bridge Courfe," and vying with Dr. Gregory, as to the size and price of his publication. The fubje&t is divided into five books, each containing a particular branch, which we fhall feparately confider. Among the definitions we find a very important one, that of weight deferred; now as most authors have been careful early to diftinguifh it as a relation of gravity, and compare the effects. of inertia and weight rather than inertia and gravity, we fee no reafon for refufing it a place which all others have affigned it; for gravity being defined, a tendency of all bodies to the earth, and inertia, a refiftance to the communication of motion, there is lefs liability to confound their effects: than thofe of weight and inertia; the latter being relations,: the former properties of matter. In the first fection, treating of the compofition and refolution of motion, the author feems to have been at very great pains to extend what we think a very fimple fubject, to an unnecellary length; and by the introduction of new terms, as refultants, compofants, &c. and the use of single letters to expreis the abstract values of forces, has rendered it very obfcure" and tedious. Letters reprefenting linear diftance, as other authors have adopted them, muft evidently be preferable to fingle quantities, because they not only well represent the quantity but alfo direction of fuch forces, and the eye catches at first glance the different relations they are intended to exprefs. Of both thefe objections we can mention a kind of fpecimen, in the proof Mr. M. gives of a plain propofition, viz. that the force compounded of several others in the fame plane is alfo in that plane. Vid. Art. 28, page 8. In the fucceeding chapters on moments we think many of the propofitions are by far too premature, and his definítion of momentum of a force (if the expreffion be not in itself a folecifm) is at least confused; and his definition expreffes, rather in the fhape of a propofition, the effective part of the given force, eftimated in a certain direction, than informs the ftudent of the meaning of the term itself. The centre of gravity very properly follows in the next chapter; being a natural force, it feems ftrange that most writers on this fubject fhould have followed a different order. The propofitions are here well arranged and explained, and feveral folutions of particular cafes exhibited, not usually found in other writers, The mechanical powers are explained in a very simple. eafy manner, and illuftrated by the moft familiar examples and excellent plates. There is an ingenious fection on tooth wheels and the bevel geer; though here is perhaps one of the the many inftances in which the author forgot that he was writing a fchool-book. Several propofitions on the tenfions of cords are alfo fubjoined, which will be found useful in illuftrating the compofition and refolution of forces. The laft fection of this book is allotted to a very ingenious and fatisfactory account of the ftrength and ftrefs of timber, and concludes with a number of mifcellaneous examples, a plan preferved through the whole five books. The fecond book treats on Dynamics, or the actions of forces on matter to produce motion, in the very outlet of which we meet with a moft glaring fault in an elementary treatise, or a direct refutation that fuch was the author's in-' tention. Defining the two kinds of forces, impulfive or inftantaneous, and accelerating or retarding forces, we have the following: "The former of thefe, i. e. impulfive force, it will, from a little confideration, be evident, can only take place in perfectly hard and inflexible bodies, which have no existence in nature; and we may obferve, that in the abstract confideration of thefe, as well as of feveral other fimilar cafes in mechanics, when metaphyfical impoffibilities instead of the natural fate of things are attended to, difficulties arife which are hardly explicable by any method of reafoning whatever; for we are certain that when finite velocity, is communicated to any natural body, the time in which it is communicated muit be of fome finite quantity, fo that, when the body acted upon begins to move from quiefcence, it will, during the action of the force, poffefs all the intermediate degrees of velocity between nothing and the velocity ultimately communi. cated." ¿rced, yait Then follows a long note refpecting the law of continuity, with an abstract of the reafonings of Bofcovich to prove its univerfality. Now whether or not it be admiffible in any author to lay down as definitions the existence of two properties, and immediately proceed to prove the non-existence of the one, by reasonings profeffedly obfcure, we can by nomeans approve of fuch difcuffions in a book fpecifically written as a fubftitute to junior ftudents, for the more dry difquifitions of fpeculative writers. The other topics of this book are, uniform and varied motions of bodies moving in the perimeters of figures; of found; bodies falling freely by gravity, moving along inclined planes or curve furfaces; the fimple pendulum; collifion of bodies; the motion of projectiles; together with fome excellent remarks and examples on the