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Specific Gravity of Bodies: By the specific gravities of bodies we mean the relative weights, which equal bulks of different bodies have to each other. And it is usual to compare them with that of water, as it is by weighing bodies in water that their specific gravities are found. A body immersed in a fluid will sink to the bottom, if it be heavier than its bulk of fluid ; if it be suspended therein, it will lose as much of what it weighed in air, as its bulk of the fluid weighs. The instrument generally used for obtaining the specific gravities is called the hydrostatical balance ; it does not differ much from the common balance. The general rule for finding the specific gravity of a solid, heavier than water, as a piece of metal, is this : weigh the body first in air, in the usual way, then weigh it when it is plunged in water, and observe how inuch it loses of its weight in this fluid, and dividing the former weight by the loss sustained, the quotient is the specific gravity of the body, compared with that of water. ample, it is usual to take a guinea, which weighs in air one hundred and twenty-nine grains, and when suspended by means of a fine hair, and immersed in water, it is found to balance one hundred and twenty-one grains and three-quarters, losing of its weight seven grains and a quarter; now one hundred and twenty-nine divided by seven and a quarter, gives about seventeen for the quotient; that is, the specific gravity of a guinea compared with that of water, is as about seventeen to one. And thus, any piece of gold may be tried, by weighing it first in air, and then in water; and if, upon dividing the weight in air, by the loss in water, the quotient comes to be about seventeen, the gold is good; if the quotient be eighteen, or between eighteen and nineteen, the gold is very fine; but if it be less than seventeen, the gold is too much alloyed with some other metal. The same principle is universal. Hence we see the reason why boats or other vessels float on water; they sink just so low, that the weight of the vessel, with its contents, is equal to the quantity of water which it displaces. The method of ascertaining the specific gravities of bodies, was discovered by


Archimedes, in the following manner. Hiero, king of Syracuse, having given to a workman a quantity of pure gold, of which to make a crown, suspected that the artist had kept part of the gold, and adulterated the crown with a baser metal. The king applied to Archimedes to discover the fraud. The philosopher long studied it in vain, and at length accidentally hit upon a method of verifying the king's suspicion. Going one day into a bath, he took notice that the water rose in the bath, and immediately reflected that any body, of equal bulk with himself, would have raised the water just as much; though a body of equal weight, but not of equal bulk, would not raise it so much. From this idea he conceived a mode of finding out what he so much wished, and was so transported with joy, that he ran out of the bath, crying out in the Greek tongue, “I have found it, I have found it !

Now, since gold was the heaviest of all metals known to Archimedes, it occurred to him that it must be of less bulk, according to its weight, than any other metal; and he, therefore, desired that a mass of pure gold, equally heavy with the crown when weighed in air, should be weighed against it in water, conjecturing that if the crown was not alloyed, it would counterpoise the mass of gold when they were both immersed in water, as well as it did when they were weighed in air. But upon making trial, it was found that the mass of gold weighed much heavier in water than the crown did : nor was this all-when the mass and crown were immersed separately in the same vessel of water, the crown raised the water much higher than the mass did ; which showed it to be alloyed with some lighter metal that increased its bulk. And upon this principle is the doctrine of the specific gravities of bodies founded.

QUESTIONS.—1. What is meant by the specific gravities of bodies ? 2. What is said of a body immersed in a fluid? 3. What is the general rule for finding the specific gravity of a solid heavier than water? 4. What example is given ? 5. How may a piece of gold be tried ? 6. Why do vessels float? 7. What incident led to the method of discovering the specific gravities of bodies ?. 8. Who made the disco. very, and how ? 9. Explain the method and the result. 10. Explain by fig. 14. the use of the hydrostatic balance. 11. Describe the hy. drometer.

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Hydraulics. Intermittent, coming by fits, not constant. Res'ervoir, a conservatory of water; a store. Vac'uum, a space unoccupied by matter. The science of Hydraulics teaches how to estimate the velocity and force of fluids in motion. Upon the principle of this science all machines worked by water are constructed, as engines, mills, pumps, and others. Water can be set in motion by its own gravity, as when it is allowed to descend from a higher to a lower level; and by an increased pressure of the air, or by removing the pressure of the atmosphere, it will rise above its natural level. In the former case it will seek the lowest situation, and in the latter, it may be forced to almost any height.

