the ocean, and decreases on advancing inland ; greater in summer than in winter ; and greater at midday than in the morning. It diminishes with the height generally ; but in particular cases, different strata are superimposed on each other, differing widely as regards dryness and humidity, and the transitions from the one to the other are often sharp and sudden. The relative humidity of the air may be regarded as the degree of approach to saturation. It is greatest near the surface of the earth during night, when the temperature, being at or near the daily minimum, approaches the dew- point ; it is also great in the morning, when the sun s rays have evaporated the dew, and the vapour is as yet only diffused a little way upwards ; and it is least during the greatest heat of the day. Between the humidity, both absolute and relative, of the air and the temperature there is a vital and all-important connection. Observation shows that when the quantity of vapour in the air is great, and also when the relative humidity is high, temperature falls little during the night, even though the sky be perfectly clear ; but when the quantity of vapour is small, or the relative humidity is low, temperature rapidly falls. On the other hand, during the day the temperature rises slowly, when the quantity of vapour is great, or relative humidity high, even though the sky be clear, but when the quantity of vapour is small, and humidity low, temperature rapidly rises. These facts are explained by the circumstance that perfectly dry air is diathermanous, that is, it allows radiant heat to pass through it without being sensibly warmed thereby. Add vapour to this air, and its diathermancy is diminished. The dia thermancy is also reduced if the temperature approach nearer to the dew-point ; in other words, if the relative humidity be increased. Hence, with an increase of vapour or with increased humidity, the effects of both solar and terrestrial radiation are much less felt on the surface of the earth the vapour screen performing, in truth, one of the most important conservative functions of the atmosphere. Since ascending currents fall in temperature as they ascend, through diminished pressure and consequent dilata tion, they increase their relative humidity; and since descending currents increase in temperature, and conse quently reduce their relative humidity, it follows that, over a region from which ascending currents rise, solar and terrestrial radiation is very considerably obstructed, but over a region upon which currents descend, radiation is much less obstructed. Most of our exceptionally hot summer and cold winter weather is to be explained in this way, on which occasions there is generally observed a high barometric pressure overspreading a comparatively limited region, on which a slow downward movement of the air proceeds. Of the solar heat which reaches the surface of the globe, that part which falls on the land may be regarded as wholly absorbed by the thin superficial layer exposed to the heating rays ; and since there is no mobility in the particles of the land, the heat can be communicated downwards only by conduction. On the other hand, the solar heat which falls on water is not, as in the case of land, arrested at the surface, but penetrates to a con siderable depth, the heating effect being in the case of clear water appreciably felt at a depth of from 500 to 600 feet. Since the heat daily received by the ocean from the sun is diffused downwards through a very considerable depth, the surface of the ocean on which the atmosphere rests is much less heated during the day than is the surface of the land. Similarly it is also less cooled during the night by terrestrial radiation. This points to a chief acting force on which the great movements of the atmosphere depend, viz., simultaneous local irregularities in the distribution of temperature in the atmosphere. The local expansion of the atmosphere by heat during the day is greatest over land, where the air is clear, dry, and comparatively calm, and least over the ocean, where the sky is clouded, and the air loaded with moisture. On the other hand, the local contraction by cold during night is greatest over land, where the air is clear, dry, and calm, or nearly so, and least over the ocean, where the air is clouded, and loaded with moisture. As familiar illustrations of atmospheric movements result ing from local expansions by heat and contractions by cold, we may refer to the land and sea breezes, and what de pend upon exactly the same principle, the dry and rainy monsoons in different parts of the globe. But the illustra tion of the principle on the broadest scale is the system of atmospheric circulation known as the equatorial and polar currents of the atmosphere, which originate in the unequal heating by the sun of the equatorial, temperate, and polar regions. The other principal motive force in atmospheric circula tion depends on the aqueous vapour. The many ways in which this element acts as a motive force will be seen when it is considered that a large quantity of sensible heat disappears in the process of evaporation, and reappears in the process of condensation of the vapour into rain or cloud ; that saturated air is specifically lighter than dry air ; and that the absolute and relative amount of the vapour powerfully influences both solar and terrestrial radiation. The question to be carefully considered here is, how in these ways the vapour produces local irregularities in the distribution of atmospheric pressure, thus giving rise to aerial movements which set in to restore the equili brium that has thus been disturbed. It is from these local irregularities using the word local in a very wide sense in the distribution of atmos pheric pressure, whether the irregularities originate in the temperature or aqueous vapour, that all winds, from the lightest breeze to the most destructive hurricane, take their rise ; for, as already stated, wind is merely the flowing away of the air from where there is a surplus of it to where there is a deficiency. In examining weather charts embracing a considerable portion of the earth s surface, such, for instance, as those published in the Journal of the Scottish Meteorological Society, vol. ii. p. 198, which include a large part of the northern hemisphere, there are seen two different systems of pressure changing their forms and positions on the globe from day to day one set being systems of low pressure marked off by concentric isobarics enclosing pressures successively lower as the central space is approached, and the other set being systems of high pressure marked off by roughly concentric isobarics bounding pressures succes sively higher towards their centres. These two systems are essentially distinct from each other, and without some knowledge of them the circulation of the atmosphere can not be understood.
1. Areas of Loiv Pressure, or Cyclones. The annexed woodcut, fig. 1, is a good representation of a cyclone which passed over north-western Europe on the morning of 2d November 18G3. The pressure in the central space is 2 8 - 9 inches, from which it rises successively, as shown by the isobarics, to 29 1, 29 3, 29 5, 297, and 29 9 inches. The arrows show the direction and force of the wind, the force rising with the number of feathers on the arrows. The two chief points to be noted are the following : (1.) The direction of the arrows shows a vorticose motion of the air inwards upon the space of lowest pressure, the motion being contrary to that of the hands of a watch. It will be observed that the winds blow in conformity with what is known as Buys-Ballot s "Law of the Winds," already III. - 5
