disturbance. Owing to various causes, but chiefly to the equal distribution of water and land, the course of the winds is by no means so regular as assumed above; but this grand system of compensating currents certainly results, although masked by circumstances in particular regions; most of these, however, give rise to minor systems of compensating currents, to which, as concerns the barometer, the same remarks will apply. Any one, indeed, may convince himself that such systems prevail in the atmosphere, by watching the progress of the clouds; and in this way he will often be able to detect more than two such currents existing together. Now, while he is observing their courses, let him for a moment suppose that the barometer is perfectly stationary, and then let him imagine one of the streams of air before him suddenly checked; the others will continue to flow on for a time in their primary direction from their inertia, and in obedience to their original impulse, and the barometer, he will conceive, must fall, because more air is carried off above it than is compensated by the current which originally maintained the balance; and, as a deficiency may be thus produced in one part of the atmosphere, an equivalent accumulation must, at the same time, result in some other part. (33) (33) Let a b c d, in the annexed diagram, represent two currents flowing in the opposite directions indicated by the darts: a barometer placed in the lower at g, will support the pressure A cause, quite competent to produce these partial effects, exists in the atmosphere of steam mingled with the atmosphere of permanent gases, and which is constantly rising from the surface of the earth, varying in force with the temperature of the waters from which it emanates. This vapour rises unchanged and transparent, till, in the gradual decreasing temperature of the air, it arrives in the upper regions at a degree of cold by which it is condensed, and, becoming visible, assumes the form of clouds. In the act of condensation, however, an immense quantity of heat is set free, which was previously combined with the steam in the latent form; and this, acting upon the surrounding air, whose capacity for heat is small, expands it, and gives an additional but unequal impulse to the current in which the process takes place. The clouds, again, are themselves subject to a new evaporation, and the vapour is carried to still higher regions, where another precipitation takes place; till, at length, large masses of the atmosphere have the natural progression of their temperature changed and their currents altered, or, perhaps, reversed. The increased temperature of the air is accompanied by a great increase in the force and quantity of the steam, the final precipitation of which takes place in the form of rain, and the atmosphere returns to its mean state through the influence of winds, which restore the original balance. The order of the phenomena corresponds with the facts that the barometer is most steady when the weather is clear, and fluctuates most with clouds and rain; and also explains the reason why, in the of both, and remain without oscillating so long as the two currents remain unchanged. But if a partial expansion, affecting the upper current alone, should urge it forward on its course on one side and check it on the other, an accumulation of the nature of a wave would take place at b, and a corresponding deficiency at a, as indicated by the curve a b, and would be accompanied by a rise of the thermometer at g, and a correspond. ing fall at f. greater disturbances of the aerial ocean, local deficiencies of the elastic fluid are restored by winds whose force is generally proportioned to the vacuum which they supply." 171. Aerial fluids are commonly distinguished into two classes, viz., vapours and gases; but the experiments of Dr. Faraday have nearly annihilated the distinction, and proved that the difference is one of degree only, and not of constitution. In vapours, strictly so called, such as the steam of water, the latent heat is retained in combination with very little force; for it abandons the water when the vapour is exposed to a lower temperature; but in gases the heat of composition is retained very forcibly, and no diminution of temperature that has ever been produced can separate it from some of them. Dr. Faraday has, however, succeeded in reducing to a liquid state many of those substances which till very recently were considered as permanently elastic fluids. His method is to generate them under strong pressure, by confining the substances from which the gases are evolved in strong glass tubes, in which their elasticity is allowed to increase till it forces their particles within the verge of cohesive attraction, when they assume the form of liquids. This operation he farther assists by the application of cold mixtures to the exterior of one extremity of the tube. When thus produced, they continue liquid while the pressure is kept up, but, on removing the pressure, instantly pass into the gaseous state. The same condensation may also be produced by mechanical force, and considerable quantities of carbonic acid have been thus reduced to the liquid state by Mr. Brunel, with the view of applying its expansive power as a mechanical agent. The cold produced by the sudden evaporation of these liquids is so great as actually to preserve some of the substances in their liquid state under the mere pressure of the air, and carbonic acid has DANIELL'S Meteorological Essays, p. 559. been even frozen by the cold produced by its own spontaneous evaporation. Mr. Brunel was the first to observe this fact, and the loss of elastic force thus occasioned was, it is believed, the cause of giving up the attempt to construct a gas-engine. § 172. More recently, some very interesting results have been obtained by M. Thilorier, who has ascertained that the expansion of liquid carbonic acid for every degree of heat exceeds that of the acid in the gaseous state even by four times, and the pressure of the vapour from 32° of Fahrenheit to 86°, rises from 36 to 73 atmospheres. A jet of liquid carbonic acid sinks the thermometer to 1300 Fahrenheit below 00. It has little capacity for heat, and from its want of conducting power, its cooling effects alone do not correspond with the depression; but when mixed with vapour of ether, M. Thilorier succeeded in forming a cold blowpipe, which froze 800 grains of mercury in a few seconds. 173. A gas, then, may be regarded as the vapour of a liquid whose elasticity, at ordinary atmospheric temperatures, is such that it is not only equivalent to the pressure of the ordinary atmosphere, but to that of many atmospheres, and which would therefore boil under the pressure of many atmospheres, and cannot consequently be preserved except under artificial pressure. The pressure required to condense the several gases is recorded in the following table: TABLE XXI.-Liquefaction of Gases. Nitrous oxyde 162 TRANSITION POINT OF LIQUIDS, ETC. Many gases have hitherto resisted the utmost degree of compression and cold to which it has been practicable to expose them, but from the generality of the law its universality is a legitimate deduction; and geologists are justified in speculating upon the existence of the atmospheric gases themselves in a state of liquidity, in the fathomless depths of the earth, under the enormous pressures to which they must there be necessarily exposed. Some gases, however, though incoercible, or requiring very high degrees of force to coerce them, by mechanical means and cold, instantly yield to the force of heterogeneous adhesion, and, as they become dissolved in water, give out their latent heat. Thus, if a current of muriatic acid gas be passed into water, it will be rapidly absorbed, and the temperature of the solution will rise considerably. 174. We have seen that water and other liquids gradually expand in volume with increase of temperature, and that, on the other hand, steam and other vapours rapidly increase in density, as the pressure and temperature to which they are exposed augments; whence it is clear that, at some particular assignable point, the liquid and elastic states must coincide, and there must be a limit beyond which a liquid, notwithstanding the pressure, ought to be wholly volatilized, or become gaseous, provided sufficient space be allowed for the expansion. M. Cagniard de la Tour, reasoning upon the general facts, was led to make some interesting experiments upon the subject. A strong glass tube, containing alcohol equal in bulk to about two fifths of its capacity, being hermetically sealed and carefully heated, the alcohol was observed to expand, till, after having attained nearly double its original volume, it suddenly disappeared, being converted into vapour so transparent that the tube appeared to be completely empty. Allowing it to cool, the |