land and ocean, mountain and valley-covered by the fluid air. We exist at the bottom of that vast continuous sea; we are born in it; and when we cease to drink of it at every inspiration, we cease to live. This ocean of the air possesses properties, in respect to the equalization of heat, which include all those of the ocean of waters, but are of greater activity. Its elasticity renders it far more expansive from any increase of temperature than water is, and therefore far more buoyant. For this reason, when heated, it becomes more readily displaced by the surrounding colder and heavier air, and powerful CURRENTS are thus established in the fluid atmosphere, by variations of temperature which would scarcely cause them to be perceived in the fluid ocean. It is incorporated with these currents of the air that excessive heat is borne up and carried away high above the earth's surface with a prevailing tendency towards the frigid zones, and cold made to creep along in contact with it towards the region of perpetual heat. Thus are the great astronomical varieties of temperature intermingled and blended by the atmosphere, nevertheless with an infinity of modifications, subject to an infinite variety of local causes, and suited to the boundless diversity of the scheme of God's creative providence. To this blending and equalizing effect of the atmosphere, the equal temperature of the ocean largely contributes. Seven-tenths of the atmosphere perpetually covers the ocean and thus partakes in that uniformity of temperature, in respect to the seasons, which peculiarly belongs to the sea. Besides a more ready formation of currents, the elasticity of the air gives it yet another property, tending eminently to the equalization of temperature, and, in fact, necessary to the equalizing power of the wind. The earth's surface may be considered as the bottom of the ocean of the air. The higher, therefore, is any portion of the atmosphere above the earth's surface, the less is its depth beneath the surface of this ocean, and the less, therefore, is the pressure upon it of the air which is above it; and being elastic and less pressed, the air is there less condensed or more rare. Thus, then, it appears that as we ascend higher, we of necessity find the air rarer or lighter; a fact which is at once verified if a barometer be carried to any elevated spot. Now this, whilst it is a necessary result of that elasticity of the air which ministers to so many other useful purposes, is also a most wise provision for this end, that it keeps the great currents of the atmosphere down to the earth's surface. It causes them to flow at the bottom of the ocean of the air instead of at the top, as do the currents of the ocean of waters. Were it otherwise, these currents would range on the extreme limits of the atmosphere, instead of on the surface of the earth. Immediately that the air becomes heated by contact with the heated earth, it would ascend in a vertical column until it reached the extreme surface of the atmosphere; along that surface it would determine a current, imperceptible on the earth's surface as are the surface-currents of the ocean in its depths. Thus we should experience no motion of the sluggish air from place to place, no "winds from the four quarters of heaven;" but a stagnant and perpetual calm, unbroken except by continual exhalations and upward currents; a dense curtain of vapour would overspread us, beneath whose shade would reign fierce vicissitudes of temperature, and the pestilence spread wide her wings. But "the goodness of God endureth continually." "Fire and hail, snow and vapours, and stormy winds fulfil," therefore, "his word;" and are "turned round by his counsels, that they may do whatsoever he commandeth them on the face of the world in the earth." The heated air ascends, indeed, by reason of the greater density of the cold air about it; but after a short ascent, it attains a region where the surrounding medium-although it may be colder-is but equally dense. There, then, its ascent terminates. It is there-instead of at the surface of the atmosphere-that it begins to spread itself, and there it determines its current. Along this region, comparatively near to the earth's surface, it distributes its heat, radiating part, and propagating the rest by contact-bearing it in a current, at first inclined downwards, to some point perhaps far removed from that where it arose, and disturbing the equilibrium of.the subjacent air wherever it passes. By the laws of hydrodynamics the motion of this current above. necessarily brings about a motion of the air beneath it; a result which is favoured by its continual tendency, as it cools, to descend. Moreover, the original displacement of this mass of air is brought about by a motion of the air around it, and the motion of this air supposes that of some other mass of air adjacent to it, that of a third, and so on. Thus every variation in the temperature of the lower air propagates a series of displacements of its mass along the earth's surface, and of currents in a region more or less elevated above it. These are the winds.