Imágenes de páginas

size of a pea, and flatted upon one side. The same kind of substances are likewise to be met with on the under side of the leaves of plants that grow in such places. These are the polypes in a quiescent state, and apparently inanimate. They are generally fixed by one end to some solid substance, with a large opening, which is the mouth; at the other, having several arms fixed round it, projecting as rays from the centre. They are slender, pellucid, and capable of contracting themselves into a very sumall compass, or of extending to a considerable length. The arms are capable of the same contraction and expansion as the body, and with these they lay hold of minute worms and insects, bringing them to the mouth, and swallowing them. The indigestible parts are again thrown out by the mouth. The green polype was that first discovered by M. Thembley; and the first appearances of spontaneous motion were perceived in its arms, which it can contract, expand, and twist about in various directions. On the first appearance of danger they contract to such a degree, that they appear little longer than a grain of sand, of a fine green colour, the arms disappearing entirely. Soon afterwards, he found the grisca, and afterwards the fusca. The bodies of the viridis and grisca diminish almost insensibly from the anterior to the posterior extremity; but the fusca is for the most part of an equal size, for two-thirds of its length, from the anterior to the posterior extremities, from which it becomes abruptly smaller, and then continues of a regular size to the end. These three kinds have at least six, and at most twelve or thirteen arms. They can contract themselves till their bodies do not exceed one fourth of an inch in length, and they can stop at any intermediate degree of expansion or contraction. They are of various sizes, from an inch to an inch and a half long. Their arms are seldom longer than their bodies, though some have them an inch, and some even eight inches long. The thickness of their bodies decreases as they extend themselves, and vice versa; and they may be made to contract themselves, either by agitating the water in which they are contained, or by touching the animals themselves. When taken out of the water they all contract so much, that they appear only like a little lump of jelly. They can contract or expand one arm, or any number of arms, inde

ndently of the rest; and they can likewise bend their bodies or arms in all possible directions, They can also dilate

or contract their bodies in various places, and sometimes appear thick set with folds, which, when carelessly viewed, appear like rings. Their progressive motion is performed by that power which they have, of contracting and dilating their bodies. When about to move, they bend down their heads and arms, lay hold by means of them on some other substance to which they design to fasten themselves; then they loosen their tail, and draw it towards the head; then either fix it in that place, or stretching forward their head as before, repeat the same operation. They ascend or descend at pleasure in this manner upon aquatic plants, or upon the sides of the vessel in which they are kept; they sometimes hang by the tail from the surface of the water, or sometimes by one of the arms; and they can walk with ease upon the surface of the water. On examining the tail with a microscope, a small part of it will be found to be dry above the surface of the water; and, as it were, in a little concave space, of which the tail forms the bottom; so that it seems to be suspended on the surface of the water on the same principle that a small pin or needle is made to swim. When a polype, therefore, means to pass from the sides of the glass to the surface of the water, it has only to put that part out of the water by which it is to be supported, and to give it time to dry, which it always does upon these occasions; and they attach themselves so firmly by the tail to aquatic plants, stones, &c. that they cannot be easily disengaged : they often further strengthen these attachments by means of one or two of their arms, which serve as a kind of anchors for fixing them to the adjacent substances. The fusca has the longest arms, and makes use of the most curious manoeuvres to seize its prey. They are best viewed in a glass seven or eight, inches deep, when their arms commonly hang down to the bottom. When this or any other kind is hungry, it spreads its arms in a kind of circle to a considerable extent, inclosing in this, as in a net, every insect which has the misfortune to come within the circumference. While the animal is contracted by seizing its prey, the arms are observed to swell like the muscles of the human body when in action. Though no appearance of eyes can be observed in the polype, they certainly have some knowledge of the approach of their prey, and shew the greatest attention to it as soon as it comes near them. It seizes a worm the moment it is touched by one of course of the fluid, such as appears at B : yet the course may descend to any depth, as at C, provided the pipe be brought back to ū. original height. If either end be in the smallest degree lower than the other, the water will sink to the level of the lower retaining brim. And if the supply be continual, the water issuing from the lowest end will mount nearly to the level of the source. This is the principle on which fountains are in general found. To effect this, however, the pipe should be small, so as to contract the issue of the fluid, and to give it greater velocity, by causing it to expose a smaller surface for the air to press upon. This contraction should not be carried to excess; else the water would want force to pass through the atmosphere, and, being subdued, would break into drops, and fall without gaining any height. The conduit-pipe is usually made about five diameters of the fountain-pipe ; under such proportions the water will ordinarily flow so freely as to give a good jet. The inelastic nature of water causes it to retain its surface perfectly level; were it otherwise, vessels would often run aground, where, at present, they find depth sufficient to float them ; and the whole body of a river would present a thousand opposing and unequal resistances ; whereas we find the resistance to be uniform. To prove this, let a piece of wood be put into a pail of water, the fluid will in every part remain equally dense, and the surface will be perfectly level. For a further elucidation of this property, we refer to Hypnostatics, wherein it will be found very conspicuOuS. The ingenious Mr. Bramah has lately applied the inelasticity of water to a variety of purposes, especially in the application of a power to remote effects.Thus, if water be filled into the pipe, A B C D, fig. 3, and that a piston be applied to A B, made perfectly tight, so that no water can possibly escape, when that piston is pressed down by means of a force capable of overcoming the friction of its sides, and the friction of the water within the tube, it will cause the water to rise in the pipe, C D, whatever may be the length of the conjunctive part, A. C.— Therefore, if a piston is inserted into the pipe C D, it will be acted upon in perfect conformity with the motion of the piston in A B ; the power to move which may be trifling, when the diameter of the pipe is small, and the purpose not relating to forcible operations. Thus, for the mere

