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the channel of a river oppose its course. This is undoubtedly one of the most important and conservative functions of the invisible moisture of the atmosphere. For if the moisture was drained out of it, and its diathermancy thereby rendered complete, the sun's rays would burn up everything by their intolerable fierceness, and during the night the escape of heat by radiation to the cold stellar spaces would be so swift and the cold so intense, that the whole living creation would be blighted by its withering touch. The earth would in truth
“ Feel by turns the bitter change Of fierce extremes, extremes by change more fierce, From beds of raging fire to starve in ice."
341. It is the imperfect diathermancy of a moist though clear atmosphere, together with its high dew-point, which prevents the temperature of the air from falling to so low a point during the night as happens when the atmosphere is clear and dry.
342. Lieutenant-Colonel Strachey has examined the Madras Meteorological Observations of several years, and compared the elastic force or tension of the vapour with the number of degrees the temperature was reduced by radiation from 6.40 P.M. to 5.40 A.M. In all the cases examined, the sky was either quite, or all but quite, free from clouds. During 1844, when the tension was 1.00 inch, the temperature fell 20.7; when the tension was between 1.00 and .90, the temperature fell 4°.5 ; between .90 and .80, 5o.4; between .80 and .70, 6°.9; and between .70 and .60, 8°.3. A tract of remarkably clear weather occurred from the 4th to the 25th March 1850, during which there were great differences in the tension of vapour. The following results exhibit the dependence of the temperature on the vapour in a clear light :
1.888.849 1.805.749.708.659.605.554 | .455
Tension of vapour,
6.40 P. M. to 5.40
69.0 7.1 8.3 89.5 10°.3 12°.6 12o.1 13°.1 169.5
Hence, then, when the quantity of vapour in the air is great, the escape of heat by radiation is obstructed, and the temperature falls little during the night; but when the quantity of vapour is small, radiation is less impeded, and the temperature rapidly falls. Also when the quantity of vapour is great, since less of the sun's heat reaches the earth's surface, the temperature rises slowly, even although the sky be perfectly clear and the sun shining brightly; but when the quantity of vapour is small the sun's rays have a freer access to the earth, and the temperature rapidly rises. From the above examples, when the tension of vapour was about .840 inch, the daily range of temperature was 190,5; but when the tension was only about .400 inch, the daily range was 43°.5. Since the temperature is hottest during the day, when the air is driest, and evaporation consequently greatest, it is evident that the extent to which the temperature falls during dry nights owing to the evaporation is comparatively small, and that the cause of night cold is radiation. Further, from observations made at Melbourne, M. Neumayer concludes that the absolute quantity of aqueous vapour in the air is in itself alone not sufficient as a criterion for the degree of radiation; but that the absolute quantity of aqueous vapour, together with a certain temperature-in other words, the relative humidity of the air—influences terrestrial radiation in such a manner that the greater the degree of relative humidity the less is the effect of radiation. In connection with this view, the effect of the dew-point in arresting the descent of the temperature must not be forgotten (par. 330).
343. In mountainous countries, where, on account of their height, much less aqueous vapour is interposed between them and the cold regions of space, radiation, both solar and terrestrial, is least obstructed. It is this which explains the
scorching heat that surprises the alpine tourist while travelling over fields of snow under a blazing noonday sun. And it is the same cause, the small amount of vapour in the air, that explains the intense heat experienced in the direct rays of the sun in the polar regions, where Captain Scoresby observed it to melt the pitch on the side of his ship exposed to the sun, while ice was rapidly generated at the other side.
