demonstration is added of thofe properties of the fines and co-fines of arches, which are the foundation of thofe applications of Trigonometry, lately introduced, with so much advantage, into the higher Geometry.. In the Spherical Trigonometry, the rules for preventing the ambiguity of the folutions, whereever it can be prevented, have been particularly attended to; and I have availed myself as much as poffible of that excellent abftract of the rules of this fcience, which Dr MASKELYNE has prefixed to TAYLOR's Tables of Logarithms. An explanation of NAPIER's very ingenious and useful rule of the Circular parts is here added as an appendix to Spherical Trigonometry. It has been objected to many of the writers on Elementary Geometry, and particularly to EUCLID, that they have been at great pains to prove the truth of many fimple propofitions, which every body is ready to admit, without any demonftration, and that thus they take up the time, and fatigue the attention of the ftudent, to no purpose. To this objection, if there be any force in it, the |