Abbildungen der Seite
PDF
EPUB

cultivated 240 acres of land in La Vendee, on chymical principles, in order to set a good example to the farmers; and his mode of culture was attended with so much success, that he obtained a third more of crop than was procured by the usual method, and in nine years his annual produce was doubled. I might also have illustrated the practical advantages of chymical science in relation to the art of extracting metals from their ores,-the conversion of iron into steel, and the metallic ore into malleable iron—the manufacture of glass, alum, copperas, blue vitriol, soda, potash, JMorocco-leather, paper, starch, varnish, and Prussianblue—the refining of sugar, saltpetre, gold and silver—the artificial formation of ice—the method of preserving fish, meat, and other articles of food, and various other processes connected with the practical departments of life, all of which are strictly chymical operations, and can be improved and brought to perfection chiefly by the knowledge and application of the doctrines and facts of chymical science. With regard to the professions of the physician, surgeon, and apothecary, it is now universally admitted, that an extensive acquaintance with the principles and facts of chymistry is essentially requisite to the successful practice of these arts. The human body may be considered as a species of laboratory, in which the various processes of absorption, secretion, fermentation, composition and decomposition are incessantly going forward. Every article of food and drink we throw into the stomach, every portion of atmospheric air we receive into the lungs, every impression we derive from the surrounding elements, every motion of the heart and lungs, and every pulse that vibrates within us, may be considered as effecting a chymical change in the vital fluids, and in every part of the animal system; the nature of which it is of the utmost importance to the medical practitioner thoroughly to investigate and understand. For, how can he be supposed to be successful in his attempts to counteract the disorders to which the human frame is incident, and to produce a chymical effect on the constitution of his patient, if he is ignorant either of the processes which are going on in the system, of the chymical properties of the substances which he throws into it, or of the effects which they will certainly produce 7 Is he is ignorant of the chymical affinities that subsist between the various articles of the Materia Medica, he may often administer preparations which are not only inefficacious, but even poisonous and destructive to his patient. When two chymical substances, each of which might be administered separately with safety, are combined, they sometimes produce a substance which is highly deleterious to the animal system. For example, although mercury and orygenized muriatic acid

have both been administered, and either of thorn may be taken separately without injury to the animal economy, yet if a medical practitioner, ignorant of the chymical affinities of such substances, and of the quality of the compound, should give both of them in conjunction, the arost dreadful consequences might ensue: since the product of this mixture, orygenized muriate of mercury, is known to be a most corrosive polson; and there can be little doubt that hundreds of lives have been destroyed, by ignorant pretenders to medical science, in consequence of the injudicious administration of such deleterious preparations. But chymistry is not the only science which is of utility in the arts which minister to the comfort and pecuniary interests of society. Geometry, trigonometry, conic sections, and other branches of mathematical knowledge; hydrostatics, hydraulics, mechanics, optics, botany, mineralogy and the other departments of the physical sciences, may be rendered of essential service to artisans and mechanics of various descriptions. All the sciences are, in some degree, connected, and reflect a mutual light upon one another; and consequently the man who has the most extensive acquaintance with science, is best qualified for carrying to perfection any one department of the useful arts. Practical Geometry is highly useful to almost every mechanic and artisan, particularly to mill-wrights, bricklayers, carpenters and masons. It teaches them to form angles of any assigned number of degrees, to draw parallel and perpendicular lines, to proportion circumferences to diameters, to divide circular rims into any number of parts, to estimate the square or cubical contents of any piece of workmanship, and to calculate the price they ought to receive for any work they perform, according to its solid or superficial dimensions. In forming estimates of the expense of any proposed undertaking, the carpenter, bricklayer, and architect must find such knowledge essentially requisite, and even the common labourer who undertakes the formation of roads, the digging of pits, and the clearing away of rubbish, will find the principles of arithmetic and geometry of important service in estimating the rate at which he can perform such operations. The following geometrical theorems, besides many others, are capable of a variety of practical applications, in many departments of the arts. “If, from the two ends of any diameter of the circle, two lines be drawn to meet in any one point of the circle whatever, such lines are perpendicular to each other,” or, in other words, they form a right angle at the point of contact.* Again, “The

