Heavy Rainfalls, And Their Causes

We measure rain by catcking il in ;i yessel called a rain-gauge, the area of whosi mouth is accurately known, and then calcúlate how deep the layer of water would have been i f all "the rain that feil into the gauge had been spread out evenly on a surface of the size of its mouth. A gallou of water would only cover a space of two feet square if spread out in u layer an inch thick. Accordingly, an inch of rain CslUng on an acre of land aniounts to 100 tons of water, and the same fall per square mile would give 60,00Ü tons. Niiw an inch oí rain falls not very 1111frequently at stations onour west coast in the course of a day, so that we can easily see why two 01 thren oays heavy rain canse iliiods in most of ourrivers. The arca drained by the ïhames is li." square English miles, or live goegmp})ical miles as slated iSir J. Herschel; and accordinglv, an inch of rain falling on tlial district wonld give ns, 4,000,000 tons oí water, uliicli must alinost all of it drain olï the high tanda and ïlood those lying lower before it ultiinately limls its way to the sea; However the wettest weather witli us is ilmost dry wlien compared witli tliat experienced lictween the troplcs during tlie rainy season. Old Dainprie, in hiij "Theory of the W'inds," says of the Island of (oignnia, which lies not far from Panama: "I have been at tuis isle three tiines, and always l'ound t verv rainy, and the rainverv violent. I remember when we touched Hiere ia our return from Cape Sharp, we boiled a kettle of chocolate before we eleaned our bark, and liaving every man liis calabash t'ull, we bogan fco sup t olí, standing all the while in ïhe rain; but I am conftdeni tháí úót a man atnots us all (lid olear liis dial, for it rained so last and Buch great drops feil tato our calabashes, that after we hatl supped olT as mucb chocolate and rainwater together as suffleed us, our calabashea were still above half f'ull." ïhe heatof the sun s the power that causes the water to rise into the air, It lias been very ti-uly said that tte world muy be considered as a giganüc stcam-engine. Xhe Ora is the sun, the boiler the sea near the equator, thecondenser the cold air of high latitudes lifee oui mvii, while the work done is tHe growth oí all vegetables and animáis. Let us look ;i littlé. eloser into this. and see how fcbe operation goeson. The action in virtue of which tlie water rises iscalled evaporation. Watt-r boils at a certata temperature, but long before it boils steani may be seen riking from the vessel in wliicli the Water ia I icing heated. In íact, steam - or, to speak more correctly, aqueous vapour - rises from every free water surface, and even from ice at all teinperatures. This steani is not tlie white thing yon see coming out of the fnnnel of a locomotive, but a perfectly transparent gas. When this is lirst eooied it fonns little drops of water called " bubble steani," and these bubbles floatiog in the air make an opaque cToud, just as a heab of pounded glass is opaque, although the glass itself may have leen quite clear and transparant. The reason is that the light is unequally refracted in passing through the air and the water on the glass, and so the rays cannot get through at all, and the cloud looks opaque. The best mode of aecelerating evanoration is lo cause a current of dry air to pass over the surf ace of the liquid from which the vapor is rising. This is precisely what takes place in the trade-wind zone at all ))eriod8 of fhe year, and also in higher latitudes wlienever an east wind is blovving. As fast as the vapor is generaled it is can iet 1 off, heing absorbed by the air in its passage, and the place of the partially saturated air is at once taken up by a fresh supply of air. whose absorbing power is as vet uniinpaired. KTow warm air can contain a niucli great er quantity of vapor than cpld air; and accordingly, as an east wind is alwavs berorning warmerand wanner the nearer it gets to the equator, we see that the trade-wiinls at the time they reach the limit of the equatoral ealms will be nearly all quite charged with ïnoisture.. Let us unv see what would cause a niass of air so charged to give up its moistnre. The simplest answerto this is. f we can cool it, "we shall, soto speak, squeeze the water out of it. This eooÜHg may be effect ed in various ays. The simplest is by causinjr the air. when ehHtgad with mnsiture, to risc up to a great height in the atmospere, wheré it Önds a very Iqw temperature prevatliiig. Every one knows that it is, as a general rule, ColdêT the highei' you ascend on a mountain: but Ijallnim ax-cnts have shown US that not iiiiricipiriitly (emperaturesextraoidnaiily low may bmet with at no very great distance from the earth's surface. InJuly, 1850, two gentlemen, M. Harrel and M. Hixio, ascended from I'aris. At the height of 6,000 feet theyentered a cloud w hich enveloped thern till thev reachcd thehcight of 2D,(KK) feet, where the ten i pera t ure was '.) z I'ahr., and on ascending 1,000 feet further the temperature feil to 4(1 - Fahr.. so that the meivury in their themometers froze. Sucli a change of temperatura as lliis, amounting to upwartls of 100, would lc siuliciciif lo render thé air, to all intciits and parposes, perfect ly ry, andas wc kimw thal eiiorinous volumes of air do riseat the equator intothe higliersliata of the atmosphere, a slight calculation will show u's how tnuch rain might !■ produced by Uiis siinjile actfon. A layer of air a foot thic&i cotering au acic of gtOUnd, and iKisscssing a temperature pf 80 , as it not iDÍte(jiiciiily dors close to the liue, couldcontain nearly nine gallons of water. Accordingly, a ooluinn BvemileShighcovering the same spatíe, would tatain l,00() tons of water il' eoniplptely saturated at that high taniperature. Now, this is not at all au iinpossilile eoiidition for those latitudes, so that, wlien we take into account the constant motion of the atmosphere, the place of air p.u lially diied being constantly taken by a (resh supply charged with aiuieous Viipor, the prdigious torrent whih puur from the sky in the wet seasons becoine quite inteligible. Darapler's account gives a lively impression ut' what thesp raina and we licur froin oiw öbserver tliat the i;ii 11 in the West Imlies faÜB '-nol. in drops, hut in streams of water. TL statements are confhmed by act nul measurements made by means of instrumente. Capt. Roussin says of ( enne, that bet ween Feb. 1 fit and Feb. 24th, 1820 tweíve feet seven inches of ïain feil. The wettest región on the earth is not, liowever, Cayenne, or even a district situated letween the tropics. Tliis distinctiou is justly claimed by Cherra Ponjee, in tlie Coésia Ilills, lying ni)itli-east of Calmita. Tbere, in .Tune 1851, at an elevation of 4,5(X) feet. 12 feet :i inches were measured, while the total aninial ratnfali is 600 inches or öl) feet! 'l'he ftverage rainfall on our own west eoasts is aboul :!n baches, so that at Cherra Ponjee, twenty times as iiiuch falls, and that within thé space of about six inonth.s. TIn: rca.son of this uiieijual distribution of the fall is that whien the monsoon changes, anl the warm and moist south-west wiinl is blowing uorth-eastwards toward ('ential Asia, it nieets a ridge of high land which it must pass over, and in i(s )assage it is chilled, and deposite its Burden of water on the western slope of