a body occupies, is called its Volume or Bulk. All bodies on or near the earth possess gravity (or weight), and tend towards the earth's centre with a force called centripetal, proportionate to their respective densities. Fluids press equally in all directions, upwards, downwards, obliquely, and laterally. Solids press only downwards. The pressure of a fluid on a lighter body or a rarer fluid, arises from the heavier fluid seeking to maintain its own level, or the level of its own weight. The extraneous body is therefore driven upwards, as in the case of gases, steam, vapour, smoke, &c.

means.

Most substances are made up of two or more others, so intimately associated, either by mechanical mixture or by chemical combination, as not to be separable, except by extraordinary The component parts are held together by an attractive force called Cohesion, which is greater in solids than in fluids, and altogether absent in gases. When solids are exposed to a certain degree of heat they change their state: some are converted into liquids; liquids into vapours; and again, by a like loss of heat, these revert to the liquid state; and liquids, by due degrees of cold, are solidified. In others, the conversion is into smoke. This property of matter is called its extension. In some cases these substances may be taken to pieces, and each part further investigated - —a property of matter which is called its Divisibility; but when they resist all separation, they are called simple or elementary.

Those substances which it most concerns us to consider in reference to the question we are investigating, are Oxygen, Nitrogen, and Hydrogen.

Oxygen is only known as gas. It is a little heavier than atmospheric air, and 740 times lighter than water. It is the principle of combustion or burning, and indispensable to the support of animal or vegetable life. All substances which are capable of combustion in common air, burn with far greater intensity in an atmosphere of pure oxygen; but were it inhaled by animals in this form, they would expire from excess of vital action.

Nitrogen, or azote, has neither colour, taste, nor odour. It extinguishes flame, cannot support life, and counteracts the activity of oxygen in that mechanical mixture of these two gases which compose our atmospheric air.

Hydrogen is the lightest and most inflammable of all material substances, being bulk for bulk more than fourteen times as light as common air. 2,000 feet of this gas will weigh only 11 lbs. ; while the same volume of common air will weigh 160 pounds. For this reason, it is used in the inflation of balloons. Phosphuretted hydrogen is generated by the decomposition of animal bodies, and has produced such phenomena as the "Will o' the Wisp," &c. Sulphuretted hydrogen is extremely poisonous, and is generated freely during the combustion of tallow or wax candles.

During consumption by fire, fermentation, or decay of or ganised bodies or vegetables, as well as during the respiration of animals, carbonic acid gas is constantly produced. It is also thrown off by vegetation during the night; but its noxious effects upon the atmosphere are neutralised by the larger quantities of oxygen which plants generate in daytime. It extinguishes light, and produces suffocation. Being heavier than air, it remains at the bottom of wells and mines, causing the "choke-damp," which, however, may be dispersed by throwing in water. A compound of this gas with hydrogen, called carburetted hydrogen, issues from stagnant waters, decomposed matter, and under the name of "fire-damp," produces explosions in coal mines.

Air from the lungs of animals, when inhaled a second time, acts as a poison, which is more or less deadly as the oxygen is more or less vitiated. A man consumes 26 cubic feet of oxygen daily, and generates nearly a cubic foot of carbonic acid hourly. Wood, much saturated, absorbs oxygen, and thus generates carbonic acid gas; and all vegetable matter emits carbonic acid at night. These are facts which naturally present serious objections to unnecessary exposure in night-boatwork on rivers, and too frequent deck-washing, and demand every possible contrivance for ventilation.

"To form just conceptions of what Ventilation is, and how it is in general to be accomplished, an engineer has to consider that the ocean of air, called the atmosphere, which rests on the surface of the earth, and at the bottom of which men live, as certain aquatic animals live at the bottom of the sea, is about 50 miles high or deep; and that the portion of this ocean which can be contaminated by any process of animal or vegetable life, or by the decomposition of organic bodies when dead, may be

regarded as less deep generally than the fiftieth part of one mile, estimated from the surface of the earth. This comparatively insignificant stratum, therefore, may be regarded as the home or lurking place of all epidemic diseases and insalubrious air; the more exact statement, indeed, being, that these are generally confined to the still much smaller portions of air contained in houses, or other enclosed places. Then the fact is to be kept in mind, that the whole mass of the atmosphere at any moment over a city or other place, is always travelling away to leeward with the speed of the wind, and is carrying with it whatever impurity may ascend from below, which impurity is then resolved quickly into the pure elementary oxygen, carbon, &c., of which all effluvia consist. Man can no more contaminate permanently the deep atmosphere over him by his proceedings at the bottom of it, than he can contaminate the Atlantic Sea, by what he may do on the shores. Then he has to learn, with the same mechanical certainty as he can substitute the pure water of a passing tide or river stream for defiled water near the shore, he may substitute pure air from the atmosphere for any air near him that has become unfit for his use."*