[merged small][graphic]

areas of all circles are in exact proportion to the squares of their radii, or half diameters.” If, for example, we draw a circle with a pair of compasses whose points are stretched 4 inches asunder, and another with an extent of 8 inches, the large circle is exactly four times the size or area of the small one. For the square of 4 is = 16, and the square of 8 is = 64, which is four times 16. And as the circumferences of the circles are in proportion to the radii, it will follow, that the length of a string which would go round the curve of the larger circle is exactly double the length of one which would go round the lesser. Mechanics, in recognising such theorems, will meet with many opportunities of reducing them to practice.—Again, there is a figure which Geometricians term a parabola, which is formed every time we pour water forcibly from the mouth of a tea-kettle, or throw a stone forward from the hand. One property of the parabola is, that if a spout of water be directed at half a perpendicular from the ground, or at an angle of elevation of 45 degrees, it will come to the ground at a greater distance than if any other direction had been given it, a slight allowance being made for the resistance of the air. Hence the man who guides the pipe of a fire-engine may be directed how to throw the water to the greatest distance, and he who aims at a mark, to give the projectile its proper direction.—To surveyors, navigators, land-measurers, gaugers and engineers a knowledge of the mathematical sciences is so indispensably requisite, that without it, such arts cannot be skilfully exercised.

[ocr errors]

In like manner the lines AD, and B D, A B and B E, will stand at right angles to each other; and the same will be the case to whatever point of the circle such lines are drawn. The practical applicatlon of this principle, in various operations, will, at once, be obvious to the Intelligent mechanic, es: pecially when he intends the two ends or sides of

fle.

any piece of machinery to stand

perpendicular to each other.

The physical sciences are also of the greatest utility in almost every department of art. To masons, architects, ship-builders, carpenters and every other class employed in combining materials, raising weights, quarrying stones, building piers and bridges, splitting rocks, or pumping water from the howels of the earth,--a knowledge of the principles of mechanics and dynamics is of the first importance. By means of these sciences the nature of the lever and other mechanical powers may be learned, and their forces estimated—the force produced by any particular combination of these powers calculated–and the best mode of applying such forces to accomplish certain effects, ascertained. By a combination of the mechanical powers the smallest force may be multiplied to an almost indefinite extent, and with such assistance man has been enabled to rear works and to perform operations which excite astonishment, and which his own physical strength, assisted by all that the lower animals could furnish, would have been altogether inadequate to accomplish. An acquaintance with the experiments which have been made to determine the strength of materials, and the results which have been deduced from them, is of immense importance to every class of mechanics employed in engineering and architectural operations. From such experiments, (which have only been lately attended to on scientific principles) many useful deductions might be made respecting the best form of mortises, joints, beams, tenons, scarphs, &c.; the art of mast making, and the manner of disposing and combining the strength of different substances in naval architecture, and in the rearing of our buildings. For example,_from the experiments now alluded to it has been deduced, that the strength of any piece of material depends chiefly on its depth, or on that dimension which is in the direction of its strain. A bar of timber of one inch in breadth, and two inches in depth is four times as strong as a bar of only one inch deep; and it is twice as strong as a bar two inches broad and one deep, that is, a joint or lever is always strongest when laid on its edge. Hence it follows, that the strongest joist that can be cut out of a round tree is not the one which has the greatest quantity of timber in it, but such that the product of its breadth by the square of its depth shall be the greatest possible.—Again, from the same experiments it is found, that a hollow tube is stronger than a solid rod containing the same quantity of matter. This property of hollow tubes is also accompanied with greater stiffness; and the superiority in strength and stiffness is so much the greater as the surrounding shell is thinner in proportion to its diameter. Hence we find that the bones of men and other animals are formed hollow, which renders them incomparably stronger aid stiffer, gives more room for the insertion of muscles,