intention of ringing a bell at D, a hundred yards distant from the pull, A, a bore of less than a quarter of an inch in diameter would answer every purpose, and would yield to the pressure of the finger, with very little exertion. On the other hand, when machinery is to be set in motion, the size of the piston, and the force whereby it is to be moved, must be proportioned to the resistance generated by friction, and by the opposition to the action of the machine. It is necessary to observe, that where the two pistons are of equal diameter, their actions will be equal; but that if the pipe, A B, be larger than CD, it will produce an increased action in the latter, which, in such case, must have a proportionate increase of altitude, and, vice versa, when the action of A B is to be greater than that of C. D.— Our readers will be sensible that a tube of less diameter can be made to contain the same quantity as that of greater capacity, only by adding to its length; and that both their areas being filled and emptied alternately by the same action, and in the same time, that which has the greatest altitude must have the greatest scope of action, and move with an increased velocity in exact ratio with the difference of the diameters. When the velocity of the machinery attached to the movementtube is to be diminished, without losing the height to which the secondary power is thus raised by the additional length of the tube, the segment on which it is made to act must be that of a larger circle, as shewn in fig. 4, where the tube, A B, is of double the diameter of that at C D, which would raise the lever, E, to the height F. Now, if this lever were the handle of a pump, requiring a considerable exercise of power, it is evident the fulcrum, G, must be placed very near to the pump. tube, H ; whereby the radius of the circle, G F, is greatly increased, and the plonge of the pump-piston, H, much diminished. If, on the contrary, the fulcrum had been at 0, i. e. dividing the distance between D and X into three parts, of which two are given to the lever, N, the plonge would be far deeper, but the power would be greatly reduced; the segment,\DF, occupying a greater angle with the fulcrum O, than it does with the fulcrum G. This is amply explained under the head of MechANics. Where water is enclosed within a vessel, or in a tube, in such manner that air cannot penetrate, it will not flow out in the same manner, as if air were admitted to supply the place of any quantity that might be required to be drawn off. Of this every person must be sensible who has ever attempted to draw wine, beer, &c. from a full cask, without opening a vent at the top, near the bung, to admit air, as the fluid might evacuate the upper part of the vessel. From this we prove, that although all fluids have a direct disposition to gravitation, they are perfectly inelastic ; if they were otherwise, we should find that, by expansion, they would be capable of filling a greater or lesser space at times; and that as the wine, &c. were drawn off below, the portion remaining in the vessel would expand, and, though less dense, would fill the whole interior. Of this property advantage has been taken to draw off liquors from one vessel to another by means of a very simple instrument called a syphon. This is a pipe of tin, copper, &c. according to its purÉ. bent at any angle, but generaly about seventy to eighty degrees, in such manner that one limb may reach down through the bung-hole of the cask to be emptied, to its very bottom; the other leg should be the longest, so that when filled it may contain a heavier body of fluid than that limb within the vessel. See fig. 5. in which the syphon, A B C, is inserted into a vessel to be emptied. In large syphons it is necessary to insert a cock at the lower end, to prevent the escape of the fluid when first filled. In small syphons it is common to put a small parallel tube, which being applied to the mouth, the end C, being immersed in the liquor to be drawn off, the operator inhales forcibly, and by thus drawing the air out of the syphon, causes the liquor to rise in its place. . The absence of air, which first caused the fluid to ascend into the tube, occasions it to remain until the finger is removed from the end A ; when the pressure of the air within the vessel causes the liquor to press through the syphon, which continues to the last to draw off the contents of the vessel, they pressing forward through the long end, A. It is proper to remark, that large syphons sometimes require to be previously filled, and then to be set in the vessels to be drawn off; but, in general, the casks, &c. can be tilted sufficiently to answer this purpose, and to bring the shorter limb nearer to a horizontal position than the longer limb, whereby the latter my possess a greater perpendicular altitude, and consequently a greater tendency to gravitation. For we trust, that, in Fig. 1, it has been demonstrated, that WOL. VI.

