Jail. 



889] 



NA TURE 



Z2J 



cloud corpiiscules, so that when finally it falls below the 

 cloud it may have a considerable size. 



Such, then, is the mode in which rain is formed in an 

 ordinary summer shower ; and the more prolonged rain- 

 tail of stormy wet weather is the result of a similar process, 

 viz. the ascent and dynauMC cooling of the moist atmo- 

 sphere. But in this case the movement is on a far larger 

 >cale, being shared by the whole mass of the atmosphere, 

 I may be, over hundreds or thousands of square miles ; 

 and to understand this movement we shall have to travel 

 somewhat further afield, and to inquire into the general 

 circulation of the great atmospheric currents set in move- 

 ment by the sun's action in the tropics, and modified by 

 the earth's diurnal rotation and the distribution of the 

 continents and oceans on its surface. 



Before, however, entering on this subject, which will 

 require some preliminary explanation, and in which we 

 shall have to take account both of ascending and descend- 

 ing currents on a large scale, 1 will draw your attention to 

 another and simpler case, in which both these classes of 

 movements are prominently illustrated, and in which they 

 exhibit their characteristic features in a very striking 

 manner. 



In the valleys of the Alps, more especially those to the 

 north of the central chain, in .Switzerland and the Tyrol, 

 ihere blows from time to time a strong warm dry wind, 

 :own as the Fdhn. It blows down the valleys from the 

 sntral chain, melting the snows on its northern face, and 

 Ithougli there is more or less clear sky overhead, all the 

 )uthern slopes of the mountains are thickly clouded, and 

 iavy rain falls on the lower spurs and the adjacent plain, 

 replaced by snow at the higher levels up to the passes and 

 the crest of the range. Cloudy weather also prevails to 

 the north in Germany, and the weather is stormy over 

 some part of Western Europe. 



It is only since the general introduction of telegraphic 

 weather reports and the construction of daily weather 

 charts have enabled us to take a general survey of the 

 simultaneous movements of the atmosphere over the 

 Ifrreater portion of Europe, that this Fdhn wind has been 

 satisfactorily explained.' It is found that when a Fohn 

 Wind blows on the north of the Alps, the barometer is low 

 somewhere to the north or north-west, in Germany, 

 Northern France, or the British Isles, and high to the 

 south-e.ist, in the direction of Greece and the Eastern 

 Mediterranean. Under these circumstances, since the 

 winds always blow from a place of high barometer to one 

 of low barometer, a strong southerly wind blows across 

 the Alps. On their southern face it is forced to ascend, 

 and therefore, as just explained, it is cooled and gives rain 

 in l.ombardy and V'enetia, and snow at higher elevations. 

 But hiving reached the crest of the mountains, it descends 

 to the northern valleys, and being by this time deprived 

 of a large part of its vapour, it becomes warmed in its 

 descent, owing to compression, absorbs and re-evaporates 

 the cloud carried with it, and is then further warmed at 

 the rate of 1° for every 183 feet of descent. Thus it 

 reaches the lower levels as a warm dry wind, its warmth 

 being the effect of dynamic heating. 



Other mountain chains afford examples of the same 

 phenomenon. A very striking instance, which much im- 

 pressed me at the time, is one that I witnessed many 

 years ago in tl»e mountains of Ceylon ; and it was after- 

 wards mentioned to me by Sir Samuel Baker, who had 

 been equally struck by it. My own experience is as 

 •follows: — In June 1861J 1 paid a week's visit to the hill 

 sanitarium of Newara Eliya, at an elevation of 6200 feet, 

 on the western face of Pedro Talle Galle, the highest 

 mojntain in the island. The south-west monsoon was 

 blowing steadily on this face of the range ; and during the 

 whole time of my stay it rained, as far as I am aware, 

 without an hour's intermission, and a dense canopy of I 



' The explanation was originally tiven by Prof. J. H.inn, of Vien;i.-i. \ 



cloud enveloped the hill face, and never lifted more than a 

 few hundred feet above the little valley in which Newara 

 Eliya is built. But on leaving the station by the eastern 

 road that leads across the crest of the range to Badulla, 

 at a distance of five miles one reaches the col or dip in 

 the ridge near Hackgalle, and thence the road descends 

 some 2000 feet to a lower table-land which str-etches away 

 many miles to the east. No sooner is this point passed 

 than all rain ceases and cloud disappears, and one looks 

 down on the rolling grassy hills bathed in the sunshine of 

 a tropical sun, and swept by the dr>' westerly wind that 

 descends from the mountain ridge. In little more than 

 a mile one passes from day-long and week-long cloud and 

 rain to constant sunshine and a cloudless sky. 



As an almost invariable rule, or at least one with (ew 

 exceptions, ascending air currents are those that form 

 cloud and rain, and descending currents are dry and bring 

 fine weather. And this holds good whatever may be the 

 immediate cause of these movements. We may now 

 proceed to consider these greater examples to which I 

 have already referred. 



In the great workshop of Nature, in so far at least as 

 concerns our earth, with but few exceptions, all move- 

 ment and all change, even the movements and energies of 

 living things, proceed either directly or indirectly from the 

 action of the sun. Nowhere is this action more direct 

 and more strikingly manifested than in the movements 

 of the atmosphere. Were the sun extinguished, and to 

 become, as perhaps it may become long ages hence, a 

 solid cold sphere, such as Byron imagined, " wandering 

 darkling in eternal space," a few days would suffice to 

 convert our mobile and ever-varying atmosphere into a 

 stagnant pall, devoid of vapour, resting quiescent on a 

 lifeless earth, held bound in a more than Arctic frost. 

 From such a consummation, despite the supposed decay- 

 ing energy of our sun, we may, however, entertain a 

 reasonable hope that we are yet far distant. 



Bearing in mind the all-embracing importance of the 

 sun, let us see how the great movements of the atmo- 

 sphere are d termined by the way in which the earth 

 presents its surface to the solar rays. 



Since the quantity of solar heat received on eacn part 

 of the earth's surface depends on the directness or 

 obliquity of his rays — in other words, on the height to 

 which the sun ascends in the heavens at noon — being 

 greatest where he is dire( t'y overhead, as in summer in the 

 tropics, it follows that the hottest zone of the earth is that 

 in the immediate neighbourhood of the equator, and the 

 coldest those around the poles. 



Did time allow, and were the necessary appliances at 

 hand, it would be easy to show you th it both as a matter 

 of experiment, and also as a deduction from physical laws, 

 there must be under su:h circumstances a tlow of air from 

 the colder to the warmer region in the lower atmosphere, 

 and a return current above. And to a certain extent we 

 have these constant winds prevailing for about 30' on 

 either side of the equator, in the trade- winds, which blow 

 towards the equator in the lower i.tmosphere, and the 

 anti-trades blowing in the opposite direction at a great 

 height above the earth's surface. 



In the neighbourhood of the equator there is a zone 

 extending right round the earth in which the barometer 

 is lower than either to the north or the south. .It is due 

 to the greater heat of the sun, and it is towards this that 

 the trade winds blow. It shifts to some extent with the 

 seasons, being more northerly in the summer of the 

 northern hemisphere, and more southerly in that of the 

 southern hemisphere ; and its average position is rather 

 to the north of the equator, owing to the fact that there is 

 more land in the northern than in ihe southern hemi- 

 sphere, and that land is more heated by the sun than the 

 ocean. 



'1 his simple wind system of the trades and anti-trades 

 does not extend right rounc' '-.e earth, nor beyond 30° or 