[graphic]

and makes them lighter and more agile, than if they were constructed of solid matter. In like manner the bones of birds, which are thinner than those of other animals, and the quills in their wings, acquire by their thinness the strength which is necessary, while they are so light as to give sufficient buoyancy to the animal in its flight through the aerial regions. Our engineers and carpenters have, of late, begun to imitate nature in this respect, and now make their axles and other parts of machinery hollow, which both saves a portion of materials and renders them stronger than if they were solid.* The departments of hydrostatics and hydraulics, which treat of the pressure and motion of fluids, and the method of estimating their velocity and force, require to be thoroughly understood by all those who are employed in the construction of common and forcing-pumps, water-mills, fountains, fire-engines, hydrostatical presses; and in the formation of canals, wetdocks, and directing the course of rivers; otherwise they will constantly be liable to commit egregious blunders, and can never rise to eminence in their respective professions. Such principles as the following:—that fluids press equally in all directions,—that they press as much upwards as downwards,--that water, in several tubes that communicate with each other, will stand at the same height, in all of them, whether they be small or great, perpendicular or oblique, that the pressure of fluids is directly as their perpendicular height, without any regard to their quantity-and that the quantities of water discharged at the same time, by different apertures, under the same heigh of surface in the reservoir, are to each other nearly as the areas of their apertures,-will be found capable of extensive application to plumbers, engineers, pump-makers, and all who are employed in conducting water over hills or vallies, or in using it as a mechanical power, by a recognition of which they will be enabled to foresee, with certainty, the results to be exnected from their plans and operations; for want of which knowledge many plausible schemes have been frustrated, and sums of money expended to no purpose. The following figures and explanations will tend to illustrate some of the principles now stated: —l. Fluids press in proportion to their perpendicular heights, and the base of the vessel containing them, without regard to the quantity. Thus, if the vessel ABC, fig. 2, has its base BC equal to the base FG of the cylindrical vessel DEFG, fig. 1, but is much smaller at the top A than at the bottom, and of the same height; the pressure upon the bottom BC is as great as

• The mechanical reader who wishes particular information on this subject is referred to the article Strength of materials in Ency. Brit: 3d edit: which was written by the late Professor Robison.

FG, when they are filled with water, or any other liquid, notwithstanding that there will be a much greater quantity of water in the cylindrical than in the conical vessel; or, in other words, the bottom BC will sustain a pressure equal to what it would be if the vessel were as wide at

the top as at the bottom. In like manner, the bottom of the vessel HIKL, fig. 3, sustains a pressure only equal to the column whose base is KL, and height KM, and not as the whole quantity of fluid contained in the vessel; all the rest of the fluid being supported by the sides. The demonstration of these positions would occupy too much room, and to many readers would appear too abstract and uninteresting; but they will be found satisfactorily demonstrated in most books which treat of the doctrines of hydrostatics. 2. The positions now stated form the soundatich of the hydrostatical paradox, namely, “that a quantity of fluid, however small, may be made to counterpoise a quantity, however great." Thus, if to a wide vessel AB, we attach a tube CD, communicating with the vessel, and pour

[graphic]

A Fig. 4.

water into it, the water will run into the larger vessel AB, and will stand at the same height C and G in both. If we affix an inclined tube EF, likewise communicating with the large vessel, the water will also stand at E, at the same height as in the other two; the perpendicular altitude •eing the same in all the three tubes, however mall the one may be in proportion to the other. This experiment clearly proves that the small column of water balances and supports the large column, which it could not do if the lateral pressures at bottom were not equal to each other.

