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Show 198 DEW OF THE DREATH . Now, as the temperature of health is very con· s1derably above the average of that of the air in temperate countries, and indeed above the average of almost any country, it follows that the expired breath, which, as has been said, is loaded with the superfluous moisture of the body, must have a tendency to produce dew upon the colder air against which it is breathed. In dry and warm states of the atmosphere that dew is not observable, though even then the breath will stain a mirror, if held near; but when the atmosphere is cold or moist, the breath of man and of all the warm blooded animals becomes visible ; and in keen frosts, a man's own breath will cover his hair with hoar frost, and even form ice upon his face. But the same heated moisture of the breath which becomes apparent ·in those cases exists in every case, whether drcumstances render it visible or not, and thus it becomes a protection against draughts or currents of air. These blow the warm and moist breath against the face ; and that instead of parching it, as the common air would do, dews gently upon it, and protects it from injury. The effect is much greater than one would suppose ; for if one stand with the head bare when the wind blows keenly, one can bear it longest by facing it. It is not a little curious that the danger of catching cold should, like all other dangers, be greatly diminished by being faced. The mists, or dews, which are formed in the higher regions of the atmosphere, are of a very different character from those that are formed on or near the surface of the earth. The earth-mist, as we may call the lower one, before it can rise upward in the air, and disturb the state of things there, has the resistance both of gravitation and cohesion to overcome ; whereas, the descent of a mist or cloud, formed in the upper part of the sky, has both of those resistances as powers acting in favour of its descent. That consideration helps to explain VAPOUR IN TH'E AIR • 199 so many of the phenomena of the weather--phenomena which are very important. for the triple purposes of pleasure, utility, and health, that every one who is to observe nature, so as either to be pleased or profited by it, should und~rstand ~hem thoroughly. WatPr can be suspended m the air without falling only when it is in very minute drops ; and as the density of the air decreases as its height above the mean surface of the earth becomes greater, the individual portions of water that it can hold without falling, at any given elevation, must be in proportion to its density at that elevation; and thus, if we suppose water to rise by evaporation from any point in perfectly still air, the vapour which arises from that point will form an inverted pyramid in the atmosphere ; and however the upper part of that pyramid may be expanded, it cannot contain more water in the highest foot of its height, than it does in the foot next the point from which the vapour rises. If, instead of a point, the vapour rises from a surfacesay that of a circular lake one mile in diameter,the vapour will, as it ascends, if there is no wind or current to carry it to one side rather than to another, spread out towards all sides ; so that when it comes to air of only half the density of that on the surface of the lake, it will extend nearly a mile all round· and as it ascends hig·her, it will spread wider and wider, till, at the upper part o! the atmosphere, where we must suppose the denstty of the air equal to nothing, it will be diffused round the whole globe. If we could see it, it would be a phenomenon of the greatest beauty; for the slope of it would not be a straight line, but a logarithmic spiral, similar to that ch?sen by those consummate artists the gothic bmlders, by means of which the arches that spring from the columns and corbels melt so beautifully and so naturally into the roof, that all notion of one part supporting another is lost to the perception, and the feeling that we have is that the roof is self .. |
