Humidity dependent on Temperature. 445 fluenced thus by the rarity of the air, and the rarity by the height, the vegetable productions of each distinct region or elevation are of a distinct character; and other peculiarities of place and climate are owing to the same cause. Because the atmospheric pressure determines the temperature of the air in different situations, as now explained, it has 641. It was explained at Art. 581 that water and other liquids, under a vacuum, rise in the form of air or vapour, with force and in quantity having a strict relation to temperature-heat being in fact the cause of their rising; and the table at Art. 687 exhibits the force, and therefore the density of watery vapour corresponding to certain temperatures. Now it is a remarkable circumstance, that vapour in the same quantity and of equal tension rises from any liquid, whether placed under the pressure of air, or where there is no air, with this difference, however, that through a space containing air it diffuses itself more slowly than if the air were not present. As regards the case of rising in air, it was for a long time supposed that the air dissolved the liquid as a liquid dissolves a salt; but it is now generally admitted that there is merely a mechanical mixture of the two gaseous fluids. If the vapour, while rising from a liquid, has not a tension or elastic force equal to the pressure of the atmosphere, the process is tranquil, and is called evaporation, and it goes on only as the vapour can diffuse itself among the particles of the air, and therefore slowly in air perfectly quiescent, but quicker as the air is moving more, or as the density of the air is less. But when the vapour, owing to greater heat, is strong enough to overcome the atmospheric pressure of fifteen pounds per inch, and the weight of any liquid over it, the phenomenon of boiling arises as already described. 642. For the reason now explained, the air of our atmosphere contains diffused through it a large quantity of invisible aëriform water; and if there were no intestine motions, and no changes of temperature in the atmosphere, the quantity of water would soon everywhere reach a maximum, or would be the greatest that the temperature of the place could support. Instead of this, however, from a variety of causes, the air is moving about constantly as winds, and the local temperatures are ever fluctuating, and when the temperature sinks, in a situation where a maximum of watery 446 Cooling by Evaporation. vapour is present, part of this is instantly reduced to the state of water again (see Art. 631); while to supply material for these phenomena, evaporation is going on wherever, over water, there is not a maximum of vapour in the air. These opposing operations of evaporation and condensation keep up that constant distribution of moisture which may be called a part of the life of nature. 643. When a given quantity of water assumes the aëriform state, it takes in and renders latent the same quantity of heat in all cases, viz., six times as much as would heat the water from the freezing to the boiling point, whether rising, for instance, from a boiling caldron, or from the surface of a cool lake. Hence we see why evaporation is so cooling a process to any liquid or moistened solid from which it is rising and as we have already shown that a rapid passing of dry air over such substance, or the placing it in a vacuum, quickens evaporation, we now see why both of these conditions accelerate the cooling. Wet linen placed in a strong wind, which does not contain a maximum of moisture, becomes dry almost immediately. A bottle of wine covered with a wet cloth and suspended in a current of air, as is practised in warm climates to prepare wine for the table, is quickly cooled; mats hung around the walls of houses in India, and frequently wetted through the day, preserve a pleasing freshness in the apartments. Sprinkling water or vinegar over a hot sick-room cools and refreshes it; and watering the streets of a city moderates in them the intensity of summer heat. In warm climates, water is cooled for drinking by being put into vessels so porous that the external surface is always moist and giving off vapour, the vessels being then suspended in a current of air, or during a calm being made to vibrate in the manner of a pendulum. Again, the rapidity of evaporation from water under the exhausted receiver of an air-pump, and particularly when some other substance which powerfully absorbs watery vapour is included in the receiver, is so great, and carries off the heat so quickly, that the mass of water freezes before much of it has been carried away. This process is used for making ice in India. 644. It is partly because air saturated with moisture, that is to say, having as much water diffused in it as can be supported in the invisible or aëriform state at the existing temperature,-lets fall a part on any reduction of the temperature, that the air of any portion of the atmosphere which has been heated by the sun during the day, and has received much moisture, lets part of that fall again during the night, and exhibits the night fogs of certain scasons, which fogs float upon the surface of the earth, until again acted upon by the beams of the next morning's sun. Fog, when farther. condensed, by groups of the minute particles uniting, forms rain; and rain when cooled to 32° becomes snow or hail. 645. A phenomenon which may be classed with dew is the moisture or dampness seen on massive walls and furniture, when with change of weather a warm moist air of higher temperature than the walls suddenly comes upon them. There is a like result when a crowd assembles in a cold church, of which the walls or other solid objects then, from not having yet acquired the new temperature of the surrounding air, condense upon themselves a copious deposition of the breath moisture. For a similar reason a decanter of wine brought from a cold cellar or from an ice-pail, into a room with company, is soon covered with thick moisture or dew; as are the glasses also into which the cool wine is poured. It is still another phenomenon of the same kind, when we see the moisture of warm breath condensed on any cold polished surface, as on the face of a mirror, or on the glasses of a carriage shut up, or on the windows of a room in winter. When the surface of a window pane is very cold, the moisture freezes on it with the appearance of beautiful arborescence. This is owing to the crystallization of the deposited water as it passes from the fluid to the solid state. 