portions are first joined together by a slender neck, which gradually fills up and disappears, the food passing from one part into the other; and thus we may form polypes, not only from different portions of the same animal, but from those of different animals. We may fix the head of one to the body of another, and the compound animal will grow, eat and multiply, as if it had never been divided. By pushing the body of one into the mouth of another, so far that their heads may be brought into contact, and kept in that situation for some time, they will at last unite into one animal, only having double the usual number of arms. The hydra fusca may be turned inside out like a glove, at the same time that it continues to eat and live as before. The lining of the stomach now forms the outer skin, and the former epidermis constitutes the lining of the stomach. See Adams on the Microscope. HYDRACHNA, a genus of insects of the order Coleoptera. Head, thorax, and abdomen united; two feelers, jointed; from two to six eyes; eight legs, ciliate and formed for swimming. The insects of this gents are inhabitants of the water, and swim with considerable swiftness: they prey on the larva of Tipulæ, and Monoculi: the eggs are red and at first spherical, but afterwards become semi-lunar; larva sixfooted and furnished with a singular proboscis. There are about fifty species. H. geographica, so called from the fancied map-like distribution of its variegations. It is one of the largest of the genus, and is occasionally seen in the clear ponds, and other stagnant waters. This is reckoned one of the most beautiful of the British insects. duced by Proust to express the chemical union of water with any substance, and especially with certain metallic oxides. The hydrate of copper is a blue-green oxide of this metal, which differs from the brown oxide, only in containing a large quantity of water, which a low red heat will expel. HYDRAULICS teach us to ascertain the velocity and impetus of fluids when in motion, and serve as the basis for computing the powers of various machinery acted up. on by running water. The first principle we shall inculcate in this service is, that water being an inelastic fluid, (though many have thrown away much time in the attempt to prove the contrary,) can only be set in motion by two causes: viz. the increased pressure of the air, as in the air-vessels of fire-engines, and by gra vitation; that is, where it is liberated from confinement, and allowed to descend to an inferior level. In the former case, water may be made to rise by machinery suited to the purpose; in the latter, it will inviolably seek a lower situation. The velocity of water, proceeding through a hole in the side of a vessel, is ever proportioned to the distance of the aperture from the level of the fluid, the square root of the intermediate space being the guide. It must, however, be recollected, that in consequence of the decrease of that space, as the water is let out, the pressure becomes gradually less; therefore the medium, or mean distance, between the surface and the vent whence the water issues, will be found, in general, a correct standard. Hence we see, that, in order to force double the quantity of water through the lowest of two aper tures, the distance must be quadrupled. For if a hole made at C in the pipe A B, fig. 1, will supply one gallon of water in a minute; to draw double that quantity in the same time, the lower hole, D, must measure from the surface, B, four times as much as from C to the surface. HYDRANGEA, in botany, a genus of the Decandria Digynia class and order. Natural order of Succulenta. Saxifragæ, Jussieu. Essential character: capsule twocelled, two beaked, containing many seeds; corolla five petalled; calyx, five-cleft, su- This establishes the above position, and perior. There are three species. proves besides, that the force is equal to the HYDRARGYRUM, an old name given velocity, as indeed we know to result in to mercury. HYDRASTIS, in botany, a genus of the Polyandria Polygynia class and order. Natural order of Ranunculaceæ, Jussieu. Essential character: calyx none; petals three; nectary none; berry composed of one seeded acini, or granulations. There is but one species, viz. H. canadensis, Canadian yellow rost. every branch of mechanism. To shew this, let the pipe, A B, be perforated in several parts, as at C DE; the first, i. e. C, be ing one foot; that at D being four feet; and that at E being seven feet below the surface, B; between E and A we will suppose only one foot interval, so that D may be in the centre of the height, A B. Draw the horizontal line, A F, and from D describe HYDRATE, in chemistry, lately intro- the semi-circle, B GA, having D G equal to DA, or D B, for its radius. Now the water will, as it flows from D, describe a parabola, and will fall upon the line, A F, at such a distance from A, as will be equal to double the radius, D G. In like manner the water flowing from the aperture, C, will reach that point, viz. K, on the horizontal A F, which may measure double the sine, CH, on the same semicircle: and the sine of the arc taken opposite to E, i. e. EL, is equal to the sine, CH, the water rushing from E will intersect, or meet, the water falling from C, at the point K. It is to be observed, that the parabolic curve of the water proceeding from C to K, has a greater tendency to gravitation than that issuing from E, which rushes with far more force, and consequently has a greater tendency to an horizontal direction. For the aperture at C is only acted upon by a column of one foot deep, i. e. from B to C, but the column of water from B to E measures seven feet. We have already stated, that the velocity is equal to the square root of the column's height above the aperture. It is the peculiar property of fluids to preserve their level, notwithstanding any varieties of course, or inequality of elevation. Thus, supposing the pipe, ABCD, fig. 2, to be bent into the form required for passing over declivities, as shown: the water will rise to the height, A D; but where the channel exceeds the level of that line, there will be a break in the 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 the 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 conduitpipe 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 HYDROSTATICS, wherein it will be found very conspicu ous. The ingenions 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, ABCD, 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, CD, whatever may be the length of the conjunc tive part, A C. Therefore, if a piston is inserted into the pipe, CD, it will be acted upon in perfect conformity with the motion of the piston in AB; the power to move which may be trifling, when the diameter relating to forcible operations. Thus, for of the pipe is small, and the purpose not 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. hand, when machinery is to be set in motion, On the other 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 CD. 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 movement-tube 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 CD, 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, GF, is greatly increased, and the plonge of the pump-piston, H, mucli diminished. If, on the contrary, the fulcrum had been at O, 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 MECHA. NICS. 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 deuse, 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 purpose, bent at any angle, but generally about 70 to 80 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, ABC, is inserted into a vessel to be emp tied. 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 sufficieatly to answer this purpose, and to bring the shorter limb nearer to a horizontal position than the longer limb, whereby the latter may possess a greater perpendicular altitude, and consequently a greater tendency to gravitation. For, we trust, that, in fig. 1, it has been demonstrated, that 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 lip. 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 offered, quite full, to the person on whom the joke is to be practiced, observ ing 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 thevessel, appearing merely as an ornament, but allowing the liquor to pass under its petals, &c. into 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 larger 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. 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 proved 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, bb the channel; B is a cavern, which by means of the arch, or rising channel, cc, becomes a syphon leading ing into D. It is obvious that, in the first instance, the water must, after filling B, rise in the channel, bb, as to be above the greatest height of c c, to cause its passing off into E, and thence ad libitum. Now the channel, c c, being of greater diameter than the channel, bb, when the former commences its operation, it will discharge more than the latter can supply, so as to keep up the discharge from ce; 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 sum mit, will recede into B, and the water must again rise to the height in bb, which shall cause it to flow over the summit of cc, 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 acrimony, among the philosophers of old; and we may say of a date by no means ancient. Common sense should have told us, what experience so amply proves, that where one body or ele ment 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 PNEU MATICS. 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, allow ing 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 sur face 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 thirty-three feet above its le vel. This is the greatest height to which water can be induced by a sucking pump. |