OCR Text |
Show 212 ACTION OF THE UPPER AIR. sphere may resist it, and dissolve it again and again, if the cause continues to act in the upper part of the atmosphere, the earth and the lower part must, in the end, give way, and rain must be the consequence. The cloud, too, or the "gum," as it is sometimes called when it is merely a tinge of cololilr without any definite and limited shape, intercepts part of the light and heat of the sun, and thus lessens the resisting power of the lower atmosphere and the earth, and that hastens the coming of the rain. The gummy appearance is probably even more suspicious than the curl-cloud, because it shows that a region higher, and therefore more sensible, is affected, and it also shows that the cause is more widely extended. The quantity of water which the air can sustain in a state of vapour, supposing the air to be of the same density, diminishes more rapidly than the temperature ; and thus when two currents of air of different densities meet, a certain degree of precipitation of moisture. always takes place; and if the difference of temperature be considerable, and the currents, or any one of them, rapid, instant rain may be the consequence, and continued rain may be the consequence of their continuance. S:pring and summer showers come on far more suddenly than the rains of autumn and winter; and. the wind always shifts before the continued rains begin to fall. The upper current may be considered as the one which more immediately produces the rain; although clouds may be borne across the horizon by the under current long before any ram actually begins to fall. But the currents of the air do. not always blow the one above the other, or the one in opposition to the other. Air moves with equal ease in all directions, whatever they may be, if the impelling force tends that way. So that there are often m.any curren. ts, moving in different directions and with different velocities within a very small space. These give rise to innumerable compound motions) th.e CURRENTS IN THE AIR. 213 . causes of which it is impossible to ~e . . any way to understand unless whe ""' parate, or m some spe~ific effect up~n visible s b nt they produce When different cu . t "' . u s ances. other they produce r~e~O~l~~t obh{·uely again.st each . and the other ro . , e mo ~on, one cucular a whirlwind !nd 1~e:~'eh The Circular motion u~ from that which . .Y ave any degree of force or stirs the lio-~~:!tt';i~s;: the rnedst blad~s of grass: water "th f: !? • on an ' or dtmples the t . m amt revolvmo- circles to that h" h wtsts up tree b th 0 ' w 1c boughs, and ra~se~ th:mrf~~~ or. wrenches off their masts of shi s . e air, o~ wrenches the spouts. As ~heotr twts~s dup th~ sea Itself in water- 1 wo wm s whtch prod th · cu ar motion of the h. l . d uce e cirstrength the . ~ lr wm are seldom of equal motion ~f the ~~~~mg folj0 1 ':s progres~ively the esp~cially land wind~ow~r u 'thand as wm~s, more vaned blow in ' w lere e surface lS much wheth~r it b ~usts, the centre of the whirlwind or a column o~ !a~:'n by a ~olumn of clust on land line, or the sam rat sea, IS very seldom a straight Th.e first whirlw~~~I:e ~or two su~cessive seconds. latwn as it moves , o ten taken m another circuscribe ~ircles in 't ctlong, and thus it is made to debe obrserved in th s progress. . The same thing may often floats down ~~at~r: a ht~le. re.volving dimple eddy of a reach an s rea!ll, t~ll lt ls taken in the many revolutio~s b~~he~~ It Will keep whirling for by that .of the eddy re l s own motion be overrun Many of the wh.irlin . reach the surface of t~ mot~ons of the air never means that we have ot . ear~h ; and so the only c.louds. These are often iJudgmg of them are the twn; and especially b £ n very wonderful commounusual thing to see th ore thunder-storms, it is no ent directions at d"fti em moving in twenty differwhi~ ling round horitz~~~~~~ rates, while some are vertical manner and oth y, ~thers tumbling in a ' ers agam moving backwards |