These substances enter into the composition of water and air, though in different proportions. Water consists of eight parts (by weight) of oxygen and one of hydrogen, and air mainly consists of one measure of oxygen and four of nitrogen. Both are fluids, and have many properties in common. The watery ocean extends over three parts of the earth's surface, and the aërial surrounds the globe to a height of 45 miles.† Both have impenetrability, inertia, and momentum. Bodies, specifically lighter, float upon their surface, and heavier bodies sink. Both gravitate towards the earth's centre, and press equally in all directions. This equality of fluid pressure on the bodies of animals

*Arnott's Physics.

The surface of the sea is estimated at 150 millions of square miles, taking the whole surface of the globe at 197 millions. The Pacific Ocean covers 78 millions of square miles, the Atlantic 25, the Indian Ocean 14, the Southern Ocean to 30 degrees 25, the Northern Ocean 5, and the Mediterranean 1 million; the Black Sea 170,000, the Baltic 175,000, the North Sea 160,000.

The depth of the ocean is one of those secrets of Nature which the ingenuity of man has failed as yet to penetrate. It has been inferred from a supposed physical relation that it is equal to the height of the mountains. Lieut. Maury does not consider this supposition, or the different reports of soundings which have exceeded 25,000 feet, to be deserving of confidence.

is sustained by the counteraction of fluids contained in their system; and when the water inside a ship has risen above the level of a leak, the entrance of more will be considerably retarded. "This equality of pressure in all directions at any point in a fluid in equilibrium, is the fundamental principle from which all the reasonings of hydrostatics are to be deduced. The particles glide over each other with perfect freedom, each particle pressing equally on all the particles that surround it, and is equally pressed upon by these. It also presses equally upon the solid bodies which it touches, and is equally pressed upon by these."*

This property of thus transmitting pressure, is one in virtue of which a liquid becomes a machine; for if a quantity be submitted to compression, the effect is equally diffused through the whole; and a given pressure, made on one inch of the surface of a fluid contained in a vessel, is instantly borne by every inch of the surface of the vessel, however large, and by every inch of the surface of any body immersed in the fluid. Thus, in the hydrostatic press, which is used in the process of forcing the preservative solution into timber, if the small piston of e (Fig. 3.) have only one thousandth of the area of the larger one, c, and be pressed down with a force of 500 lbs., it will cause the large one to rise with a force of 1000 times 500 lbs. In the construction of the Britannia Bridge, a press of this kind was made use of, which lifted a weight of 1,144 tons. The internal diameter of the great cylinder was 22 inches, and that of the ram or smaller piston, 20 inches.

d

Fig. 3.

f

Water is of the same density throughout, and is incompressible. Air has different degrees of density, and is elastic. To the general law that heat causes expansion and cold contraction, water is but partially obedient, while air obeys that law under all conditions. A cubic foot of distilled water weighs 1000 ounces avoirdupois. Sea water contains from 3 to 4 per cent. of salt. Fresh water boils at 212° Fahrenheit, and begins to freeze at 32°. Sea water freezes at 2810. At this point, the atoms become fixed

Tomlinson's Nat. Philosophy.

in crystals, and continue to expand until they form ice, which being 8 parts in 100 lighter than water, floats on its surface.* Nine cubic inches of water become ten by freezing; and a cubic inch, confined and frozen, expands with a force equal to nearly 13 tons,- - a fact which, if not considered in the arrangement and management of those pipes and valves which in ships commu. nicate with the sea, may lead, and has led, to dangerous accidents.†

Bodies of greater density than water, when completely immersed, lose just as much of their weight as that of the quantity of water they displace; a consideration intimately affecting all such operations as the carrying of guns, anchors, rudders, &c., underneath boats, or weighing sunken vessels, &c.

CHAP. II.

DISPLACEMENT AND SHAPE.

BODIES which float on the surface of water are borne upwards by a force equal to the weight of the fluid displaced; but those bodies whose bulk does not displace a quantity of water equal to their weight will sink, unless floated in a vessel sufficiently large to displace a quantity of water heavier than their united weights. A ship, therefore, destined to carry a certain amount of tonnage, must be so constructed as to displace a quantity of water equal to that amount, and her own weight besides. This consideration involves the distinct connection of hydrostatics with naval architecture, especially as affecting the subject of displacement.

* Liebig tells us that "during the act of freezing, the temperature remains at 230 Fahrenheit. Nevertheless water may be cooled as low as 50 without becoming solid, if the fluid be in a state of perfect rest, but that the least disturbance is sufficient to effect congelation."