Abbildungen der Seite
PDF
EPUB

A few years ago, a friend of mine, when in Ireand, performed this experiment to convince an English gentleman, who called in question the principle, and who laid a bet of fifty pounds that it would not succeed. A hogshead, above 3 feet high, and above 2 feet wide, was filled with water; a leaden tube, with a narrow bore, between 20 and 30 feet long, was firmly inserted into the top of the hogshead; a person, from the upper window of a house, poured in a decanter of water into the tube, and, before the decanter was quite emptied, the hogshead began to swell, an 1, in two or three seconds, burst into pieces, while the water was scattered about with immense force. Hence, we may easily perceive what mischief may sometimes be done by a very small quantity of water, when it happens to act according to its perpendicular height. Suppose, that in any building, near the foundation, a small quantity of water, only of the extent of a square yard, has settled, and suppose it to have completely filled up the whole vacant space, if a tube of 20 feet long were thrust down into the water, and filled with water from above, a force of more than 5 tons would be applied to that part of the building, which would blow it up with the same force as gunpowder." The same effect may sometimes be produced by rain falling into long narrow chinks, that may have inadvertently been left in building the walls of a house; which shows the importance of filling up every crevice and opening of a building, and rendering the walls as close and compact as possible. Hence, likewise, similar processes in nature, connected with pools of water in the bowels of the earth, may occasionally produce the most dreadful devastations. For, should it happen, that, in the interior of a mountain, two or three hundred feet below the surface, a pool of water thirty or forty square feet in extent, and only an inch or two in depth, was collected, and a small crevice or opening of half an inch in breadth were continued from the surface to the water in the pool; and were this crevice to be filled with rain or melted snow, the parts around the layer of water would sustain a pressure of more than sir hundred tons, which might shake the mountain to its centre, and even rend it with the greatest violence. In this way, there is every reason to believe, partial earthquakes have been produced, and large fragments of mountains detached from their bases. The principles now illustrated are capable of the most extensive application, particularly in all engineering and hydraulic operations. It is on the principle of the lateral and upward pressure of fluids that the water, elevated by the New River water-works, in the vicinity of London, after having descended from a bason in a vertical

[ocr errors]

pipe, and then, after having flowed horizontally in a succession of pipes under the pavement, is raised up again through another pipe, as high as the fountain in the Temple Garden. It is upon the same principle that a vessel may be filled either at the mouth or at the bottom indifferently, provided that it is done through a pipe, the top of which is as high as the top of the vessel to be filled. Hence, likewise, it follows, that when piers, aqueducts, or other hydraulic works for the retention of water, are to be constructed, it becomes necessary to proportion their strength to the lateral pressure which they are likely to sustain, which becomes greater in proportion to the height of the water to be sustained. Walls, likewise, designed to support terraces, ought te be sufficiently strong to resist the lateral pressure of the earth and rubbish which they are to sustain, since this pressure will be greater as the particles of earth, of which the terraces are composed, are less bound together, and in proportion as the terraces are more elevated. The increase of pressure in proportion to the depth of any fluid likewise shows the necessity of forming the sides of pipes or masonry in which fluids are to be retained, stronger towards the bottom, where the pressure is greatest. If they are no thicker than what is sufficient for resisting the pressure near the top, they will soon give way by the superior pressure near the bottom; and if they are thick enough in every part to resist the great pressure below, they will be stronger than necessary in the parts above, and, consequently, a superfluous expense, that might have been saved, will be incurred in the additional materials and labour employed in their construction. The same principle is applicable to the construction of flood-gates, dams, and banks of every description, for resisting the force of water. When the strength and thickness requisite for resisting the pressure at the greatest depth is once ascertained, the walls or banks may be made to taper upwards, according to a certain ratio founded on the strength of the materials, and the gradual decrease of pressure from the bottom upwards; or, if one side be made perpendicular, the other may proceed in a slanting direction towards the top. From the principles and experiments now stated, we may also learn the reason why the banks of ponds, rivers, and canals blow up, as it is termed. If water can insinuate itself under a bank or dam, even although the layer of water were no thicker than a half-crown piece, the pressure of the water in the canal or pond will force it up. In fig. 8, let A represent the section of a river or canal, and BB a drain running under one of its banks; it i. evident, that, if the bank C is not heavier than the column of water BB, that part of the bank must inevitably give way. This effect may be prevented in arañcial canals by making the sides very tight with clay heavily rammed down, or by cutting a trench EF, about a soot and a-half wide, along the bank of the river or canal, and a little deeper than the bottom of the canal, which being filled up with earth or clay