646. Many instruments have been contrived, with the name of hygrometers,* for indicating the quantity of water in the atmosphere. A prepared human hair forms part of one of those formerly used; the lengthening or shortening of the hair, according to the quantity of moisture absorbed into it, is caused to move an index like that of a wheel-barometer, to mark the degrees. This, however, and other common hygrometers, are only philosophical toys; but Professor Daniell, in his 'Meteorological Essays,' described a correct and simple instrument for the purpose, depending on the formation of dew as explained above. The explanation in a few words is, that when the temperature of a body in the atmosphere falls below that at which the quantity of watery vapour in the air around it can be maintained in the aëriform or invisible state, dew forms on the body. 647. Daniell's hygrometer consists of a bent tube with two bulbs containing ether vapour, and in one bulb liquid ether in which a small thermometer is immersed. By causing ether to evaporate from the exterior of one bulb, the liquid ether is cooled in the other *ypos, moist, and μeтpov, a measure. 448 Wet-bulb Thermometer. and at a certain point, called the dew point, this bulb receives a thin film of aqueous vapour from the cooled air around it. The temperature of the liquid ether, is indicated by the thermometer immersed in it. This bulb is darkened in order that the deposited vapour may be rendered rore visible. The temperature of the air is at the same time observed, and the greater the difference, or the lower the degree before the deposit of moisture takes place, the dryer the atmosphere. 648. The Wet-bulb thermometer is also used as an hygrometer for determining the dew point or the dryness or dampness of the air. It consists of two thermometers joined together and equally graduated. The bulb of one is covered with some material which can imbibe and hold water, while the bulb of the other is freely exposed to the air. The wetted bulb is of course exposed to cooling by evaporation, owing to which the mercury in this bulb sinks to a lower degree than in the other. Before wetting the bulb the thermometers should stand at the same degree. By exposure for a few hours such an evaporation and cooling will have taken place that the mercury in this thermometer will have fallen below the other, and the difference marked, will depend on the degree of moisture in the air. If there is no difference it will show that the air is saturated for that temperature, and there has, consequently, been no evaporation. If there is a difference of 10° or 12° it will show that the air is comparatively dry. This instrument is a useful adjunct to the barometer in judging of the probability of the fall of rain. 649. A great fall of the barometer marks a diminished pressure in the atmosphere around, with a consequent dilatation of the air and fall of temperature, as explained in a former part of this work; and if the air at such a time hold a maximum of moisture, a part of this must become visible as fog or rain. Thus a fall of the barometer, a fall of temperature, and a fall of rain, often occur as associated phenomena. Illustrating this by experiment, we find, that on the extraction of common air from the receiver of an air-pump, a thin cloud or mist generally appears in it with the first strokes of the piston :-the reason being that the still remaining air, because cooled by the rarefaction, absorbs heat from the invisible vapour in combination with it, and renders the water visible. The mist may then be removed by the continued action of the machine, or may be re-dissolved by the usual quantity of air being re-admitted. We understand from this why rain happens much more frequently Dissolving Mists. 449 anong mountains than on extended plains. When air saturated with moisture approaches a mountain ridge to rise over it, for every foot that it rises, it escapes from a degree of the pressure which it bore while lower down, and in then dilating, it becomes colder, and lets fall part of its moisture. It is the rain copiously produced in mountainous regions from this and other causes which constitutes the supply of the many rivers there, and which, with periodical changes of wind, occasions the extraordinary annual overflowing of such rivers as the Nile and the Ganges. 650. Those who have visited the Cape of Good Hope, will recollect a striking phenomenon illustrative of our present subject, observed there when the wind blows from the south-east. Cape Town and the bay in which ships anchor are on the west side of the Cape. Beyond the city, as viewed from the bay, there is a mountain of great elevation, called from its extended flat summit, the Table Mountain. In general its rugged steeps are seen rising in a clear sky; but when the south-east wind blows, the whole summit becomes enveloped in a cloud of singular density and whiteness. The inhabitants call the phenomenon the spreading of the tablecloth. The cloud does not appear to be at rest on the hill, but to be rolling rapidly onward; yet to the surprise of the beholder, it never descends, for the snowy wreaths seen falling over the precipice towards the town below, vanish completely before they reach it, while others are formed on the other side to replace them. The reason of the phenomenon is this. The air constituting the wind from the south-east having passed over a vast extent of the southern ocean, comes charged with as much invisible moisture as its temperature can sustain. In rising up the side of the mountain it is rising in the atmosphere, and is thereby gradually escaping from a part of the pressure lately borne; and on attaining the summit it has dilated so much, and has consequently become so much colder, that it lets go part of its moisture. This then appears as the cloud just described; but it no sooner falls over the brow of the mountain, and again descends in the atmosphere to where it is pressed, and condensed, and heated as before, than it is re-dissolved and disappears :-the magnificent apparition dwelling only on the mountain top. The foregoing reasoning explains why, along the sides of mountain ridges, clouds are generally seen floating at a certain height only, and therefore in strata nearly horizontal. The water is separated from the air at a certain temperature, which corresponds with the |