HYDROSCOPE
, an instrument anciently used for the measure of time. It was a kind of water-clock, consisting of a cylindrical tube, conical at bottom: the cylinder was graduated with divisions, to which the top of the water becoming successively contiguous, as it trickled out of the vertex of the cone, pointed out the hour.
HYDROSTATICAL Balance, a kind of balance contrived for the exact and easy finding the specific gravities of bodies, both solid and fluid, and thereby of estimating the degree of purity of bodies of all kinds, with the quality and richness of metals, ores, minerals, &c, and the proportions in any mixture, adulteration, or the like.
This is effected by weighing the body both in water, or other fluid, and out of it; and for this purpose one of the scales has usually a hook at the bottom, for suspending the body by some very fine thread. And the use of the instrument is founded on this theorem of Archimedes, that any body weighed in water, loses as much of its weight as is equal to the weight of the same bulk of the water. Thus then is known the proportion of the specisic gravities of the solid and fluid, or the proportion of their weights under the same bulk, viz, the proportion of the weight of the body weighed out of water, to the difference between the same and its weight in water. Hence also, by doing the same thing for several different solids, with the same fluid, or different fluids with the same solid, all their specific gravities become known.
The instrument needs but little description. AB is a nice balance beam, with its scales C and D, turning with the small part of a grain, the one of them, D, having a hook in the bottom, to receive the loop of a horse hair &c, E, by which the body F is suspended. GH is a jar of water, in which the body is immersed when weighing.
The pieces in the scale C denote the weight of the body out of water; then, upon immerging it, put weights in the scale D to restore the balance again, and they will shew the specific gravity of the body.
There have been various kinds of the Hydrostatical balance, and improvements made on it, by different persons. Thus, Dr. Desaguliers set three screws in the foot of the stand, to move any side higher or lower, till the stem be quite upright, which is known by a plummet hanging over a fixed point in the pedestal. Desag. Exp. Philos. vol. 2, p. 196. And for sundry other constructions of this instrument, designed for greater accuracy than the common sort, see Martin's Phil. Britan. or Gravesande's Physices Elem. Math. tom. 1, lib. 3, cap. 3, &c.
The specific gravities of small weights may be determined by suspending them in loops of horse hair, or fine silken threads, to the hook at the bottom of the scale. Thus, if a guinea suspended in air weigh 129 grains, and upon being immersed in water require 7 1/5 grains to be put in the scale over it, to restore the equilibrium; we thus find that a quantity of water of equal bulk with the guinea, weighs 7 1/5 grains, or 7.2; therefore dividing the 129 by the 7.2, the quotient 17.88 shews that the guinea is so many times heavier than its bulk of water. Whence, if any piece of gold be tried, by weighing it first in air, then in water, and if, upon dividing the weight in air by the loss in water, the quotient be 17.88, the gold is good; if the quotient be 18 or more, the gold is more fine; but if it be less than 17.88, the gold is too much alloyed with other metal. If silver be tried in the same manner, and found to be 11 times heavier than water, it is very fine; if it be 10 1/2 times heavier, it is standard; but if less, it is mixed with some lighter metal, such as tin.
When the body, whose specific gravity is sought, is lighter than water, so that it will not quite sink; annex| to it a piece of another body heavier than water, so that the mass compounded of the two may sink together. Weigh the denser body, and the compound mass, separately, both in water and out of it, thereby finding how much each loses in water; and subtract the less of these two losses from the greater; then say, As the remainder is to the weight of the light body in air, so is the specific gravity of water to the specific gravity of the light body.
HYDROSTATICAL Bellows, a machine for shewing the upward pressure of fluids and the Hydrostatical paradox. It consists of two thick boards, A, D, each about 16 or 18 inches diameter, more or less, covered or connected firmly with leather round the edges, to open and shut like a common bellows, but without valves; only a pipe, B, about 3 feet high is fixed into the bellows at e, Now let water be poured into the pipe at C, and it will run into the bellows, gradually separating the boards, by raising the upper one. Then if several weights, as three hundred weights, be laid upon the upper board, by pouring the water in at the pipe till it be full, it will sustain all the weights, though the water in the pipe should not weigh a quarter of a pound; for the pipe or tube may be as small as we please, provided it be but long enough, the whole effect depending upon the height, and not at all on the width of the pipe: for the proportion is always this, As the area of the orifice of the pipe is to the area of the bellows board, so is the weight of water in the pipe to the weight it will sustain on the board.