the pressure of a fluid is in proportion to its perpendicular height. We must caution the reader, that as a column of water of thirty-three feet in perpendicular height is equal to the weight of the atmosphere pressing on the surface of such a column, it follows, that no syphon exceeding that length will act, because the power would be less than the weight to be raised. A comical display of the properties of the syphon is seen in what is called “The cup of Tantalus;” the designation of which is derived from fabulous history, wherein we are told, that Tantalus, king of Phrygia, was condemned by Jupiter to suffer perpetual hunger and thirst, amidst a profusion of delicacies, which always receded when applied to his lips. To imitate this disappointment, a syphon, having its two limbs parallel and contiguous, is fixed into the middle of a cup double its height; one limb receiving the liquid at the bottom of the interior, and the other discharging it through the centre of the bottom, as seen in fig. 6. Thus, when the outlet is stopped by means of a finger applied thereto, the cup may be of. fered quite full to the person on whom the joke is to be practised, observing that the syphon will not act until the liquor in the cup exceeds the level of its bend, when the whole will be drawn through the tube. This whimsical contrivance is rendered yet more diverting, by having the syphon so contrived, that its action may commence only when the cup is inclined a little, as is usual when a person is about to drink; and if only a small flower, &c. be at the bottom of the vessel, appearing merely as an ornament, but allowing the liquor to pass under its petals, &c. in. to a tube made through one of two handles, and brought under the bottom. Many springs are derived from natural syphons, existing in the sides of mountains, &c. at various depths, and to various extents. Some springs, situated on the tops of hills, near to large ones, supply water all the year, others only periodically, when they usually flow in profusion. In either case, the ignorant multitude rarely attribute the supply to the proper cause. We shall demonstrate from whence it originates. When various caverns, in which water is either pent up or received, lay in a regular descent, one below the other, the water will naturally pass from one to the other, and cause a regular flow, more or less abundant, according as the source may be more or less abundantly supplied. C c

