own level, is one of the greatest practical importance. It is by a knowledge of this law of nature, that we are enabled to bring water from a great distance in pipes, and distribute it over a whole town, not only in the lower, but in the upper floors of the houses, provided they be not above the level of the surface of the water in the reservoir from which it emanates. From ignorance of this principle, or of the mode of its application, the ancients thought themselves under the necessity of erecting magnificent and costly aqueducts, over which the water was conducted.-The effect of fluids upon floating bodies shall be noticed in the next article upon specific gravity.

SPECIFIC GRAVITY.

IT is a consequence of the pressure of the particles of a fluid, that any lighter body immersed in it is borne up to the surface, a body of equal weight floats in it, and a heavier one is retarded in its descent by the resistance of the fluid depriving the body of part of its gravity. In consequence of this resistance, every body suspended in water loses as much of its weight (which it had when weighed in air), as is equal to the weight of the quantity of water which it has displaced. From what was formerly said on the subject of the impenetrability of bodies, it is also plain that every body which sinks in water displaces as much of the fluid as is equal to its own bulk. It is bulk alone, not weight, which is in this matter to be considered. A cubic inch of the heaviest metal will evidently displace no more of the fluid, than a cubic inch of the lightest substance. These properties of fluids have been found of great service in ascertaining what is called the specific gravity of bodies. A pound of lead, every one is easily brought to acknowledge, is just equal in weight to a pound of feathers; and a very large quantity of water has greater weight than a very small quantity of quicksilver. Still, however, we say that the specific gravity of the lead is greater than that of the feathers, and the specific gravity of the water less than that of the mer

cury. Two substances are said to have an equal specific gravity, when a quantity of the one has precisely the same weight with a quantity of the other of the same bulk. On the other hand, if a cubic inch (for example) of one substance weigh more than a cubic inch of another, the former is said to have a greater specific gravity than the other. You will readily perceive that it must be an extremely useful thing to adopt some one substance as a standard, by which the specific gravity of all others may be compared. Now the properties we have been considering, as well as some other circumstances connected with water, have led to its general adoption for this purpose. Its use in this way is said to have been originally suggested to an ancient philosopher of the name of Archimedes, by an incident as familiar, and apparently as little likely to lead to any philosophical discovery, as the apple which our Newton saw fall in his orchard. Hiero, the king of Syracuse, had put into the hands of a workman a certain quantity of gold, of which he was to make for him a crown. When the crown was finished and given to the king, he had reason to suspect that his gold had been improperly adulterated, and applied to Archimedes for his assistance in detecting the imposture. After many attempts for this purpose, the philosopher was about to abandon the object altogether, in despair of being able to accomplish it, when the fortunate incident occurred, to which we have alluded. Stepping into the bath one day, as was his custom, he happened to observe, that the water rose as he plunged into it, and that it did so in proportion to the bulk of his body. He immediately perceived, that any other body of the same bulk would have raised the water equally, but that one of equal weight, if of less bulk, would not have produced so great an effect. To his discerning mind this train of thought suggested a solution of the question, which he had undertaken to solve to the king: and he was so overjoyed, that he is said to have run into the street, just in the state in which he leaped out of the bath, exclaiming, "I have found it out; I have found it out." He now got two masses, one of gold and the other of silver, each of equal weight with the

crown, and, having filled a vessel very accurately with water, plunged into it first the silver mass, and marked the quantity of water which overflowed. Next he plunged the gold mass, and found that a less quantity now overflowed than before. Hence he inferred that, though the masses were equal in point of weight, the bulk of the silver was greater than that of the gold. He then plunged the crown into the water, and found that it displaced more of the fluid than the gold had done, and less than the silver, which led him to infer and report to the king, that it was neither pure gold nor pure silver, but a mixture.-In order to ascertain the specific gravity of a body which sinks in water, it is in the first place weighed in air; it is next weighed in water, and the weight which it now has in water is subtracted from that which it formerly had when weighed in air; the difference between these two weights shows the weight of the water displaced by the body, because we have seen that every body, when weighed in water, loses weight exactly equal to that of the water displaced: and therefore it is quite plain, that, by dividing the weight of the body in air, by the difference between that weight and its weight in water, we ascertain how many times the weight of a certain bulk of water is contained in the weight of a similar bulk of the substance which we are weighing; or, in other words, we determine the specific gravity of that substance. Thus, if we find that a stone weighs in air 600 grains, and in water only 400, we know that a quantity of water, equal in bulk to the stone, would in air weigh 200; and dividing the weight of the stone in air (600) by the weight of the water (200), we find the stone to be three times heavier than the water. If then we call the specific gravity of water 1, the specific gravity of the substance which we have been weighing is 3. If we call the specific gravity of water 1000, the specific gravity of the substance is 3000. It evidently matters not in this inquiry, whether you call the specific gravity of water 1, or 1000, or any other numerical denomination, for the object of the inquiry is not to ascertain the actual weight of any particular quantity of the substance, but only its specific gravity ;

