veniences of an opposite kind would occur. The air would be too thin to breathe; the weight of our bodies, and of all the substances surrounding us, would become too slight to resist the perpetually-occurring causes of derangement and unsteadiness; we should feel a want of ballast in all our movements. Things would not be where we placed them, but would slide away with the slighest push. We should have a difficulty in standing or walking, something like what we have on ship-board when the deck is inclined; and we should stagger helplessly through an atmosphere thinner than that which oppresses the respiration of the traveller on the tops of the highest mountains." The force of gravity and the projecting force explain all the motions of the solar system. "To understand the operation of these in relation to each other," remarks Dr Duncan in his Sacred Philosophy of the Seasons, "first suppose a small body, placed at rest, in the neighbourhood of a large one, also at rest, both of them in empty space. The mutual attraction would immediately begin to operate, and they would move toward each other till they met, and that with a rapidity proportioned to the quantity of matter contained in each, the larger most powerfully attracting the smaller. Suppose, again, that the smaller body, instead of being placed at rest, was projected with a certain velocity in a direction different from that in which the other was situated. That velocity might be very great; and then, after a mutual disturbance, arising from the reciprocal attraction, the moving body would disengage itself from the other, and fly off into boundless space; or, it might be very small, and then the vis inertia would be overcome, and the two bodies would fall toward each other, and unite. But, if the projecting force were in every way proportioned to the gravitating force, the consequence would be, that there would be a certain point in the progress of the moving body in which the two powers would balance each other, and then that body would continue to move, indeed, but in a constant circuit round the attracting force, which would thus become its centre of motion; and its course would be in an oval or in a circle, according to the force and direction of the motion originally impressed. This is, in fact, the principle by which the whole planetary system is governed. The sun, a body of vast dimensions, is the centre of attraction to which all the planets gravitate, and into which they are prevented from falling only by the opposite power of inertia. The two forces, with the nicest exactness, balance each other; the former, in reference to this principle, being called the centripetal, and the latter the centrifugal motion; and thus the great machine is kept in action, and the planets wheel steadily in their respective orbits, and perform their various beneficent functions. I may add, that this is the principle, not of the solar system only, but the system of the universe. 'O, unprofuse magnificence divine! O, wisdom truly perfect! thus to call From a few causes such a scheme of things,- An universe complete!'" Attraction, or the mere force of gravity, produces the earth's tides. The moon attracts the hemisphere of the earth next to it more than the hemisphere opposite to it, and the central parts of the hemisphere more than the outer parts; or, in other words, always draws most strongly toward itself the portion of the earth which is nearest, and least strongly the portion which is most remote. Waters obey attraction rather in molecules than in mass,-having no firm cohesion like land to obey it only in one rigid body; and therefore those directly under the moon in the up-hemisphere rise above the level of the surrounding ocean, and those directly under it in the down-hemisphere are left behind, so as also to appear to rise. And as their rise in both hemispheres is a constantly progressing one, in the course of the earth's constant revolution, it constitutes a double daily tide, a continual series of great waves at the distance from one another of half the globe. If the waters obeyed instantaneously, the crest of the tide would always be right below the moon; but they are hindered by their own inertia, and by the friction of things in contact with them, from obeying even promptly, so that the tidecrest never arrives at any spot till some time after the moon has passed its meridian. The sun acts in the same way as the moon, but with only one-third as much power; and when sun and moon attract in the same direction, as happens at new moon and at full moon, they produce the conjoint full wave called a spring-tide, and when they attract in transverse direction, as happens at the moon's first and third quarters, they produce the disjoint half wave called a neap-tide. Tides, as a whole, however, are considerably modified by several things in the varying action of gravity, which we cannot take space to explain; and those of some localities are annulled, those of others vastly mag nified, and those of almost all in some way or other affected by the earth's own relations of sea and land. The tides of the Baltic and the Mediterranean, for example, are scarcely appreciable; those of the central parts of the Pacific are only about two feet high; those of the Bay of Fundy rise to the amazing height of seventy feet; and those of the river Amazon are a series of ascending and descending currents, modifying one another in wondrous diversity over a distance of five hundred miles from the ocean. Those of the Solway Frith, however, are to the full as curious as any, and may be briefly noticed as an interesting specimen. The tide of the Atlantic rolls right against Ireland, and sweeps round its ends so as to enter the Irish Sea simultaneously at the northern and the southern inlets. Two tides are thus generated; and they rush into collision with each other at the extremity of Galloway, and form a sudden tumbling heap of waters, which flies furiously off to the long, broad, sandy hollow between Scotland and Cumberland. This is the Solway tide, -so swift and headlong as to be rather a career than a current, rather a sweeping torrent than a wavy flow. A spring-tide, but especially a tide which runs before a stiff breeze from the south or the south-west, careers along at the rate of from eight to ten miles an hour. It is heard by the people along the shore upwards of twenty miles before it reaches them, and approaches with a hoarse and loud roar, and with a brilliance of demonstration incomparably more sublime than if the wide sandy waste were densely scoured with the fleetest and the most gorgeously appointed invading army of horsemen. Before the first wave can be descried from the shore, a long cloud or bank of spray is seen, as if whirling on an axis, and evanescently zoned and gemmed with mimic rainbows, and the rich tintings of partial refractions, sweeping onward with the speed of a strong and steady breeze. Then follows a long curved white and flowing surf. And when the magnificent banner of spray and this surfy pioneer have made distinct announcement, finally and suddenly appears the majestic van of the tide, a speckled and deeply dimpled body of waters, from three feet to six feet high abreast, rolling impetuously forward, and bringing closely in its rear a tumbling and tempestuated mass of marine vales and hillocks, glittering and gorgeous all over with the most fitful play of the prismatic colours. The force of tides, of streams, of wind, of steam, and even of galvanic currents, as a power in nature which man employs for driving his machines, is too obvious and too well known to need any remarks. Yet the simple pressure of still water and the common pressure of the atmosphere, though also generally known, may be selected to show how beneficently great powers slumber, all ready for use, in quantities where an unreflecting mind would never suspect their existence. The pressure of any body of still water is in proportion, not at all to either its own volume or its own surface, but solely to the extent of the bottom on which it rests, together with the height of a column of the water over that bottom. In a funnel-shaped-vessel, for example, if the neck be the bottom, the pressure is equal only to the weight of the small column resting on the neck, and if the expanded end be the bottom, the pressure is equal to a column of the entire diameter |