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 well-beaten road of routine practice to the more laborious paths which better knowledge would point out.

§ 167. 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 nonelastic, 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 to the equator upon the surface of the earth, and opposite currents from the equator to the poles, in the upper region 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.

§ 168. 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 heats of the torrid zone, as well as the intense colds 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 coasts of Africa, and being deflected from the shores of the Gulf of Mexico, is carried northwards 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.

§ 169. 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 (§ 153). When the whole mass of accumulated fluid, such as that in a deep lake, has been cooled down to the temperature of 40° by the perpendicular circulation which we have just considered, the vertical movement ceases, and the surface water becoming lighter as the temperature further falls, soon sets into a sheet of ice. The subjacent water is preserved from the further influence of the cold by the cessation of the circulation, and its almost perfect non-conducting power. If,

K

like mercury, oil, and other liquids, its density went on augmenting to its freezing point, the cold air would continne to rob the mass 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.

We are naturally struck with this wonderful proof of design in a superintending Providence: for although proofs of the most perfect contrivances abound in every stone which we tread beneath our feet, and in every breath of air which we draw, we here see that the Almighty in his working is not rigidly bound by the laws which He has framed for the order of the material universe, but that He can maintain that order and effect his beneficial purposes by exceptions to those laws, when it seems fit to his perfect wisdom.

This is the course in 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 the saline, which, increasing the density of the lower strata, arrest their circulation, and thus preserve them from the superficial cold.

HEAT OF COMPOSITION.

§ 170. Heat and Temperature we have hitherto used as nearly synonymous terms, and all the effects of the subtle force, to which we have been directing our attention, have been accompanied by its free development, and have been measured by our sensations, and by the thermometer and pyrometer. We have now to trace it, entering, as it were, into the composition of bodies, losing its character of temperature, and becoming latent to our instruments and our feelings.

Equal volumes of the same liquid, at different temperatures, afford, upon mixture, the mean temperature of the two. A pint

of water at 50°, being mixed with a pint at 100°, a thermometer immersed in the mixture will indicate a temperature of 75°. This result has already, indeed, been adduced in confirmation of the accuracy of the instrument (§ 143). If, however, a measure of quicksilver at 100° be agitated with an equal measure of water at 40°, the resulting temperature of the two will not be 70°, or the mean, but 10° lower, or 60°; so that the quicksilver will lose 40°, whereas the water will only gain 20°: yet the water must contain the whole heat which the quicksilver has lost. Hence, it appears that water has a greater capacity for heat than quicksilver: it requires a larger quantity of heat to raise it to a given temperature. The confirmation of this view may be obtained by the converse of the experiment; for if a measure of water at 100° be agitated with an equal measure of quicksilver at 40°, the resulting temperature will be 80°: the water will fall 20° in temperature, but in this fall will give out sufficient heat to raise the quicksilver 40°.

The same comparison may be made by weight, and will lead to the same conclusion. Thus, if a pound of quicksilver at 40° be agitated with a pound of water at 156°, the resulting temperature will be 152°.3: the water will lose 3°.7 of temperature, but enough heat will be evolved to raise the metal 11203. Now, the proportion of 3°.7: 112°.3, is the same as 0.033 1; hence, adopting water as the standard of comparison, we call the specific heat of quicksilver 0.033, designating by the term specific heat the heat peculiar to the species of matter compared with the standard.

Again: If a pound of water at 100°, and the same weight of oil at 50°, be mixed together, the resulting temperature will not be the mean, 75°, but 83°; the water, therefore, will lose 16°, while the oil will gain 33°, or reversing the temperatures, the mean will be 66°, so that the oil will give out 33°, and the water will rise only 16°. Hence, the heat which will raise the temperature of oil 20, will raise an equal weight of water only 10; and the specific heat of oil will therefore be 0.5.

§ 171. This different capacity of different bodies for heat must have a considerable influence upon their rates of heating or cooling: those which have the highest specific heat increasing or diminishing their temperatures most slowly under equal circumstances. Thus, if equal weights of water and quicksilver be placed at equal distances before a fire, the metal will be

more rapidly heated than the water; and again will cool down a certain number of degrees more rapidly when exposed in a cold place. Conversely, the specific heats of different bodies may be determined by carefully observing the time in which they cool down a certain number of degrees, and comparing them with water under similar circumstances. This method is susceptible of great accuracy, and may obviously be applied where mixture is impossible.

§ 172. A third method of ascertaining specific heats was devised by MM. Lavoisier and La Place, who contrived an apparatus for the purpose, to which they gave the name of Calorimeter. This instrument was liable, however, to some practical objections, which have limited its use. The principle, upon which it was constructed, will afford another illustration of the nature of the phenomenon (40). A certain weight of water, for instance, was surrounded with ice in a convenient vessel, and in passing from the temperature of 212° to 32°, the quantity melted was found to be a pound; an equal weight of oil in cooling down through the same range of temperature thawed only half a pound: and from this experiment we arrive at the same conclusion, as from mixture and cooling, that the specific heat of water being reckoned as 1°, that of the oil is only 0.5°.

(40) The calorimeter consists of two similar metallic vessels, the one contained within the other, and kept separate by small pieces of wood. The interval between the two is filled with ice, broken small, and packed close. By constantly renewing this ice as it melts by the heat of the atmosphere, the interior vessel will be kept constantly at the temperature of 32°. The water which is formed is removed by a stop-cock placed at the lower part of the interval between the two vessels. Within the interior vessel another still smaller is suspended, formed of iron net, designed to hold the body to be cooled. The interval between this third vessel and the second is also filled with ice: and the water which this latter produces in melting, flows out of the lateral stop-cock into a vessel which receives it, that it may be accurately weighed.