Fig. 8. E

well moistened with water, forms, when dry, a kind of wall through which the water cannot penetrate. By inattention to such circumstances many disasters have happened, and much expense needlessiy incurred; and, therefore, the scientific principles to whiéh I have now adverted ought to be known, even by labourers of the lowest rank employed in operations carried on for the improvement of the country. To the want of a recognition of these principles may be attributed the failure of the following scheme, and the disaster with which it was attended:—After the diving-bell was invented, it was considered desirable to devise some means of remaining for any length of time under water, and rising at pleasure without assistance. “Some years ago, an ingenious individual proposed a project, by which this end was to be accomplished. It consisted in sinking the hull of a ship made quite water-tight, with the decks and sides strongly supported by shores, and the only entry secured by a stout trap-door, in such a manner, that, by disengaging from within the weights employed to sink it, it might rise of itself to the surface. To render the trial more satisfactory and the result more striking, the projector himself made the first essay. It was agreed that he should sink in twenty fathoms water, and rise again without assistance at the expiration of 24 hours. Accordingly, making all secure, fastening down his trap-door, and provided with all necessaries, as well as with the means of making signals to indicate his situation, this unhappy victim of his own ingenuity entered, and was sunk. No signal was made, and the time appointed elapsed. An immense concourse of people had assembled to witness his rising, but in vain; for the vessel was never seen more. The pressure of the water at so great a depth had, no doubt, been completely under-estimated, and the sides of the vessel being at once crushed in, the unfortunate projector perished before he could

even make the signal concerted to indicate his distress.”* Many other applications of the principles of hydrostatics might have been mentioned, but what has been now stated may serve to exemplify the practical utility of an acquaintance with such principles, not only to engineers and superintendants of public works, but to mechanics and artificers of every description. The science of Pneumatics, which treats of the mechanical properties of the atmosphere, will likewise be found useful to mechanics and artists of various descriptions, to whom it is, in many cases, of importance to know something of the effects of the resistance, the pressure, and the elasticity of air. The construction of barometers, syphons, syringes, and air-pumps, depends upon the pressure of the atmosphere, and likewise water-pumps, fire-engines, and many other hydraulic machines; and, consequently, the constructors of such instruments and engines must frequently act at random, if they are unacquainted with the nature and properties of the atmosphere, and the agency it exerts in such mechanical contrivances.f Even the carpenter and the mason may be directed, in some of their operations, by an acquaintance with the doctrines of pneumatics. When two pieces of wood are to be glued together, they are first made as even and smooth as possible; the glue is then applied to one or both of the surfaces; they are then pressed together till the glue has become thoroughly dry. The use of the glue is to fill up every crevice in the pores of the wood, so as to prevent the admission of any portion of air between the pieces; and then the atmos

* Herschel's “Discourse on the Study of Nat. Philosophy.”

* As an illustration of the importance of being acquainted with the atmospheric pressure, the fol. lowing anecdote may be here inserted :-A respectable jo. of landed property, in one of the middle counties of Scotland, applied to a friend of mine, a Lecturer on Chymistry and Natural Philosophy, in order to obtain his advice respecting a pump-well which he had lately constructed at considerable expense. He told him, that, notwithstanding every exertion, he could not obtain a drop of water from the spout, although he was quite sure there was plenty of water in the well, and although. he had plastered it all around, and blocked up 6 very crevice. When my friend inspected the pump, he suspected that the upper part of the well was airtight, and, consequently, that the atmospheric pressure could not act on the surface of the water in the well. He immediately ered a hole to be bored adjacent to the pump, when the air rushed in with considerable force; and, on pumping, the water flowed copiously from the spout. e gentleman was both overjoyed and astonished; but it is somewhat astonishing, that neither lie, nor his neighbours, nor any of the workmen who had been employed in its construction, should have been able to point out the cause of the defect; but, on the other hand, should have taken the very opposite means for remedying it, namely, by plastering up every crevice, so as to produce a kind of vacuum within the well. This and similar facts show how little progress scientific knowledge has yet made, even among the middle classes of the community

w

[graphic]

