PREFACE. MECHANICAL SCIENCE is, beyond all doubt, one of the most useful branches of education which any age, or any country, can boast of. It is mechanical science that has been chiefly instrumental in raising England to that proud and enviable situation which she now holds in the scale of nations, and in making her the emporium of the trade of the whole world. Any attempt, therefore, to simplify or add to our stock of knowledge on this branch, however unsuccessful, may be freely pardoned. The work which we now lay before the public consists principally of three parts. The first part is both theoretical and practical, and contains the elementary principles of Statics and Dynamics, in which the articles are all regularly numbered; for we considered this necessary in point of reference, as the propositions have a dependance upon each other. But the other parts being mostly practical, we did not consider such an arrangement necessary in them. In the parts on falling bodies, &c. we have given practical rules; and each example is done both by the formulæ and by the rules which are deduced from them. On account of the great usefulness of algebraic formula, we would advise every one to make himself expert in substituting numbers for the letters; and this may be very easily effected, even by those who know nothing of algebra except the signs and the notation. The second part contains an account of the Steam-Engine, with its various proportions, in which is given a more extensive view of Parallel Motion than is to be found in almost any other work. This is a subject which has not only very much perplexed the practical mechanic, but also many whose scientific acquirements are considerably above mediocrity. The learned vi Professor Millington, at page 289 of his Epitome of Natural Philosophy, has given a method for finding the length of the radius rod, which may easily mislead the practical mechanic. His method is to make the radius rod always equal to A B, (see Fig. 5 to Steam Engine) which will only hold when the point B is in the middle of A C', or, which is the same, when A B = B C, and the end of the radius rod is put on at D; for if it be put on at any other point E or e, either above or below the point D, it will cease to hold in this case also. But the learned professor seems not to have considered this subject with that attention which is necessary; for he says that the point D will describe a right-lined motion, which it transmits to C; but this is evidently false, for since one end of the radius rod is fixed, the other end D will evidently describe a circle. Also, we have demonstrably proved that the point F, and not the point D, describes the same kind of a line as the point C, where the piston rod is attached. cylinders, and n A knowledge of the proportional friction of cylinders is useful to the mechanic, though not mentioned in the body of the work. The proportion which the friction of a large cylinder bears to the friction of any number, the sum of the areas of which is equal to the area of the large cylinder, may be easily shewn as follows. Let d = the diameter of one of the small the number of them; then d2 n = area of the large one in circular inches; and √d2 n = d √ n = the diameter of the large cylinder. But the friction is proportional to the circumference of the cylinder; therefore the friction of the small cylinders may be represented by 3.1416 d n, and the friction of the large cylinder may be represented by 3-1416 dn. Hence the friction of the large cylinder to the friction of all the small ones :: 3.1416 dn: 3·1416 d n :: √n :n, which, in the case of four small cylinders, becomes 1 to 2 that is, the friction of four cylinders is double the friction of one cylinder, the area of which is equal to the sum of the areas of all the four. ; The third part contains the first principles of Hydrostatics and Hydraulics, the Specific Gravities of bodies, and an extensive collection of problems to elucidate these subjects; at the end of which is added a short dissertation on Rail-roads. But we would refer those who wish to have a profound knowledge on this sub vii ject to Mr. Wood's excellent practical Treatise on Rail-roads also, much valuable information may be had by consulting Mr. Tredgold's work on the same subject. We have appended tables of various kinds, viz. a table of areas of circles, from 1 inch diameter up to 80 inches ;-parallel motion tables, in calculating which we were assisted by Mr. Robert Milburn, engineer, Friars Goose, who, by an ingenious contrivance, tried a great number of them experimentally, none of which ever deviated sensibly from a vertical straight line;-also tables on cast and wrought iron, with an example to each to shew how they are calculated. We have given, lastly, a table of the diameters of piston rods for high pressure engines, pressure from 20 to 50 lbs. per square inch: these tables are calculated at upwards of double pressure. The works which we have consulted on Mechanics are those of Dr. Gregory, and his edition of Hutton's Course; also the works of Whewell, Bridge, Marrat, and Emerson. The articles on Central Forces are principally taken from Gregory's Mathematics for Practical Men. Those parts which are taken from the invaluable works of Barlow and Tredgold are noticed in the body of the work. We have omitted all those parts on the forms of the teeth of wheels, considering that for want of room we would not be able to do justice to them. On this subject, see the works of Gregory, Buchanan, Camus, &c. In conclusion, as we have occasionally pointed out the errors of other writers, we sincerely hope that our readers will deal as candidly with any which may have inadvertently crept into this work; and they may rest assured that we will cheerfully acquiesce in the discovery of our blunders, for it is not our wish to mislead the practical mechanic by that which is false, clothed in the garb of truth. |