motion, in almost every direction, the rapidity with which the ascending and descending currents of the air upon which they are borne intermingle; and upon looking across a heated body, as a chimney or a hot poker, the ascent of the heated column of air may clearly be detected.

The force with which it rises is indicated by the steady and tapering form of the flame of a candle, which consists of aeriform matter in a state of ignition. This heated current generally ascends perpendicularly in the surrounding cool medium, but may be diverted from its course by holding over it a long tube, in the axis of which it will flow, though greatly inclined from the perpendicular. The flame of a gas-burner may even be directed downward by connecting it with the shorter arm of a tube, bent twice, at right angles. The longer arm must have the air within it rarefied, by burning a little spirit of wine in a sponge, when an ascending current will be established, which will draw down the ignited gas, the heat of which will afterward maintain the draught in the chimney.

133. The expansive and ascensional force of a body of hot air may also be illustrated by confining it in a bag of any light material, such as tissue paper. If the aperture of such a bag be held over the flame of a piece of sponge, steeped in spirit of wine, it will become distended and ascend in the air. was the power which Montgolfier first applied to the construction of a balloon, and in dependance upon this principle Pilatre de Rosier first committed himself to float upon the atmosphere.

Such

§ 134. Some very interesting properties of aerial currents may be illustrated by placing a lighted taper in a flat dish, and covering it with a bell-glass, with a long chimney immediately over the flame. If a little water be placed round the edge of the glass in the dish, the air will be prevented from entering it, and the flame will speedily be extin

guished for want of a fresh supply. If, however, the taper be again lighted, and the chimney of the bell-glass be divided into two channels by a diaphragm down the middle, and it be again placed over the taper, it will continue to burn for any length of time. The heated and light particles of the air, which tended to ascend in the tube in the former arrangement, but were impeded by the opposite tendency of the cold particles to descend, will pass out by an ascending current on one side of the diaphragm, while a contrary current of fresh air will continually compensate them on the other side; the direction of these currents is readily rendered visible, by mingling some cloudy or smoky substance with the descending current. This separation of contrary currents is of considerable importance in the ventilation of mines, and many deep shafts are divided by boarded partitions, to ensure the descent of fresh air upon this principle.

We are indebted to the expansion of air for the ventilation of our rooms and for the ascending currents in our chimneys, by which our fires are maintained. If we hold the flame of a candle to the upper part of a door opening into a heated room, we shall not fail to find, by the direction in which it is bent, that a current of light heated air passes outward; while, by lowering it to near the bottom, we shall be able to detect a counter-current of denser cold air rushing inward with equal velocity.

The knowledge of these principles, it cannot be denied, ought to have led to a more perfect system of ventilation than is commonly adopted in our public buildings, where crowds assemble for business or for pleasure; but this, unhappily, is not the only instance in which indolence prefers the wellbeaten road of routine practice to the more laborious paths which better knowledge would point

out.

135 The convection of heat is a process of the

utmost consequence also in some of the grand operations of nature; and it is principally by the circulation of fluids, elastic and non-elastic, of which their perfect mobility renders them susceptible, that the distribution of temperature over the earth is regulated, and those great inequalities guarded against, which would certainly result from the heating of the solid strata alone. Thus the atmosphere with which the globe is surrounded, moderates the extremes of temperatures both in the arctic and equatorial regions. When the surface becomes heated by the direct rays of the vertical sun, the stratum of air reposing on it is rarefied and ascends; its place is supplied by the denser air pressing in from the colder parallels, and by a constant succession of these operations the heat is moderated, which would otherwise become intense. The constant current of the trade-winds owes its primary impulse and direction to this cause. The constantly-ascending warm air, in its effort to maintain that equilibrium and equality of pressure which results from the laws of hydrostatics, must flow over towards the colder climates, and carry with it the heat with which it is charged, and thereby mitigate the extremes of cold. Currents from the poles and the equator upon the surface of the earth, and opposite currents from the equator to the poles, in the upper regions of the atmosphere, will therefore constantly exist, although their directions may be variously modified by inequalities upon the earth's surface, and other disturbing influences. The course of these grand compensating currents has often been traced by accidental circumstances. Some years ago, during an eruption of a volcano in the island of St. Vincent, in the West Indies, it was observed that the ashes were carried against the course of the winds: i. e., against the course of the current upon the surface of the earth. The force of the explosion, or the ascensional force of the

ascending column of heated air had, in fact, carried them into the upper stream, which was flowing in the contrary direction. Those also who have made the ascent of the Peak of Teneriffe, have constantly observed the wind blowing in the contrary direction on the summit, to that which prevailed at the foot of that mountain.

§ 136. Water is not less useful in this respect in the economy of nature. When a current of cold air passes over the surface of a large collection of water, it abstracts from it a quantity of heat; the specific gravity of the water is thereby increased, and the cooled portion sinks. Its descent forces up a portion of warmer water to the surface, which again communicates heat to the air passing over it; and this process may be continued for a considerable time, proportioned to the depth of the water. Currents are also formed in the ocean similar to those in the elastic atmosphere. The water which descends, when unobstructed, must spread at the bottom of the sea, and the denser water of cooler latitudes will flow towards the equator, and produce compensating currents upon the surface in opposite directions; and thus the ocean again tends to moderate the excessive heat of the torrid zone, as well as the intense cold of the polar climates. The Gulf Stream, as it is named, is an example of this action. This great current sets across the Atlantic from the coast of Africa, and, being deflected from the shores of the Gulf of Mexico, is carried northward to the banks of Newfoundland, in the neighbouring latitudes of which it may generally be detected by its effects upon the thermometer. An immense volume of warm water is thus transported to the cold regions of the north, where it cannot but have a considerable influence upon the climate. It is probable that the temperature of the westerly winds of our own island in winter is much influenced by this cause.

137. And here we must not fail to observe the important and beneficial purpose effected by that singular exception to the general law of expansion by heat, which fixes the point of greatest density in water nearly eight degrees above its point of congelation (§ 121). When the whole mass of accumulated fluid, such as that in a deep lake, has been cooled down to the temperature of 400 by the perpendicular circulation which we have just considered, the vertical movement ceases, and the surface water becoming lighter as the temperature farther falls, soon sets into a sheet of ice. The subjacent water is preserved from the farther influence of the cold by the cessation of the circulation, and its almost perfect non-conducting power. If, like mercury, oil, and other liquids, its density went on augmenting to its freezing-point, the cold air would continue to rob the masses of its heat till the whole sank to 32°, and it would suddenly set into a solid rock of ice, and every living animal within it would perish. In these climates a lake so frozen could never again be liquefied; for the process of thawing necessarily beginning above, the heated and light water would lie upon the surface, and effectually prevent the convection of heat to the lower strata.

This observation is, however, only fully applicable to accumulations of fresh water: for the waters of the "great deep," another protection has been provided. Saline matter in solution in water, it has been ascertained, lowers both the point of freezing and the point of maximum density. The ocean, on that account, and because of its great depth, which renders it an almost inexhaustible store of heat, resists freezing still more effectually than the deepest natural reservoirs of fresh water, and is scarcely known to freeze, except in latitudes where the most intense cold prevails. Even then, it is the watery particles alone which congeal, to the exclusion of