-Moseley. LESSON XII.-TUESDAY. THE MECHANICAL POWERS—INCLINED PLANE, AND WEDGE. The inclined plane may be considered as a right-angled triangle resting on its base. This is called the base of the plane, and the hypotenuse and perpendicular are respectively termed the length and the height of the plane. The following illustration illustrates the use of the inclined plane as a mechanical power:-Suppose it is required to move a cask weighing a quarter of a ton from the ground into a warehouse, the floor of which is six feet high, while the distance measured on the ground between the warehouse and the cask is sixty feet. This may be accomplished in two ways. It may be rolled to the door of the warehouse, and then lifted perpendicularly to the floor, or it may be rolled up any contrivance laid in a slanting direction from the floor to the cask, thus avoiding the perpendicular lift. In both cases the effect produced is the same; but by the first method two powers are employed, one comparatively weak to roll it to the bottom of the warehouse, the other equal to the weight of the cask to lift it to the floor; while the second method only involves the employment of one uniform power. By this contrivance, which forms one of the mechanical powers called an inclined plane, one man is frequently able to raise a weight which twenty men could not lift perpendicularly. Waggons by which timber is brought from forests are loaded in this way. Casks of merchandise are frequently rolled in and out of carts and let down into cellars by the same means. In the inclined plane, when the power acts in the direction of the length of the plane, it bears the same proportion to the weight as the height of the plane bears to the length. If the plane is thirty feet long and six feet high, a power of six pounds will sustain a weight of thirty pounds. A horse, drawing a load up a hill which rises one foot in twenty, is, in reality, lifting the twentieth part of the load, in addition to the resistance which would be presented on a level road. Since the power and weight vary directly as the height and length of the plane, it follows that the longer the plane is for any given height the less will be the power required. Hence, a driver, in ascending a hill, makes his horse wind from one side of the road to the other; and it is not uncommon for a horse to do this instinctively, without the guidance of the driver. By this means the length of the road is increased without increasing its steepness, and consequently the load is more easily moved. The wedge is commonly regarded as a distinct mechanical power, but it is only a double inclined plane. It is generally in the form of an isosceles triangle, a form obtained by placing the Bases of two equal and similar right-angled triangles together; and therefore, if the bases of two equal inclined planes are joined together, a wedge is formed. The slant of the wedge is called its length, and the end which is struck is called its height. The inclined plane is fixed and the resistance moveable; but the reverse is the case in the application of the wedge, the resistance being fixed and the wedge itself moveable. There is also a difference in the action of the power; in the inclined plane it acts as a pressure, but the wedge is acted on by percussion. The wedge is extensively used where very great weights are to be moved. A ship, about to be launched, is raised by means of wedges driven under the keel, by which the force of gravity or weight of the ship is overcome. Wedges are also used for splitting timber, and in quarries for tearing asunder large blocks of stone. The wedge acts in these cases in opposition to the force of cohesion. Friction, the great drawback to the effective application of most of the mechanical powers, is the only means by which the wedge is rendered applicable. Were there no friction between the sides of the wedge and the resisting surfaces it would spring out after every stroke of the hammer with a force equal to that with which it was struck. Knives, chisels, needles, awls, and all cutting and piercing instruments, are examples of the wedge. The inclined plane and wedge are among the simplest of mechanical powers, and consequently appear to have been employed in very remote times. The huge blocks of stone occupying elevated positions in some Egyptian structures are supposed to have been hewn and raised by means of these powers. LESSON XIII.-WEDNESDAY. TO THE RAINBOW. Triumphal arch, that fill'st the sky To teach me what thou art. Still seem, as to my childhood's sight, For happy spirits to alight Betwixt the earth and heaven. Can all that Optics teach, unfold When Science from Creation's face And yet, fair bow, no fabling dreams, When o'er the green undeluged earth |