If the soil through which it passes be close and retentive, the water will then be occasionally raised, as well as lowered, in proportion to the weight of the incumbent fluid, and will rise, if so guided by the channel through which it passes, even to the height of the source, as may be roved by what has already been shewn in fig. 2. Thus, after various changes of altitude, the fluid may escape at any height not above that source; or it may be carried away to any depth. The place where it issues forth is called a spring. Fig. 7. exhibits such a current, which we will suppose to have a perpetual supply. But the intermitting spring may also have a regular supply. This is occasioned by the existence of caverns connected by syphons, as we may see by reference to fig. 8, where A is the source, b b the channel: B is a cavern, which by means of the arch, or rising channel, c c, becomes a syphon leading into D. It is obvious that, in the first instance, the water must, after filling B, rise in the channel, b b, so as to be above the greatest height of c c, to cause its passing off into E, and thence ad libitwn. Now the channel, c c, being of greater diameter than the channel, b b, when the former commences its operation, it will discharge more than the latter can supply, so as to keep up the discharge from c c : therefore, after B has been exhausted so far as to allow air to pass from it into c c, a certain quantity in that channel, which has not gained the summit, will recede into B, and the water must again rise to the height in b b, which shall cause it to flow over the summit of c c, before the spring can again appear to be supplied. Yet the flow from the source was never diminished. The existence, or otherwise, of a vacuum, or void space, was long agitated, and that too with no small degree of acri. mony, among the philosophers of old; and we may say of a date by no means ancient. Common sense should have told us, what experience soamply proves, that where one body or element retires, another must supply its place, else the whole creation would inevitably be torn asunder. It is, indeed, well known, that the elasticity of the air, which could be rarified ad infinitum, if we had the means of effecting the process, enables it to occupy large spaces on emergency, or to contract within the narrowest bounds. See PNEUMAtics. Under ordinary circumstances, however, we consider the air as being of a particular standard,

namely, that a column ascending to the summit of our atmosphere corresponds in weight with a column of water of thirty-three feet in height, allowing the bases, i.e. of the air and of the water, to be equal. Thus we find that where the air is withdrawn, by means of suckers, pistons, valves, &c. from within a pipe, of which the lowest part is immersed in the water contained in a well, &c. the fluid will rise to the height of thirty-three feet within the pipe, supplying the place of the air thus withdrawn. This is effected by the pressure of the atmosphere on the surface of the water; whereby it is forced into the space formerly occupied by the air. Generally speaking, it is not a sudden operation; for unless the well be very shallow, it will require many strokes of a pump to withdraw so much air as may so far rarify the residue within the pipe, as to allow the water to rise thirtythree feet above its level. This is the greatest height to which water can be induced by a sucking pump. In this contrivance the piston, A, see fig. 9, has a valve, which, as the rod draws up, is closed by the pressure of the air above it; but in descending it opens, and allows the water, which had flowed into the lower part, whence the air was withdrawn, to rush through as the piston is raised again, the weight of the water forcibly oppresses the valve, until it finds a lateral passage at B, whence it issues, and in this manner any quantity may be raised. If the water has a direct issue, as in the common spouts of pumps, no further apparatus is wanted; but if it is to be retained, or pass through any other pipes more elevated than the debouchure, B, there must be a small angular projection, as shewn by the dotted lines, to admit the valve C, also pointing upwards. In dry weather, or when the pump is not much used, the leather binding of the piston, as also the valves, will become dry; therefore it is necessary, on such occasions, to throw in a pail-full or two of water, to moisten them; else the air will pass downwards as the piston rises, and prevent that exhaustion on which the ascent of the water depends. It is generally necessary to have a valve at the bottom of the pipe, to keep in the water drawn into it, in order that the labour may be decreased; and that, if the pumping be intermitted, there may be less trouble in bringing up the water within reach of the piston. Where the water lays near the surface, a lifting-pump may be used. This is nearly the same with the former; but requires the piston should be forced down beneath the level of the water in the well. In this it is not so indespensably necessary that the leather on the piston should fit so close; though it is the better for so doing. In the lifting-pump, the whole depends on actually raising the water from the well as though it were done by means of a bucket; this occasions many to apply that designation to the piston. The same precautions are necessary, if the water is to be passed into any pipe, as has been stated regarding the debouchure of the sucking-pump. The forcing pump has a solid piston, as seen at Ain fig. 10, which, after the water has passed the valve at B, is pressed down, and causes the fluid to pass into the conducting pipe C, where there is also a valve d, to prevent its return. The valve at B closes as the piston descends, while that at d rises, to allow its escape from the main pipe. When the piston rises, the water follows, as in the two former instances, through the lower valve B, while the smaller valve at d is also closed by the super-incumbent water in the conduit, e, and by the attraction of the piston the water rushing after it to prevent a vacuum. In this kind of pump, the piston must fit extremely close; both on account of the intended attraction of the fluid from below, and to prevent its escape upwards, when the piston is pressed downwards. The whole of those inventions, which raise water by alternate risings and fallings of only one piston, are subject to the inconvenience of having the water issue in jerks, which, in some instances, would prove highly inconvenient. To remedy this, a cistern should be placed near the debouchure, or spout, whence a small stream would flow with much less variation than from the spout itself. But the best mode of regulating the issue of water is by aid of an air-vessel, as in a fireengine See PNEUMAtics. To detail all the varieties of pumps that are in use would be both beyond the limits of this work, and of no real utility to the reader : we shall, therefore, enter upon the description of the valves in general estimation, and then proceed to give a brief account of hydraulic machinery. The most common kind of valve consists of a piece of stiff leather, such as is applied for soles in shoes, and is generally known by the name of pump-leather— On its upper side a piece of milled lead