Hence if a man stand upon the upper board, and blow into the pipe B, he will raise himself upon the board; and the smaller the pipe, the easier he will be able to raise himself; and then by putting his singer upon the top of the pipe, he can support himself as long as he pleases, provided the bellows be air-tight.
Mr. Ferguson has described another machine, which may be substituted instead of this common Hydrostatical bellows. It is however on the same principle of the Hydrostatical paradox; and may be seen in the Supplement to his Lectures, p. 19.
HYDROSTATICAL Paradox, is a principle in Hydrostatics, so called because it has a paradoxical appearance at first view, and it is this; that any quantity of water, or other fluid, how small soever, may be made to balance and support any quantity, or any weight, how great soever. This is partly illustrated in the last article, on the Hydrostatical bellows, where it appears that any weight whatever may be blown up and supported by the breath from a person's mouth. And the principle may be explained as follows: It is well-known that water in a pipe or canal, open at both ends, always rises to the same height at both ends, whether those ends be wide or narrow, equal or unequal. Thus, the small pipe GH being close joined to another open vessel AI, of any size whatever; then pouring water into the one of these, it will rise up in the other, and stand at the same height, or horizontal line DF in both of them, and that whether they are upright, or inclined in any position. So that all the water that is in the large vessel from A to I, is supported by that which is in the small vessel from D to I only. And as there is no limit to this latter one, but that it may be made as fine even as a hair, it hence evidently appears that any quantity of water may be thus supported by any other the smallest quantity.
Since then the pressure of fluids is directly as their perpendicular heights, without any regard to their quantities, it appears that whatever the figure or size of the vessels may be, if they are but of equal heights, and the areas of their bottoms equal, the pressures of equal heights of water are equal upon the bottoms of these vessels; even though the one should contain a thousand or ten thousand times as much as the other.
Mr. Ferguson confirms and illustrates this paradox by the following experiment.
Let two vessels be prepared of equal heights, but very unequal contents, such as AB and CD; each vessel being open at both ends, and their bottoms E and F of equal widths. Let a brass bottom G and H be exactly fitted to each vessel, not to go into it, but for it to stand upon; and let a piece of wet leather be put| between each vessel and its brass bottom, for the sake of closeness. Join each bottom to its vessel by a hinge D, so that it may open like the lid of a box; and let each bottom be kept up to its vessel by equal weights W, hung to lines which go over the pulleys P, whose blocks are fixed to the sides of the vessel at f, and the lines tied to hooks at d, fixed in the brass bottoms opposite to the hinges D. Things being thus prepared and fitted, hold one vessel upright in the hands over a bason on a table, and cause water to be poured slowly into it, till the pressure of the water bears down its bottom at the side d, and raises the weight E; and then part of the water will run out at d. Mark the height at which the surface H of the water stood in the vessel, when the bottom began to give way at d; and then, holding up the other vessel in the same manner, cause water to be poured into it; and it will be seen that when the water rises in this vessel just as high as it did in the former, its bottom will also give way at d, and it will lose part of the water.
The natural reason of this surprising phenomenon is, that since all parts of a fluid at equal depths below the surface, are equally pressed in all manner of directions, the water immediately below the fixed part Bf will be pressed as much upward against its lower surface within the vessel, by the action of the column Ag, as it would be by a column of the same height, and of any diameter whatever; and therefore, since action and reaction are equal and contrary to each other, the water immediately below the surface Bf will be pressed as much downward by it, as if it were immediately touched and pressed by a column of the height Ag, and of the diameter Bf; and therefore the water in the cavity BDdf will be pressed as much downward upon its bottom G, as the bottom of the other vessel is pressed by all the water above it. Lectures, p. 105.