phere, with a force equal to 15 pounds on every square inch, presses the pieces firmly together. A knowledge of this principle will suggest the propriety of filling up every opening or crevice, and continuing the pressure for some time, as the air, wherever it gains admission, has a tendency, by its elastic force, to loosen every species of cement. The same principle might direct bricklayers and masons, in building either stone or brick-walls, in suggesting the propriety of filling up every crevice with the most tenacious cements, so as to prevent the access of the external air to the interior of the walls. For there can be no question that the firmness and stability of our houses and garden-walls depend, in part, upon the pressure of the atmosphere, after the interior crevices are thoroughly filled up. An extensive knowledge of this science would likewise direct them to the proper mode of constructing the flues of chimneys, so as to prevent that most disagreeable of all circumstances in dwelling houses, smoky chimneys. From ignorance of the effects of heat, of the experiments that have been made on rarefied air, and their relation to our common fires,-of the proper dimensions of funnels, of the effects of winds and currents of air, of the proper height and width of chimneys, of the method of promoting a good draught, and making the air pass as near the fire as possible, and various other particulars requisite to be attended to in the construction of fire-places and their flues; many dwelling-houses have been bungled, and rendered almost uninhabitable. The workmen, n such operations, without any rational principle to guide them, carry up funnels in the easiest way they can, according to the practice of “use and wont,” and leave the tenants or proprietors of the houses they erect to get rid of their smoke in the best way their fancy can contrive. Whereas, were chimneys and their flues constructed according to the principles of science, they might be rendered, almost with certainty, completely esficient for the purpose intended. To all who are acquainted with the nature and properties of elastic fluids, it must be obvious, that the whole mystery of curing smoky chimneys consists in finding out and removing the accidental causes which prevent the heated smoke from being forced up the chimney by the pressure of the cool or heavier air of the room. These causes are various ; but that which will be found most commonly to operate is, the bad construction of the chimney in the neighbourhood of the fire-place. “The great fault,” says Count Rumford, “ of all the open fire-places now in common use is, that they are much too large, or rather it is the throat of the chimney, or the lower part of its open canal, in the neighbourhood of the mantle, and immediately over the fire, which is too large.” The following is a

condensed view of some of the rules given on this subject, by this ingenious practical philosopher, and which are founded on the principles of science, and on numerous experiments:–1. The throat of the chinmey should be perpendicularly over the fire; as the smoke and hot vapour which rise from a fire naturally tend upwards. By the throat of a chimney is meant the lower extremity of its canal, where it unites with the upper part of its open fire-place. 2. The nearer the throat of a chimney is to the fire the stronger will be its draught, and the less danger of its smoking; since smoke rises in consequence of its rarefaction by heat, and the heat is geater nearer the fire than at a greater distance from it. But the draught of a chimney may be too strong, so as to consume the fuel too rapidly; and, therefore, a due medium must be fixed upon, according to circumstances. 3. That four inches is the proper width to be given to the throat of a chimney, reckoning across from the top of the breast of the chimney, or the inside of the mantle to the back of the chimney, and even in large halls, where great fires are kept up, this width should never be increased beyond 44 or 5 inches. 4. The width given to the back of the chimney should be about onethird of the width of the opening of the fire-place in front. In a room of a middling size, thirteen inches is a good size for the width of the back, and 3 times 13 or 39 inches for the width of the opening of the fire-place in front, 5. The angle made by the back of the fire-place and the sides of it, or covings, should be 135 degrees, which is the best position they can have for throwing heat into the room. 6. The back of the chimney should always be built perfectly upright. 7. Where the throat of the chimney has an end, that is to say, where it enters into the lower part of the open canal of the chimney, there the three walls which form the two covings and the back of the fire-place should all end abruptly, without any slope, which will render it more difficult for any wind from above to force its way through the narrow passage of the throat of the chimney. The back and covings should rise 5 or 6 inches higher than the breast of the chimney. 8. The current of air which, passing under the mantle, gets into the chimney, should be made gradually to bend its course upwards, by which means it will unite quietly with the ascending current of smoke. This is effected with the greatest ease and certainty, merely by rounding off the breast of the chimney, or back part of the mantle, i.stead of leaving it flat or full of holes and corners. Fig. 1 shows the section of a chimney on the common construction, in which d e is the throat. "Fig.2 shows a section of the same chimney altered and improved, in which d i is the reduced throat, four inches in the direction of di, and thirteen inches in a line parallel to the mantle.

Masons, bricklayers and others, who are interested in this subject, would do well to procure and study Count Rumford's “Essay,” which was originally sold for two shillings. His directions have seldom been accurately attended to in this country by those who have pretended to improve chimneys on the principles he has laid down, partly from carelessness, and partly from ignorance of the elements of science. When the grate is not set in its proper place, when its sloping iron back is retained,— when no pains have been taken to make its ends coincide with the covings of the fire-place,— when the mantle, instead of having its back rounded off, is a vertical plane of iron, cutting a column of smoke which rises beneath it; and, above all, when the throat of the chimney, instead of four, is made, as we often see, fourteen inches wide,-not one of the Count's directions has been attended to, and his principles have as little to do with the construction of such a chimney as with the building of the dykes of Holland, or the pyramids of Egypt.

