So that in those movements of the Earth which determine the varying quantitiesof light and heat which any portion of it receives from the Sun, there goeson a quadruple rhythm: that causing day and night; that causing summer andwinter; that causing the changing position of the axis at perihelion andaphelion, taking 21,000 years to complete; and that causing the variationof the orbit's eccentricity, gone through in millions of years. §84. Those terrestrial processes directly depending on the solarheat, of course exhibit a rhythm that corresponds to the periodically changingamount of heat which each part of the Earth receives. The simplest, thoughthe least obtrusive, instance is supplied by the magnetic variations. Inthese there is a diurnal increase and decrease, an annual increase and decrease,and a decennial increase and decrease: the latter answering to a period duringwhich the solar spots become alternately abundant and scarce. And besidesknown variations there are probably others corresponding to the astronomicalcycles just described. More obvious examples are furnished by the movementsof the ocean and the atmosphere. Marine currents from the equator to thepoles above, and from the poles to the equator beneath, show us an unceasingbackward and forward motion throughout this vast mass of water -- a motionvarying in amount according to the seasons, and compounded with smaller likemotions of local origin. The similarly-caused general currents in the air,have similar annual variations similarly modified. Irregular as they arein detail, we still see in the monsoons and other tropical atmospheric disturbances,or even in our autumn equinoctial gales and spring east winds, a periodicitysufficiently decided. Again, we have an alternation of times during whichevaporation predominates with times during which condensation predominates;shown in the tropics by strongly marked rainy seasons and seasons of drought,and in the temperate zones by changes of which the periodicity is less definite.
The diffusion and precipitation of water furnish us with examples of rhythmof a more rapid kind. During wet weather lasting over some weeks, the tendencyto condense, though greater than the tendency to evaporate, does not showitself in continuous rain; but the period is made up of rainy days and dayswhich are wholly or partially fair. Nor is it in this rude alteration onlythat the law is manifested. During any day throughout this wet weather aminor rhythm is often traceable; and especially so when the tendencies toevaporate and to condense are nearly balanced. Among mountains this minorrhythm and its causes may be studied to advantage. Moist winds, which donot precipitate their contained water in passing over the comparatively warmlowlands, lose so much heat when they reach the cold mountain peaks, thatcondensation rapidly takes place. Water, however, in passing from the gaseousto the liquid state, gives out heat; and therefore the resulting clouds arewarmer than the air that precipitates them, and much warmer than the highrocky surfaces round which they fold themselves. Hence in the course of thestorm, these high rocky surfaces are raised in temperature, partly by radiationfrom the enwrapping cloud, partly by contact of the falling rain-drops. Consequentlythey no longer lower so much the temperature of the air passing over them,and cease to precipitate its contained water. The clouds break; the sky beginsto clear; and a gleam of sunshine promises that the day is going to be fine.
But the small supply of heat which the cold mountains' tops have received,is soon lost: especially when partial dispersion of the clouds permits radiationinto space. Very soon, therefore, these elevated surfaces, becoming as coldas at first, begin again to condense the vapour in the air above, and therecomes another storm, followed by the same effects as before. In lower landsthis action and reaction is less conspicuous, because the contrast of temperaturesis less marked. Even here, however, it may be traced, not only on showerydays, but on days of continuous rain; for in these we do not see uniformity: always there are fits of harder and gentler rain.
Of course these meteorologic rhythms involve corresponding rhythms inthe changes wrought by wind and water on the Earth's surface. Variationsin the quantities of sediment brought down by rivers that rise and fall withthe seasons, must cause variations in the resulting strata -- alternationsof colour or quality in the successive laminae. Beds formed from the detritusof shores worn down and carried away by the waves, must similarly show periodicdifferences answering to the periodic winds of the locality. In so far asfrost influences the rate of denudation, its recurrence is a factor in therhythm of sedimentary deposits. And the geological changes produced by glaciersmust similarly have their alternating periods of greater and less intensity.
