with the key to the climates of all parts of the globe in the different seasons of the year; for the direction of prevailing winds thus becomes known, and these are warm or cold, and dry or wet, as determined by the regions from which they blow. This subject will be more fully stated in the chapter on WINDS.



301. The Two Atmospheres of Air and Vapour.---The gaseous envelope surrounding the earth may be considered as composed of two distinct atmospheres—an atmosphere of dry air, and an atmosphere of vapour. The dry air (oxygen and nitrogen) is always a gas, and its quantity is constant from year to year; but the vapour of water does not always remain in the gaseous state, and the quantity present in the atmosphere is, by the processes of evaporation and condensation, varying every instant.

302. According to the strength of the force of cohesion drawing the particles of matter together, as compared with the repulsive energy of heat driving them asunder, so is the body solid, liquid, or gaseous.

In solids and liquids the cohesive force is in excess, whilst in gases it is absent. It is the absence of cohesion which is the distinctive characteristic of gases and vapours. If a little water be poured into a vessel it will only rise to a certain level, and leave the rest of the vessel unoccupied. On the contrary, gases and vapours completely fill the vessel in which they are, showing that, instead of cohesion, there is a mutual repulsion among their particles. Since then, the particles constantly tend to recede from each other, it follows that they will exert an outward pressure on the sides of the vessel, and that the amount of this pressure will be proportioned to the repulsive force, or to the elasticity of the gas.


303. From the circumstance that a gas completely fills the vessel which contains it, it will be seen that its volume is determined by the pressure to which it is subjected. The law of the compression of gases was discovered by Boyle and Marriotte. It is generally known as Marriotte's law, and is as follows: At the same temperature, the volume occupied by the same gaseous mass is in inverse ratio to the pressure which it supports. Consequently the density and tension of a gas are proportioned to the pressure—that is, air under a pressure equal to that of two atmospheres will only occupy half the bulk it occupied when under the pressure of one atmosphere; under the pressure of three atmospheres, one-third of that bulk, &c. At the pressure of 770 atmospheres, air would become as dense as water. But by doubling the pressure we double the elasticity.

304. This law is true for air at all pressures and temperatures which have hitherto been tried ; but it does not hold good for the vapour of water, either as respects pressure or temperature. Under low pressures the vapour of water follows the law; but under high pressures the space occupied is less than would have been if the law had been observed, because part of the vapour then passes into the liquid state, thus occupying less space.

305. Since it is the repulsive force of heat which drives the particles of bodies asunder, it is evident that an increase of temperature will add to the elasticity of gases, and a decrease of temperature diminish it. Conversely, if part of the pressure exerted on a given quantity of gas be removed, the gas will increase in bulk and fall in temperature; but if the pressure be increased, the volume of gas will be less, and its temperature higher. Since the pressure is diminished as air ascends and increased as it descends in the atmosphere, it follows that currents of air become colder as they ascend and warmer as they descend. This law applies to air at all temperatures, but it does not apply to the vapour of water.

306. There is another property of gases and vapours by which they are distinguished from liquids. If mercury, water, and oil be poured into a vessel, they will settle accord

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ing to their densities,-mercury on the bottom, oil on the top, and water between the two; and they will remain in these relative positions without exhibiting any tendency to mix together. But if gases of different densities be put into the same vessel, they will not arrange themselves according to their densities, but will ultimately be diffused through each other in the most intimate manner. Each


tends to diffuse itself as in a vacuum,—the effect of the presence of other gases being only to diminish the rate of expansion and thus retard their mutual diffusion. This equal intermixture occurs with all gases and vapours which do not act chemically on each other, and when once such a mixture is effected it remains permanent and uniform. Of this, common air, which is a mixture of oxygen and nitrogen, is an example. As regards the atmosphere, the law is, that vapour diffuses itself through the dry air, the presence of the air having only the effect of retarding the rate of its diffusion.

307. If the vapour of water remained permanently in the atmosphere—that is, was not liable to be withdrawn from it by being condensed into rain—the mixture would be as complete and uniform as that of the oxygen with the nitrogen. But the equilibrium of the vapour atmosphere is being constantly disturbed by every instance of condensation, by the ceaseless process of evaporation, and by every change of temperature. From these considerations, and from the circumstance that dry air greatly obstructs the free diffusion of the vapour, it follows that the law of the independent pressure of the vapour and of the dry air of the atmosphere does not absolutely hold good ; but that from the constant effort of the vapour to attain to a state of equilibrium there is a continual tendency to approach this state. And since the independent and equal diffusion of the dry air and the vapour is, owing to these disturbing causes, never reached, it follows that the observations of the hygrometer only indicate local humidity. Hence they should never be regarded as anything more than approximations to a correct indication of the quantity of vapour in the atmosphere over the place of observation. It should, however, be added, that though in exceptional cases the amount of vapour indicated may be wide of the mark, yet, in long averages, a very close approximation will be obtained, except in confined localities which are exceptionally damp or exceptionally dry.

308. Vapour is continually passing into the air from the surface of water and moist surfaces at all temperatures by the silent process of evaporation. Evaporation also takes place from the surface of snow and ice. In evaporation the vapour is supplied only from the surface of the water; hence the extent of surface in contact with the air greatly influences the amount and rate of evaporation. By the increase of temperature the elastic force of the vapour in the atmosphere is increased, and with it the rate of evaporation. The atmosphere can contain only a certain amount of vapour, according to the temperature; hence, when it already has in suspension its full complement, or when it is saturated with moisture, evaporation ceases. Conversely, evaporation would be greatest when the air is perfectly dry or free from vapour. Since currents of air remove the saturated air and substitute dried air to the evaporating surface, evaporation is much more rapid in windy than in calm weather. Though the quantity of vapour required to saturate a given space is the same, whether that

space be occupied with air or be a vacuum, yet the time occupied in completing the saturation increases with the pressure on the surface of the fluid. When water evaporates into a vacuum, the maximum density of the vapour is acquired at once ; but when it evaporates into air it is not acquired till some time has elapsed. And since every addition to the vapour increases the pressure, the rate of the evaporation is under these circumstances continually diminishing,

309. Evapometer. — The instrument for measuring the quantity of water which the atmosphere can take up from the surface of water in the form of vapour in a given time is called an Evapometer. In its simplest form it consists of an evaporating dish, about 5 inches in diameter, with an overflow pipe a little below the surface fitting into a bottle,

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