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Sikes's hydrometer is now generally employed, especially since its adoption by the commissioners of his majesty's customs. This instru ment has but nine shifting weights, applicable upon the upper part of the stem, and is used with a set of tables, or a sliding rule sold with it, for computing compensation for different temperatures. The scale is divided into ten principal divisions, each of which is subdivided into five parts, and by the separate application of the weights in succession completes the range of strength from pure alcohol to water, each weight being equivalent to ten principal divisions. This hydrometer, with the weight marked 60, screwed on to the lower stem, is so adjusted as to sink to the line mark P on the scale of the instrument when placed in proof spirit, of the temperature of 51° Fahrenheit, and, by the addition of the square weight on the top of the stem, it sinks to the same point in distilled water of the same temperature. This weight being just one-twelfth part of the entire weight of the whole hydrometer, together with its bottom weight No. 60, causes the scale to show the difference between water and proof spirit, which, the act states, shall weigh exactly twelve-thirteenth parts of an equal bulk of distilled water.

Mr. Meikle's hydrometer consists of a glass tube, open at both ends, and bent into a kind of double syphon, having four parallel legs; so that the open ends are pointed in the same direction or upwards, as shown in fig. 13, plate I. The manner of using it is very simple. Let one of the ends be stopped with a finger or cork, and water be poured into the other. This fluid will only rise a small way into the second leg, because of the included air. Next stop the other orifice, and open the one first closed; and, having poured into the latter the liquid whose specific gravity is to be tried, open the top of the water tube; then, the instrument being held upright, the two liquids will arrange themselves so as to press equally on the included air. This pressure will be measured by the difference in the heights of the two columns of either liquid, multiplied by its specific gravity, so that, by dividing the difference of the two columns of water by the difference of those of the other liquid, we obtain the specific gravity of the latter; that of water being unity. The difference between the columns may be measured by applying any scale of small equal parts, or the glass may be attached to a graduated plate furnished with verniers, &c. The longer the columns of liquids employed, the more accurate the process. The expansion of the glass, or its capillary action, cannot affect the result, nor is it influenced by the expansion of the scale; the only correction required will be to reduce the observations to one temperature.

There are other methods of judging of the strength of spirituous liquors, which though useful are not accurate, such as the taste, the size and appearance of the bubbles when shaken, the sinking or floating of olive oil in it, and the appearances that it exhibits when burned; if it burns away perfectly to dryness, and inflames gunpowder, or a piece of cotton immersed in it, it is considered as alcohol; the different spirituous

liquors leave variable proportions of water, when thus burned in a graduated vessel.

There is the greatest difficulty in ascertaining what is meant by the terms proof spirit. Dr. Thomson, quoting the act of parliament of 1762, states, that at the temperature of 60°, the specific gravity of proof spirit should be 0.916; and he also observes, that proof spirit usually means a mixture of equal bulks of alcohol and water; but the specific gravity of such a mixture will, of course, depend upon that of the standard alcohol, which is not specified. It appears from Gilpin's Tables that spirit of the specific gravity 916, at 60°, consists, by weight, of 100 parts of alcohol, specific gravity 825, at 60°, and 75 of water; and, by measure, of 100 parts of the same alcohol, and 61.87 of water.

One of the most accurate and convenient methods of obtaining the specific gravity of fluids is by what is called a thousand grain bottle. This is sold by most of the philosophical instrument makers, together with a weight, which is an exact counterpoise for the bottle when filled with distilled water; its magnitude being adjusted by grinding down the length of its neck, until it holds exactly 1000 grains of water at 60° of Fahrenheit. This instrument consequently requires no computation, but is simply to be filled with fluid, and placed in one scale of a balance, while its counterpoise is placed in the other. If the fluid put into it is lighter, than water, it will appear deficient in weight, and as many grains must be added to the scale that contains it as will restore the balance. This at once shows that the specific gravity of the fluid under examination is negative, or less than the standard, and consequently must be a fractional number; but, should the fluid be heavier than water, the bottle will preponderate, and weights must be put into the opposite scale, when their amount will be positive, and must be added to the amount of the standard. For example: if the bottle were filled with sulphuric ether, it would require 739 grains to be placed in the same scale to restore the balance, consequently its specific gravity would be thus expressed 0.739. Had it been filled with seawater, which is rather denser than that which is distilled, twenty-six-hundredths, or rather better than one-fourth of a grain must have been added in the opposite scale, and these, as already explained, must be added to the standard 1.000 to express the specific gravity of such water, which would be thus written 1.026. Sulphuric acid again, being still heavier, would, in like manner require 875 grains, and would accordingly be expresscd 1·875.

