of that end,-ay, and if the neck be prolonged upward by a tube of any length filled with water, the pressure will still be as great as if the whole were a cylinder.of the same circumference as the bottom. Or, if in a vessel or cavity of any kind a tube rise from the surface and be filled with water, the pressure on the bottom will be the same as if the whole body of the water stood as high as that in the tube. Hence the famous experiment of bursting a strong cask by means of a few ounces of water. Let such a cask be filled with water; let a long tube of only a few ounces in capacity be screwed tightly into a hole in its top; let this tube be filled with water, and the cask will burst. A wellknown contrivance called the hydrostatic bellows, makes an ounce of water balance a thousand ounces of any solid substance; and a machine of the size of a common tea-pot, constructed on the same principle, can cut a bar of iron as easily as a slip of pasteboard.

The average common pressure of the atmosphere at the level of the sea and of low valleys, as we stated in the chapter on the Air, is about fifteen pounds on every square inch. This is not only a great force, but an all-pervading one on earth; and it works in thousands of ways that most men little dream of. It keeps close things tight, cohering things firm, and living things elastic; and is either the main force or a necessary subordinate one in most of the physical resistances and movements in the world. So strongly does it plug an orifice in any artificial vessel containing liquid, as, for example, a cask or a well-formed tea-pot, that none of the liquid, no matter what its weight be, can escape unless another hole, however small, elsewhere communicate with the atmosphere to

let in the counteracting pressure. So sternly does it put together, and hold together, two perfectly polished surfaces, such as of the smoothest marble or the smoothest ground glass, coated thinly with oil, that, if particles of air cannot get access between them, no power short of about fifteen pounds for every square inch of this extent can pull them perpendicularly asunder. This pressure is, no doubt, the grand acting power in all cohesion by means of glues and lutes and mortars, these substances serving principally to fill all pores which counter-pressing air could penetrate, and so occasioning all the exterior to be jammed fast by a force of fifteen pounds on its every square inch.

This force also co-operates with frost to seal up the underground waters in winter. The moisture of land, even in Siberia, is seldom truly frozen to a greater depth than two feet; but, when converted into ice on some soils to even but a tenth of that depth, it acts like a cement, just in the same way as mortar does in walls, and occasions all the moisture below, all the tricklings of water in the capillary tubes of the earth's interstices, to be locked fast by atmospheric pressure. Hence is it that, during even the comparatively mild, brief frosts of Britain, the wells and rills and rivulets, in many parts of the country, become almost dry; and hence, too, is it that, on the recurrence of a thaw, notwithstanding that slowness of the liberation of frozen water which we pointed out when speaking of chemical action, so ample and refreshing a supply of liquid bursts speedily into the water-courses from the relaxing earth.

Every kind of what is popularly called suction, arises from atmospheric pressure. A man dips his

lips in water, and then draws the air within his mouth into his lungs, and immediately the water is pressed up into his mouth. But were he not to let his lips touch the water, he might pull with all his power for a twelvemonth, and not obtain a drop. An infant, in the same way, makes a vacuum round the nipple, and the atmospheric pressure on the parts adjacent forces out the milk. A boy's sucker, too, simply prevents the access of the exterior air, or lies on a smooth stone in the same way as two polished oiled surfaces of glass lie on each other; and if it have perfect contact to the extent of just one square inch, it will lift a weight of almost exactly fifteen pounds.

Limpets, periwinkles, and other molluscs, select a smooth surface of rock, and fill their shells with such expansions of their own body, or such quantities of water, as completely exclude all air; and then the shells are held fast to the rock at the same rate of power as the boy's sucker on the stone. Two-valved molluscs, also, such as oysters and mussels, exclude all air, and therefore have their shells held so firmly together, that no power short of about fifteen pounds for every square inch of their surface can force them open. But let any one of them have a little hole ground into it, so as to let in the air and counteract the pressure, and it will readily open. Common houseflies, and other small creatures which have the power of walking on a vertical smooth surface, or the power of walking along a ceiling with their feet uppermost, owe that power to the very simple contrivance of squeezing the air from between their feet and the surface they walk on, so as to let themselves be held fast by atmospheric pressure.

The air within substances, no matter how small in quantity, counterpoises the air without, and therefore holds them in perfect equilibrium. But for this, a porous substance would be at once pressed compact or crushed to powder; and a cavitous one would be at once squeezed flat or shattered to atoms. Let a syringe be tightly fitted through the cork of a tightlycorked, flat, empty phial, and let it be made to draw out the interior air, and the phial will speedily be smashed to pieces. The counterpoise of air within and air without, however, keeps things more secure than if no pressure whatever existed, just as a particle in the centre of a mass of any matter is more secure than a particle at the circumference. And, in the case of animals, it also produces elasticity and vigour, or at least serves jointly with the varying weight and varying expansion of the air within the lungs and throughout the body to act like sinews to the whole frame, and like wings to the legs and arms.

Atmospheric pressure forces the mercury up the tube of the barometer, and water up the bore of the common pump. It is the power by which the syphon conveys water from one reservoir to another over any intervening obstacle of less height than thirty-two feet. It acts in the forcing-pump, in the fire-engine, in the steam-engine, and more or less in every other machine which is worked in any way by water or vapour or air. It is the force which was brought into play in the great experiments, a few years ago, with what were called atmospheric railways. And though it was so undeftly used in these experiments as to sink into less favour than steam, it is nevertheless in itself so magnificent a force, so cheap, so facile, so everywhere

available, so perfectly free from all risk and nuisance, ard so purely, so directly, so beautifully a gift of the Divine beneficence, that it must surely, at no distant day, in a more advanced stage of mechanical science, become the choice moving power in many great classes of machinery.

The elasticity of the air is a force, in some respects, as wonderful as the pressure. It is the force, as we saw in a former chapter, which co-operates with heat to produce all the winds. It is the force which enables fishes to sink or rise in the water; for they have an air-bladder which, whenever they please, they can so contract as to make them sink, or so dilate as to make them rise. It is also, no doubt, one of the tremendous forces of natural rock-splitting, of land-slips, of the upheaval of islands and mountains, of the eruption of volcanoes, and of the rumbling and rending of earthquakes. And, as well as the atmospheric pressure, it is a facile, all-prevailing, mighty power, abundantly capable of being turned to strong, manifold, valuable, mechanical account. Ordinary air, at the level of low valleys, can be compressed into a space sixty times less than what it naturally occupies, and at the same time is capable of expansion into a space thirteen thousand times greater; so that between its extremes, under high compression and in complete vacuity, lies a force of elasticity or explosiveness similar to the power of gunpowder.

The interdiffusiveness of aeriform substances is a force widely different from pressure and elasticity, and possesses beauties and wonders peculiar to itself. It maintains the gases of the atmosphere in equable diffusion, irrespective of their weight. It makes the

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