theory and prac tice of gunnery. The laft fection is on central forces, in which fome propofitions of Newton's Principia are fhortly explained and clearly illuftrated. On the whole, we think the arrangement of the fubjects under this head extremely proper, and deferving the attention of more laborious compilers. The 3d and 4th books are on the fubject of Hydrostatics, with that of Pneumatics, which, as they have ever been held diftin&t branches of natural philofophy, we cannot fee why they fhould now be claffed under a work ftrictly meant to treat of mechanics; unless Mr. M. had chosen some title for his work of more extended meaning. As they are, however, we willingly give him credit for the precifion and neatness with which thofe principles are elucidated which often confuse if not mislead the young fcholar. We cannot, however, help remarking, that there is the fame objection to his introducing the fpeculations of more advanced philofophers in his definitions; fpeculations introduced in our very entrance on a fubject, when we can form no folid judgments, will always confound, never produce conviction. Thus, as in all our reafonings concerning fluids, we muft, for the proper comprehenfion of the fubject, make the diftinction of them into elaftic and non-elaftic, compreffible and incompreffible, the introducing of Mr. Canton's enquiries in fuch a place, if not frivolous, muft rather tend to fhow the author's reading than to facilitate the young ftudent's progrefs. The propofitions are illuftrated, and many ingenious contrivances investigated and explained; among the latter we think that of the ancient Clepfydra will be found not the least useful and entertaining. The 5th and laft book is purely mathematical; after the first fection on finding the centre of gravity of bodies, we are presented with the centrobaryc, or method of difcovering the contents, areas, furfaces, &c. of bodies formed by their revolution, deduced from the expreffion which preceding fluxional investigations have given. This method, though fimple, we have rarely met with in any author, and yet it certainly ought to find its place in other treatifes of this kind. The investigation of the centres of gyration, percuffion, &c. follow; the times of emptying veffels, &c. There is alfo a good section on the motion and maximum effects of machines, with the principle of D'Alembert for eftimating the effective parts of velocities communicated; and laftly, fome ingenious and practical remarks on the effects of friction,: wheel-carriages, and the ufe of fly-wheels. With the exception of fome few, the whole are very fimilar in their demonftrations to fuch as we have feen in other authors, and almoft almost entirely depending on the fluxional calculus, which we think will lead the juvenile mind into inquiries too ab frufe for his comprehenfion. On the whole, the fubject is of fuch a nature, that where an individual can add but little to the mafs of previous information, whoever undertakes to give it in a more enlarged or comprehenfive view must neceffarily be indebted for the body of his work to the labours of others, and the author muft efcape the odium of plagiarifm, even though we difcover the frequent use of foreign aid in the fimilarity of his demonftrations. Mr. Marrat would, however, have incurred a confiderable portion of our cenfure in this refpect, from the very frequent identity we have obferved with the works of others, had he not difclaimed all precedence in this walk of fcience, by profeffing to follow, in the humbler line of inftructing the juvenile part of the community. As he has been at confiderable pains to bring to their notice the utmost limits of the subject in a compreffed form, and to combine with the theory fuch portion of what is really and practically useful, even though there be lefs originality in the matter than might have been expected, we must give him ample credit for having fimplified, and illuftrated by a copious and judicious felection of examples, the matter he has treated; and we think his work will ftand a fair chance, if not of being ranked among the fpeculations of profounder theorists, of finding its merited place among the useful philofophy of our practical mechanicians. ART. IV. Poems, by William Robert Spencer. Svo. Cadell and Davies. p. 10s. 6d. 1811. SEVERAL of thefe Poems have before been printed, and that called "Leonora,' as well as "The Year of Sorrow," have long and defervedly been favourites with the public. The characteristics of Mr. Spencer's compofitions; are cafe, elegance, tenderness, and a true claffical tafle. They who are fond of that defcription of poetry, in which thefe, diftinctions form the effential and prominent ingre dients, will be delighted to poffefs these productions in one handfome volume, and we can have but few readers, it may may be prefumed, who will not thank us for the infertion of the following fpecimens : "THE BLUSH.-AN ENIGMA.. "When first o'er Pfyche's angel breaft "No limbs my airy charms obfcure, "I fleep with beauty, watch with fear, "Love's funshine, beamed from brightest eyes, Than my aufpicious light, which flies Like meteors o'er the heaven of beauty.” |