344. When observations with black-bulb thermometers, showing the force of solar radiation in different parts of the world, are stated, they appear at first sight absurd, if not contradictory. Thus at Port Louis, Mauritius, in lat. 29° 9'56" south, the highest reading of the black-bulb in vacuo was 125° in 1864, and 130° in 1865. Now in Scotland, a thermometer placed in these circumstances, particularly in the eastern districts, would register higher temperatures than those, during at least four months of the year; and in the case recorded by Captain Scoresby, the heat of the sun's rays in melting the pitch must have been about 130°, that is, as high as occurred in the Mauritius during two whole years. An interesting collection of facts of solar radiation in different latitudes and at different elevations is given in Daniell's *Essays' (2d ed., p. 208 et seq.) From the facts adduced, the conclusion is drawn that the power of solar radiation in the atmosphere increases from the equator to the poles, and from below upwards,—a result which these remarks on the vapour of the atmosphere explain. It follows that hygrometric observations ought to accompany all observations on solar radiation, in order to render them intelligible.
345. The above considerations explain in part the nervous derangement and general unhealthiness produced by the east wind in spring; for as the air is then very dry, the part of the person exposed to the sun's rays is greatly heated as compared with the part in shadow, and this strain on the physical frame few constitutions except the most robust can bear without positive discomfort. On the other hand, exposure to the sun's rays in the tropics is, on account of the thick screen of vapour above, not attended with the intense heat which might have been expected. Nothing is more common than for natives of the West Indies and other warm moist climates to complain of the, to them, intolerable heat of the sun in our British climate in spring and summer. *
* In the above remarks on the diathermancy of the atmosphere in reference to the influence of its moisture on solar and terrestrial radiation, I have used the word “ vapour” to include all states of the moisture which are, in a popular sense, invisible—in other words, as inclusive of every form in which water may be suspended in the air, except those of mist, fog, cloud, and rain-drops. In 1862, Professor Tyndall made experiments on the vapour of water, from which it was concluded, that while the dry air of the atmosphere was diathermanous, the pure vapour was not so. As this result seemed to give a very satisfactory explanation of the observed effects of solar and terrestrial radiation, it was very generally adopted by meteorologists. In the first edition of this work it was assumed throughout.
But in June 1867, Professor Magnus of Berlin published a more exten. sive series of experiments, by which it was shown that the results obtained differ not according as the air which is forced through the tubes is moist or dry, but according to the condition of the sides of the tube through which the currents of dry air and moist air are forced. Hence, while Professor Tyndall's experiments, so far as they go, are good, they do not warrant the conclusion which was drawn from them, since the results are shown to be due to the condition of the sides of the tubes used, and not to any diathermical difference between moist air and dry air. Others have confirmed Professor Magnus's experiments.
The observations of solar and terrestrial radiation in relation to the moisture of the atmosphere are probably to be accounted for by the presence of moisture in states other than that of pure vapour, which are imperfectly diathermanous,--such as moisture in a vesicular state, and at the same time invisible, and in states intermediate between the vesicular and that of pure vapour. The conditions under which atmospheric moisture may be formed and maintained in these states, so as to account for the varied and often surprising phenomena of radiation, are very imperfectly known, and some of them perhaps not even yet suspected.
346. Mists and fogs are visible vapours floating in the air near the surface of the earth. They are produced in various ways,— by the mixing of cold air with air that is warm and moist, or generally by whatever tends to lower the temperature of the air below the dew-point.
347. During a calm clear night, when the air over a level country has been cooled by radiation, and dew begun to be deposited, the portion of the air in contact with the ground is lowered to the dew-point, and thus becomes colder than the air above it. Since in these circumstances there is nothing to disturb the equilibrium and give rise to currents of air, and there being no cause in operation which can reduce the temperature much below the point of saturation, the air within a few feet of the surface remains free from mist or fog. But if the ground slopes, the cold air, being heavier, must necessarily flow down and fill the lower grounds; and since it is colder than the saturated air which it meets with in its course, it will reduce its temperature considerably below the point of saturation, and thus produce mist, or radiation fog, as it is sometimes termed. When a lake, river, or marsh fills up the valley, the air, being thereby more saturated, often gives rise to denser fogs; and, on the other hand, when the low grounds are sandy or dry, mist is less frequently produced.
348. When an oceanic current meets a shoal in its course, the cold water of the lower depths is brought to the surface,