A bottle, however, holding 1000 grains is often inconveniently large, and a small and thin globular phial, with a piece of thermometer tube ground into it by way of stopper, will be found more useful such a phial should not weigh more than from fifty to sixty grains, and may contain between 400 and 500 grains of water. To use it it should be accurately counterbalanced in a delicate pair of scales, and then filled with distilled water, and the stopper thrust in, the capillary opening in which allows a little to ooze out, and prevents the likelihood of bursting the

phial; it is then to be wiped clean and dry, and again carefully weighed, by which the quantity of water it contains is ascertained; the water being poured out it is next filled with the liquid whose specific gravity is required, taking care that it is of the same temperature as the water; we then weigh as before, and divide the weight by the former weight of water, the product gives the specific gravity required. Thus, suppose the phial to contain 425 grains of water at the temperature of 45°, it will be found to hold 5737-5 grains of pure mercury of the same temperature; and 5737·5 ÷ 425 = 13.5 the specific gravity of mercury. Or, supposing the liquid lighter than water, such as alcohol, of which we may assume the pnial to contain 350-5; then 350-54250824, the specific gravity of the alcohol under trial.

The following table is given by Mr. Gilpin, in the eighty-fourth volume of the Philosophical Transactions, aud is of essential use for taking the specific gravities both of solids and fluids, by enabling the operator to reduce the weight or bulk of the distilled water, employed in any case, to that which it would have at any other common temperature, and particularly to 60°,

which is the usual standard.

Thus, for example, since the specific gravity of water at 47° is 1.0008 grains, and at 60° is 100000, (and consequently 10008 grains, at 47°, are equal in bulk to 100000 grains at 60°), it follows that it would require 252-708 grains at 47, to equal the space of a cubic inch; for 100000 10008: 252.506 (the weight of a cubic inch at 60°), : 252-708.

TABLE of the SPECIFIC GRAVITY of WATER, at every Degree of Temperature, from 30° to

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Ammoniacal

Carbonic acid.

Carbonic oxide

Carbureted hydro

gen

Chlorine

Chlorocarbonous

1.222 Lard

4.000 to 4.230

Do. common, from 3.576 to 3.700

Lead, glance or galena from
Derbyshire, from 6.565

tum, from 0.905 to 1.650

to 7-786 Mineral tallow

Gases,-Atmospheric air 1.000 Limestone, compact, from Myrrh (a resin)

0.590

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2.386 to 3.000 Naphtha from 0.700 to 0.847 1.527 Magnesia, native, hydrate 0.972 of

Do.

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Nitre 1.900 2.330 Obsidianum from 2-348 to 2.370 carbonate of, Oils, Essential-Amber 0.868 0.972 from 2.220 to 2.612 2.500 Malachite, compact, from

Anise-seed 0.986
Carraway-seed 0·904
Cinnamon 1.043

3.572 to 3.994|

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TABLE OF SPECIFIC GRAVITIES. —Continued

Orpiment from 3:048 to 3:500] Do. carbonate of,

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It may be proper to add, that Mr. S. L. Kent, to whom we are indebted for much valuable information on this subject, has nearly completed the most extensive and accurate series of observations on the specific gravity of mineral bodies that has ever been attempted.

Having in the preceding portion of our article examined the nature of hydrostatic pressure, we may now proceed to treat of fluids in motion, and the structure of hydraulic machines.

follow, it will continue to flow out of the vessel, through the tube BC, as long as the aperture A is under the surface of the liquor. Or if the syphon be at first filled with the fluid, and the aperture stopped with the finger until the aperture A is immersed, the event will be precisely the same. During the process of sucking, the air in the tube is rarefied, and the equilibrium destroyed; consequently the water must be raised into the less leg A B, by the preponderating pressure of the atmosphere. The syphon being thus filled, the atmosphere presses equally on each extremity, so as to sustain any equal quantity of water in each leg; but the air not being able to sustain all the water in the longer leg, and being more than able to sustain that in the shorter leg, with the excess of force, there

One of the most simple instruments for raising water is the syphon; a bent tube which owes its operation to the pressure of the atmosphere. The ordinary syphon is represented at fig. 1, plate II., HYDROSTATICS and HYDRAULICS; and consists of a crooked tube ABC, of such a length, and with such an angle, or so bent at the vertex, as that, when the orifice A is placed on a horizontal plane, the height of AB may not exceed thirty-two or thirty-three feet. For common uses a foot or half a foot high suffices. If now the less arm A B be immersed in water, or any other liquid, and the air before, it will raise new water into the shorter sacked out of it by the aperture C till the liquor leg; and this new water cannot make its way

but by protruding the first before it by this means the water is continually driven out at the longer leg, as fast as it is raised by the

shorter.