is rivetted firmly, and the part where it is to be fixed on the frame, or shell, of the piston is grooved, for the purpose of giving it pliancy, that it may work up and down, as if on a hinge. Fig. 11, shows the plan ; and fig. 12, the profile of this valve, which is cheap, simple, and easily repaired, though it has the defect of being liable to choke, and of not rising high enough to allow a sufficient passage for the water. Fig. 13, shows a button-valve, which is merely a piece of turned metal A, having a shank, B, of about eight inches or a foot in length according to the depth of the block, a z. The shank passes through the bar C, at the bottom of the block, and is prevented from coming up too high by the stud or nut o, at its bottom. When the water rises, it forces up the button A, and passes through the hollow in the block, of which the superior part is expanded so as to fit the bottom, which, being the frustrum of a cone, necessarily fits close into the expanded part as the water presses it, after having passed upwards in consequence of the descent of the piston, which may either be solid, as in a forcing-pump, or valved, as in a lifting or a sucking-pump. This valve may be applied to a piston, as well as to that part of the pipe which retains the water, that may be within reach of the piston’s action. An improvement has been made to this valve, by adding a ball of some weight to the bottom of the shank B, and excavating the button, in order to reduce its weight in proportion: this insures the regular descent of the button to its seat. The butterfly-valve, exhibited in fig. 14, varies from the two former, in having two semicircular flaps appended by hinges to a bar passing over the centre of the excavated piston. This valve is peculiarly eligible, because if one part should be stiff and adhere to the piston, the other will play with an increased effect, though not equal to the action of both valves. The simplest valve with which we are acquainted is the sphere, which is made of metal, and fits into a semi-spherical cavity on the top of the piston or block. When the piston (if it be on that) rises, the sphere falls into the socket; but when the piston is depressed, the rush of water from below forces the sphere upwards. The only inconvenience attendant upon this valve, which is shown at fig. 15, is, that its diameter, being nearly equal to that of the bore, leaves a very narrow passage for the water. This, however, might perhaps be obviated, by making an

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