A knowledge of the science of Optics, which explains the nature of vision, and the laws by which light is refracted and reflected, is essentially requisite to the makers of telescopes, microscopes, and all other dioptric and catoptric instruments, in order to carry them forward to their highest pitch of improvement. And yet how often do we find many of those employed in the construction and manufacture of such instruments glaringly deficient in the first principals of optical science? One maker imitates the instruments of another without discrimination, and while he sometimes imitates the excellencies, he as frequently copies the defects. Hence the glaring deficiencies in the construcion of the eye-pieces of most of our pocket te

lescopes, and the narrow field of view by which they are distinguished, which a slight acquaintance with the properties of lenses would teach them to obviate. By a moderate acquaintance with the principles of this science, any ingenious mechanic might, at a small expense, be enabled to construct for himself many of those optical instruments by which the beauties of the animal and vegetable kingdoms, and the wonders of distant worlds have been explored. Although, in the hands of mathematicians, the science of optics has assumed somewhat of a forbidding appearance to the untutored mind, by the apparently complex and intricate diagrams by which its doctrines have been illustrated, yet it requires only the knowledge of a few simple facts and principles to guide an intelli. gent mechanic in his experiments, and in the construction of its instruments. In order to the construction of a refracting telescope, it is only requisite to know, that the rays of light, passing through a convex-glass, paint an image of any object directly before it, at a certain point behind it, called its focus; and that this image may be viewed and magnified by another convexglass, placed at a certain distance behind it. Thus, let CD, fig. 1, represent a convex-glass, whose focal distance CE is 12 inches; let AB represent a distant object directly opposite; the rays of light passing from this object, and crossing each other, will form an image of the object AB, at EF, in an inverted position. Let GH represent another convex-glass, whose focal distance is only one inch. If this glass is placed

at one inch distant from the image EF, or 13 Fig. 1.

[graphic]
[graphic]

hoches from the glass CD, and the eye applied at the point S, the object AB will be seen turned upside down, and magnified in the promortion of 1 to 12, or twelve times in length and breadth. This forms what is called an Astronomical telescope; but, as every thing seen through it appears inverted, it is not adapted for viewing terrestrial objects. In order to fit it for viewing land objects, two other eye-glasses, of the same focal distance, (namely, one inch,) are requisite; the second eye-glass IK is placed at 2 inches from GH, or double their focal distance, and the glass NO at the same distance from IK.” By this means a second image IM is formed in an upright position, which is viewed by the eye at P, through the glass NO, and the object appears magnified in the same proportion as before. The magnifying power of a telescope of this construction is found by dividing the focal distance of the object-glass by the focal distance of the eye-glass. Thus, if the object-glass be

36 inches in focal distance, and the eye-glass 11 inch, the magnifying power will be 24 times; if the focus of the eye-glass be 2 inches, the

Fig. 2.

* This is not the best form of a terrestrial eyepiece; out it may serve for the purpose of illustration. The eye-piece now most generally used, con. of four lenses, combined on a different principle.

magnifying power will be 18 times, &c.—LM is the telescope fitted up for use. A compound microscope might likewise be easily constructed by any ingenious artizan or mechanic, by attending to the following illustrations and directions. Fig. 2 represents the glasses of a compound microscope. AB is the glass next the object; CD is the amplifying glass for enlarging the field of view; EF is the glass next the eye. When a small object, as GH, is placed below the object-glass AB, at a little more than its focal distance from it, a magnified image of this object is formed by the glass AB at GH, which is magnified in proportion as the distance GG exceeds the distance of AG. This magnified image of the object is magnified a second time by the glass EF, to which the eye is applied at K. This instrument, when fitted up for use, is represented in fig. 3, where LM represents a box or pedestal on which it stands,

[ocr errors]

NO the stage on which the objects are placed, over the opening i, which is supported by 3 pillars fixed to the top of the box. P is a tube which is supported by 3 pillars fixed into the stage. Into this tube the tube R slides up and down for adjusting the focus. The small tube u, which carries. the object-glass, is connected with the tube R, and slides up and down along with it. S is the tube which contains the two eye-glasses, and which may be made to slide up and down into the tube R, for increasing the magnifying power when occasion requires. T is a mirror, fixed on the pedes

tal, capable of moving up and down, and to the

[graphic]
[graphic]
« ZurückWeiter »