The syphon is a pipe used to draw off water, wine, or other fluids, from vessels which it would be inconvenient to move from the place in which they stand. It is made of tin or copper, and bent in such a manner that one limb may reach down through the hole in the top of the vessel to be emptied, to its very bottom; the other limb should be the longest, so that when filled it may contain a heavier body of fluid than that within the vessel. The pressure of the atmosphere being taken off from that part of the surface of the liquor within the tube, the liquor rises above its natural level, and flows through the longer limb, and the contents of the vessel are drawn off to the last. There are intermittent springs in various places of the world, which have been explained on the principle of the syphon. A passage for the water may have been formed in the soil, and when the internal cavity has been filled with water, so as to begin to run off by this passage, the pressure of the atmosphere will make the water flow till all is carried off. Of course the spring then ceases until the cavity is again filled with water, when the same phenomenon is repeated. Fluids may be conveyed over hills and valleys in bent pipes, to any height which is not greater than the level of the spring whence they flow. The Romans, either from their ignorance of the pressure of fluids, or from their love of magnificence, conveyed water across valleys by

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straight-lined aqueducts, which were supported by immense arches or columns.

The common pump consists of a large tube or pipe, called the barrel, whose lower end is immersed in the water which it is designed to raise. A kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down by means of a metallic or wooden rod. In the piston, there is a valve, or little door, which opening upwards, admits the water to rise through it, but prevents its returning. A similar valve is fixed in the body of the pump.

When the pump is in a state of inaction, the two valves are closed by their own weight; but when the piston is made to ascend, it raises a column of air which rested upon it, and produces a vacuum between itself and the lower valve; the air beneath this valve expands and forces its way through it; and the water, relieved from the pressure of air, ascends into the pump, being forced up by the weight of the surrounding atmosphere. When the piston now descends it is forced into the water, which, as it cannot repass through the lower valve, must rise through the valve of the moveable piston, by the ascent of which, it is lifted up and runs off at the spout. There must never be so great a distance as thirty-three fret from the level of the water in the well, to the valve in the piston, for in that case, the water would not rise through the valve, because the pressure of the atmosphere will not sustain a column of water above that height. But when the water has passed the valve in the moveable piston, it is not the pressure of the air on the reservoir which makes it ascend; it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom.

The forcing pump is not only used to raise water from a well to the surface of the earth, but likewise to force it into reservoirs on the tops of buildings, from which pipes are laid to convey it to different parts as conveniency requires. It differs from the common pump by having the upper piston solid, and a pipe joined to the barrel just above the lower piston, through which the water passes into what is termed the air vessel. In the pipe which leads to the air vessel there is a fixed valve, which opens upwards and prevents the return of the water. Through the upper part of the air vessel a tube is inserted, which reaches nearly to its bottom. Now the air which is above the water in the vessel being

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confined, and condensed into a smaller bulk than its natural space, presses by its elasticity upon the surface of the water, and forces it violently up the tube in a continual stream. It is upon this principle that the engine for extinguishing fires is constructed.

QUESTIONS.—1. What does the science of hydraulics teach ? 2. What machines are constructed on the principles of this science? 3. What are the different ways in which water may be set in motion ? 4. What is the syphon ? 5. Describe the manner of its conveying Huids. 6. How are intermittent springs caused ? 7. Describe the common pump and show how it raises water. 8. How high can water be raised in a common pump? 9. Describe the forcing pump. 10. What engine is constructed on the principle of the forcing pump? 11. Describe the common pump by fig. 21. and show its action. 12. Forcing pump by fig. 22. and show how it acts in forcing up water,


The Diving Bell, and Steam Engine. Vertically, in a direction perpendicular to the horizon. Appara'tus, utensils and appendages belonging to a machine.

if you take a glass tumbler, and plunge it in water with we mouth downwards, you will perceive that very little water will enter into it. The air which fills the glass prevents the entrance of the water ; but as air is compressible, it cannot entirely exclude the water, which, by its pressure, condenses the air in a slight degree. Upon this simple principle machines have been invented, by which people have been able to walk about at the bottom of the sea, with as much safety as upon the surface of the earth. The original instrument of this kind was much improved by Dr. Halley, more than a century ago. The machine was made of copper in the shape of a bell. The diameter of the bottom was five feet, that of the top three feet, and it was eight feet high. To make the vessel sink vertically in water, the bottom was loaded with a quantity of leaden balls. Light was let into the bell by means of strong spherical glasses fixed in the top. Barrels, filled with fresh air, were made sufficiently heavy, and sent down, from which a leathern pipe communicated with the inside of the bell, and a tube with a stop at the upper part et out the air which had become unfit for breathing. The

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