or, in other words, what proportion the weight of any bulk of this substance bears to the weight of a similar bulk of water. When, indeed, we have found the specific gravity of any substance, it becomes a very easy matter to calculate the weight of any given bulk of it, as the weight of a cubic foot of water is exactly 1000 ounces; and therefore, if the specific gravity of water be accounted 1000, the number which denotes the specific gravity of any other substance, denotes also the number of ounces in a cubic foot of that substanceIn order to ascertain the specific gravity of a substance, which will not by itself sink in water, some heavier body must be attached to it, of which the weight, both in air and water, had previously been ascertained, and the difference marked; the bodies thus attached are weighed together, first in air, and afterwards in water, and the difference found; from this difference is subtracted the difference between the weight of the heavier body in air and water, and thus is obtained the weight lost by the lighter body in water; by which its weight in air must be divided, in order to ascertain its specific gravity. Thus, in order to find the specific gravity of a light wood, of which I have a portion weighing five ounces, I must attach to it a body of greater specific gravity, as, for example, a stone. I find that the stone which I employ, weighs 12 ounces in air, and only 6 ounces in water, making a difference of 6 ounces. I find that the stone and wood, when attached, weigh 17 ounces in air, and only one ounce in water, making a difference of 16 ounces. From this difference (16) between the weight of the attached bodies in air and water, I deduct the difference formerly found (6), between the weight of the heavier body in air and water, and thus obtain the difference (10) between the weight of the lighter body in air and water. By this, difference (10) I divide the weight of the lighter body in air (5), and thus find the specific gravity to be, if that of water be accounted 1: or 500, if the specific gravity of water be accounted 1000.-The specific gravity of a fluid may be found by ascertaining first the difference between the weight of a bottle, when filled only with common air, and when filled with water, by

which the weight of the water is found: then ascertaining the difference between the weight of the same bottle, when filled with common air only, and when filled with the fluid, whose specific gravity we wish to ascertain, by which the weight of the quantity of the fluid in the bottle is found; and thus the proportion is easily calculated between the specific gravity of the water, and the other fluid. If, for example, the other fluid in the bottle has been found to weigh twice as much as the water, its specific gravity is obviously just double that of water. In practice, however, recourse is generally had to an instrument called a hydrometer, for the purpose of ascertaining the specific gravity of fluids. This instrument is so constructed, that the specific gravity of the liquid is estimated by the depth, to which the hydrometer sinks in it. The farther this instrument sinks, the lighter is the specific gravity. It is upon a similar principle, that, in order to ascertain the strength of brine for salting meat, it is not uncommon to place an egg in the boiling water, and continue to put salt into it until the egg swim.

MECHANICAL PROPERTIES OF AIR.

AËRIFORM fluids differ from liquids principally in respect of the superior elasticity of the former, which hence are distinguished by the name of elastic fluids. Atmospheric air and all the various kinds of gases are of this description. They differ from each other in their chemical properties: but the Mechanical properties of all elastic fluids are the same. Though the air, by which we are continually surrounded, and without which we would cease to live, is invisible to the eye, its presence is sufficiently manifested by its effects. By the motion of a lady's fan you immediately feel that you have putit in agitation; by briskly moving a switch you hear its sound; in pushing the rammer of a child's pop-gun, plugged at the opposite extremity, you feel its resistance; by immersing a phial under water, you see the bubbles, which it forms in making its escape. We are now to consider some of its leading mechanical