Whatever be the inclination of the tube EF, still the perpendicular altitude will be the same as that of the other tubes, although the column of water must be much longer than those in the upright tubes. Hence it is evident, that a small quantity of a fluid may, under certain circumstances, counterbalance any quantity of the same fluid. Hence also the truth of the principle in hydrostatics, that “in tubes which have a communication, whether they be equal or unequal, short or oblique, the fluid always rises to the same height.” From these facts it follows, that water cannot be conveyed by means of a pipe that is laid in a reservoir to any place that is higher than the reservoir. These principles point out the mode of conveying water across valleys without those expensive aqueducts which were erected by the ancients for this purpose. A pipe, conforming to the shape of the valley, will answer every purpose of an aqueduct. Suppose the spring at A, fig. 5, and water is wanted on the other side of the valley to supply the house H, a pipe of lead or iron laid from the spring-head across the valley will convey the water up to the level of the spring-head; and if the house stand a little lower than the spring-head, a constant stream will pour into the cisterns and ponds where it is required, as if the house had stood on the other side of the valley; and, consequently, will save the expense of the arches BB, by which the ancient Romans conducted water from one hill to another. But,

if the valley be very deep, he pipes must be made very strong near its bottom, otherwise they will be apt to burst; as the pressure of water increases in the rapid ratio of 1, 3, 5, 7, 9, &c. and is always in proportion to its perpendicular height.

3. Fluids press in all directions, laterally and upwards, as well as downwards. That fluids press laterally may be seen by boring a hole in the side of a cask containing any liquid, when the liquid will run out in consequence of the lateral pressure. The upward pressure is not so obvious, but is clearly proved by the following experiment, with an instrument generally termed the hydrostatic below-j-This machine con

sists of two thick oval boards, about 18 inches long and 16 inches broad, united to each other by leather, so as to open and shut like a pair of common bellows, but without valves. Into this instrument a pipe B, several feet high, is fixed at D. If we pour water into the pipe at its top C, it will run into the bellows and separate the boards a little. If we then lay three weights, each weighing 100 pounds, upon the upper

[graphic]
[graphic]
[ocr errors]

will run into the bellows, and raise up the board with all the weights upon it. And though the water in the tube should weigh in all only a quarter of a pound, yet the pressure of this small force upon the water below in the bellows shall support the weights, which are 300 pounds; nor will they have weight enough to make them descend, and conquer the weight of water, by forcing it out of the mouth of the pipe. The reason of this will appear from what has been already stated respecting the pressure of fluids of equal heights, without any regard to the quantities. For, if a hole be made in the upper board, and a tube be put into it, the water will rise in the tube to the same height that it does in the pipe; and it would rise as high (by supplying the pipe) in as many tubes as the board would contain holes. Hence, if a man stand upon the upper board, and blow into the bellows through the pipe, he will raise himself upward upon the board; and the smaller the bore of the pipe is, the easier will he be able to raise himself. And if he put his finger on the top of the pipe he may support himself as long as he pleases. The uses to which this power may be applied are of great variety and extent; and the branches

Fig. 7.

of art dependent upon it appear to be yet intheir infancy. By the application of this power the late Mr. Bramah formed what is called the Hydrostatic Press, by which a prodigious force is obtained, and by the help of which, hay, straw, wool, and other light substances, may be forced into a very small bulk, so as to be taken in large quantities on board a ship. With a machine, on this principle, of the size of a tea-pot, standing before him on a table, a man is enabled to cut through a thick bar of iron as easily as he could clip a piece of pasteboard with a pair of sheers. By this machine a pressure of 500 of 600 tons may be brought to bear upon any substances which it is wished to press, to tear up, to cut in pieces, or to pull asunder. Upon the same principle, the tun or hogshead HI, fig. 7, when filled with water, may be burst, by pressing it with some pounds additional weight of the fluid through the small tube KL, which may be supposed to be from 25 to 30 feet in height. From what has been already stated, it necessarily follows, that the small quantity of water which the tube KL, contains, presses upon the bottom of the tun with as much force as if a column of water had been added as wide as the tum itself, and as long as the tube, which would evidently be an enormous weight

[graphic]
[graphic]
« ZurückWeiter »