The syphon will raise a stream of water to a considerable altitude in every situation where a little descent can be procured, but, while the operation continues, no water can directly be taken out of the stream above the lowest part of the tube. When, however, the two open ends of a syphon are closed, a quantity of water may be let out of the highest part, and its place supplied by introducing a like quantity of no use: all the avenues for the purpose being then closed, and the stream suffered to flow through the tube, the useless water will be displaced, and a fresh quantity may be soon after drawn off. This mode of exchanging fluids may be useful in furnishing a supply for domestic purposes; but there are some cases in which the water drawn off by this arrangement would not be thought sufficiently pure. To effect the same end the following apparatus has been suggested. In the upper part of the syphon EE, fig. 2, plate II., are inserted two small pipes, and their apertures in the inside of the tube should be divided by a projecting piece a quarter of an inch thick; wherever the pipes are inserted, the piece must be placed in such a position that the current will strike against one of its flat sides. The pipe which opens on that side of the obstacle, or dam, struck by the stream, may be called the water-pipe, and that on the other side the air-pipe. Insert their other ends into a circular vessel, the air-pipe opposite to c must rise to near the top of this vessel, but the waterpipe o need not rise above the place of its insertion; a cock perfectly air-tight must be fixed in each pipe between the vessel and syphon; the vessel must also have a tube in its lower part for letting out water. This tube must have a cock fixed in it, or a valve, covered with leather, to close its lower end. To hasten the delivery of the water in this vessel, the external air may be admitted in such a manner as is most convenient.

The communication between the vessel and syphon being intercepted by turning the cocks in the pipes c o, and the branches closed at their lower ends, the tube may be filled with water through an aperture in the top. After this aperture is closed, and a stream of water let into the cistern o for supplying the syphon, the ends of the branches may be opened, and a continued stream will flow through the tube.

When it is required to fill the vessel c o with water, exclude the external air, and open the pipes between it and the syphon. The vessel will soon be filled, and the water may be let out by opening the tube for that purpose; after which the small pipes are again closed by turning the cocks.

In estimating the discharge by a syphon, the head of water must be reckoned equal to the difference between the levels of the surface of the water, and of the lower orifice. The reason of this will be obvious, when it is recollected that the length of the shorter leg is only measured to the surface of the water, however far it may reach below; and that, as the action of the

instrument is dependent on the discharging leg being the longer of two, the greater the difference in favor of this leg, the greater will be the force employed in promoting the discharge.

This

The improved syphon of M. Buten is shown at fig. 3, where A B is the long branch, with a bulb at A, and DC the short branch. syphon requires neither to be blown into, nor any suction. It is sufficient to fill the long branch A B, and the ball A with the liquid, and to plunge the short branch C D into the liquid to be decanted. The bulb A, in emptying itself, draws off the liquid in contact with the short branch, and, though the bulb itself is partly empty, the flow is unremitting.

Another improved syphon by M. Hempel, is shown at fig. 4 It has the same advantages as that of M. Buten, and is more easily constructed on a large scale. A part of the liquid to be decanted is poured into the funnel A, at the top of the tube A B, which is fitted into the short branch of the syphon. As soon as the flow commences, through the long branch D C, the tube A B is withdrawn, and the flow of water continues.

The syphon is also occasionally disguised to form a philosophical toy, called Tantalus's cup, from the well known fable. This cup has a hollow stem, as at y, fig. 5, into which a small glass syphon z is cemented in such a manner that the upper bend of the syphon may be a little below the top of the cup, and the shortest leg a may very nearly touch it. On pouring water into the cup it will rise to its proper level in the tube a, but still the cup will hold water, but, on attempting to fill it, the water will still rise up the tube a until it reaches the bend, which it will flow over and thus fill the whole of the longer leg zb, and the instant this is effected the water will flow continuously from b until the cup is quite emptied. The upper part of the syphon za may be concealed by placing a hollow image of a man over it, with the chin on a level with the bend of the syphon, when the cup will hold water until filled to the chin of the figure, but the water will begin to flow away as soon as it reaches that point.

The strange appearance of intermitting springs, or springs which run for a time and then stop altogether, and after a time run again, and then stop, is entirely occasioned by the channels in which the water flows being formed like syphons. Thus if A B C fig. 6, plate II., represents a hill or mountain, in which there is a hollow E F G, and a channel bent like a syphon F H B leading out of it. The water collected from the rills d will fill the hollow, and, as soon as it rises to the line O P, of the same height with H, it will rise to H in the channel, and then flow out through B, till the whole runs off to the level of F. It will then cease to flow until the hollow is again filled to the level O P, when it will flow again, and so on. Some springs called variable or reciprocating, do not cease to flow, but only discharge a much smaller quantity of water for a certain time, and then give out a greater quantity. This is owing to the hollow being supplied from another hollow, which is situated higher up, and has a common runner going to join the stream below the bend II; for this

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