in a chorus in Judah, at the words, 'The Lord devoureth them all.' The sounds, sinking into an abyss of harmony, are penned with an effect worthy of the great Beethoven himself."

CHAPTER VIL

FORCE AND MOTION.

FORCE-GRAVITY-ATTRACTION-VIS INERTIA-PROJECTION-RELATIONS OF THE EARTH'S ATTRACTION TO THE MASS AND POWER OF ANIMALS-GRAVITATION-THE MOTIONS OF THE PLANETS-THE TIDES THE PRESSURE OF STILL WATER-THE PRESSURE OF THE ATMOSPHERE-EFFECTS OF ATMOSPHERIC PRESSURE ON MINERALS, ANIMALS, AND MACHINES-THE ELASTICITY OF THE AIR-THE INTERDIFFUSIVENESS OF THE ATMOSPHERIC GASES.

FORCE is anything which alters motion, or produces it, or tends to alter or to produce it. It may be in equilibrium, holding a body in a state of rest; or it may act in any of the ways, or with any of the effects, popularly called pressing, rubbing, hitting, lifting, throwing, driving, pulling, splitting, and swelling. It may be viewed as remote or primary,—and then it exists grandly in the power of an animal's will over its muscles, and infinitely in the power of the all-creative mind of the Eternal over all physical things; or it may be viewed as proximate or secondary, and then it exists grossly in the action of machines, more refinedly in the action of animal strength, and most definitely in heat, in electricity, in chemical affinity, and, above all, in what is called weight.

Yes, apart from the power of mind, the grand originator and sustainer of motion is weight. But what

is this? We cannot tell. Philosophers call it gravity, and dilate upon its laws, but either say nothing about its nature, or treat it only in vague and conflicting speculations. As probable a theory as any identifies it with magnetism, or with something near akin to magnetism; but this, like everything else on the subject, is mere theory. Its properties or effects, however, are quite obvious, and perhaps appear all the more wonderful that the thing itself is unknown.

Two bodies, if free, or in the degree in which they are free, move toward each other by their mutual gravity-called in this respect their mutual attraction. But, when of different sizes, they move in proportion to their respective bulk,-the smaller one moving much, and the larger one moving little. Hence do all pieces of matter set free in the air, in all parts of the world, fall straight to the ground, so that those in antipodal regions proceed direct toward each other, and none anywhere can fly off into space; and hence do even the largest appear to make all the motion of coming into contact with the ground,-for, though the earth really advances to meet them, it is so immensely bulkier than they, that the amount of its motion is incomputable and inappreciable. Whatever the pieces of matter be, even suppose them such widely different things as a ball of iron and a feather, if set free at the same height in the air, they tend to fall at the same rate of speed; and they are comparatively accele rated or retarded only by the different way in which they encounter the resistance of the air, so that, when dropped into a vessel or chamber which has had all air artificially pumped out of it, they actually descend through it with precisely equal velocity. All falling

bodies, however, move slightly and slowly at the commencement of their fall, and gain increase of speed as they descend, and move always quicker and quicker till they reach the ground; and hence the vast pelting, pattering power of hail-showers, which come from no higher an elevation than a mile or two,-and hence the enormous depth to which small star-stones, descending, perhaps, through thousands of miles, have plunged into strata of sand or clay.

Any body in a state of rest, if never disturbed, will for ever remain in a state of rest; and any body, if set in motion by some other force than that of attraction, and never in any way interfered with, will for ever continue in motion. In other words, a body rests or moves only in relation to one or more other bodies, and has not any powers of its own to make itself either move or stop. This is called its vis inertia; and any force which sets it in motion otherwise than by attraction is called a projecting force. But every such force, or rather all motion produced by it, is modified by gravity. No body in any part of the universe, so far at least as men have been able to observe or conjecture, can move in such entirely free space as to be uninfluenced by other bodies. And certainly none in our world can move an inch without being mightily controlled both by the resistance of the air and the attraction of the earth. Any body projected aloft by the strongest force of man, or the strongest force of any engine he can possibly contrive, is so steadily resisted by the air and so powerfully attracted by the earth, that it gradually loses its upward speed, soon comes to a pause, and after an instant of equilibrium yields wholly to the earth's attraction, and begins to descend

And the regular

in the manner of a falling body. fusion of the forces which guide it, beginning with the full power of the projecting, and ending with the full power of the attracting, occasions it to describe the beauteous curve which all eyes admire and all tasteful artists delight to study.

All animals have mass and power in a certain relation of excess to the earth's attraction; for, otherwise, they either could not move at all, or would move with great difficulty, or with inconvenient lightness, in jerks and jolts, losing their balance, and wanting all necessary self-control. Other planets differ so greatly in size and density from the earth, that, in consequence merely of their correspondingly different force of attraction, all foose things on their surface, animate and inanimate, must have widely different forms and relations, and probably, also, widely different combinations of elements, from those of the earth. If our world were to be exactly what it is, with simply such an increase of bulk as to make it as large as Jupiter, it would attract all its animals with eleven times greater force than it does, and would therefore destroy them. "The lightness of the fawn, the speed of the hare, the spring of the tiger, could not exist with the existing muscular powers of those animals. For man to lift himself upright, or to crawl from place to place, would be a labour slower and more painful than the motion of the sloth. The density and pressure of the air, too, would be increased to an intolerable extent, and the operation of respiration, and others which depend upon these mechanical properties, would be rendered laborious, ineffectual, and probably impossible. If, on the other hand, the force of gravity were much lessened, incon