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Show 214 THlJNDER STORMS. and forwards hetween the larger masses, as if inviting them to come together. Th~se currents and commotiOns are always most conspicuous when the clouds are congregating before thunder-storms; and when they appear in several masses of strata, the one above the other, there are as many currents of air of different temperatures, moving in different directions, and mingling together. In these cases there is often no general motion of the mQ.ss of the atmosphere, in all that part of its height which the masses of cloud occupy ; and it is frequently, generally indeed, a dead calm on the surface of the ground, while the motionless state of the thin white curl-clouds that appear through the openings shows that there is not much apparent agitation in the upper air. Nothing is more deceptive, however, than the apparent lightness and cleanliness of these white curls. Theirs is the region of . atmospheric sensibility; and their great height diminishes to our view both their magnitudes and their motions : and though they appear to be above the gathering storm, the probability is that they are the real agitators in the whole,-unless there be some cause in the surface of the earth, snch as one place scorched to almost absolute dryness, while another retains its average degree of moisture. ~ large city, a barren moor, or an arid down m_ay, m very hot weather, which has been l<?ng ~ontmue.d, be the means of producing those motrons m the mr, the result of which is a thunder-storm; but thunderstorms that have that origin are generally very local, and of short duration. If the storm has its origin in the upper regions of the air, its primary cause must be at a g~eater distance, and consequently more powerful, as 1t can propagate its action tl_1rough a greater v<?lume of air. The storm itself IS therefore more widely ex~ tended, and of longer duration ; and indeed it generally brings a change of the weather. In those THUNDER-STORMS, 215 cases, it is of no conseq t storm whether it be th~e~fe d<? the moti<?n of the from the distant lace le rsturbed au comes from that place !nd a~t~r a sted~dy current comes other. But ther'e I. s a veryu mpoanrk Ids turbed air at the d·~ . ~ha~ge of the weather; for if th ed. Iuerenc_e m the lt brmgs broken or rainy weath e . tstudr~ed au come, ~urrent come, it drives th b ~r' an If the steady It has been a tract of dr e a weathe! away. If appears, and then a thur~:~S~~her, zn)ld If curl-cloud b_e sure of a tract of bad rm o o_ws, we may tmued alternations of sh weather ; and If after conbleak winds thunder e owers and warmth, and cold certa~n that the weather ~~r~l;e may be equally case Is, however, in the tern ar up .. The former the most frequent. The dr p~r~te latitudes, by_ far !Uuch quicker conductor ol hair tl~hfine weath~r IS a m broken weather ; and ea an the J?Oisture both reflects and radiate:hen the earth Is dry it earth produces cold b hea~, whereas the wet is no attraction of coh!s_evago[atwn. Besides, there the water that forms a c~~~d ~ w~~t tbhe dry earth and earth or water and th t ' w I e etween the wet same attraction' of coh a. w~ter, there is the very cumulated in the sky e~~n {which clouds are acto the moist surface . an e c _oud thus comes down those thunder-show~rs t~a avOids the_ dry ; and even surface of the earth folio~ ~t~elthfrr causes on the the courses of the rivers . IC { ~afy woods and c.ountries, though the local· th Ev;n m mountainous times strike the eak un er-clouds do some-plough up trench~s in {hthey ~luch mor~ frequently abound in moist eat e ose e evated heights which with water Tfiat ( ar~hd and are always saturated which are perfectly ~~t m fed all the occurrences derstand them) i~ . ura are when we once un-clond strikes it u;u~l~~h}Y11 benegcial. Where the faJl,g upon those heio-l t ads, an the water which wade as if it fe11 on th~ spe~:: not so soon run to |
