236 



THE FARMER'S MAGAZINE. 



A DR(3P OF RAIN-WATER. 



BY CUl'HBEllX W. JOHNSON, ESQ., I'.R.S. 



In two previous papers (ante pp. 98 and 140), I 

 have endeavoured to trace the origin and progress 

 of the aqueous portion of the air we breathe. We 

 have followed that vapour as it rose into the atmos- 

 phere from the surface of the earth : we have marked 

 its equally insensible descent in dew. I propose in 

 this concluding little section to invite my readers 

 to note some of the phenomena which attend the 

 deposition of that atmospheric moisture, in a state 

 which is more evident to our senses, either fluid in 

 drops of rain, or solidified in the hailstone or the 

 snowflake. In tracing together these things, 

 we shall hardly advance a step, without remarking 

 facts of abounding interest to the skilful agri- 

 culturists of our country. 



We have seen that the atmosphere at a tempera- 

 ture of 65°, when saturated with moisture, contains 

 about 8 grains of water. We find that at a lower 

 temperature its power to retain the water decreases ; 

 and, therefore, when the warm saturated air has its 

 temperature diminished, that it has to part with a 

 portion of its water, which then ceases to be insen- 

 sible moisture, it appears as cloud, or fog, or rain. 



We can only trace the effects of thus cooling a 

 warm and saturated current of air, by following 

 the lower cun-ents of our atmosphere. We find, 

 that certain of these, flowing over the warm surface 

 of equatorial seas, soon become saturated with 

 moisture. Then again we note that if these great 

 currents meet, in their course, mountains of con- 

 siderable height, that then the warm current is 

 directed upwards, becomes rapidly cooled, and so 

 unable to retain the moisture with which it is 

 saturated, and torrents of rain are then showered 

 down on the sides and tops of such elevated moun- 

 tain ranges. It would be reasonable to conclude 

 that this was owing to the cooler atmosphere with 

 which these warm sea-borne currents mingle as 

 they ascend the mountain sides : various careful 

 observations made at different heights show that 

 this is the case. M. Gay Lussac, from remarks 

 made during balloon ascents, concluded that the 

 temperature of the atmosphere decreases one de- 

 gree for every 352 feet of elevation. In ascending 

 the higher mountains of our earth, we find that the 

 mean temperature gradually decreases until we 

 arrive at the line of perpetual snow. That point of 

 eternal frost varies in altitude from the level of the 

 sea near the poles, to about 15,000 feet near the 

 equator. 



For the better understanding of this portion of 

 our inquiry, let us follow the course of a constant 

 warm sea breeze or trade wind, and see how by its 

 course being thus slanted up the sides of highly 

 elevated mountain ranges, it places certain portions 

 of their sides in almost never-ending rain, and 

 other portions in a state of almost perpetual dryness. 

 To this end let us trace the great south-east trade- 

 wind of the Atlantic, follow it across the ocean, on 



to the shores of Southern America, witness the end- 

 less profusion of rain it showers down on the 

 eastern sides of the Andes, follow it over the peaks 

 of eternal snow which occupy their summits, and 

 thence, after it has thus been deprived of its mois- 

 ture, trace it as it descends from these elevated 

 ranges, over the low heated arid districts of Peru, 

 until it again disperses itself over the warm surface 

 of the Pacific, to again absorb that moisture which 

 it will again deposit on colder surfaces of the globe. 

 This course of the Atlantic south-east trade-wind, 

 and the rain phenomena attending its course, 

 Maury has described in language which I need not 

 attempt to vary {Fhj. Geo. Sea, p. 204). He says 

 that " the coast of Peru is within the region of per- 

 petual S.E. trade winds. Though the Peruvian 

 shores are on the verge of the great South Sea 

 Boiler, yet it never rains there. The reason is plain. 

 The south-east trade winds in the Atlantic Ocean 

 first strike the water on the coast of Africa. 

 Travelling to the north-west, they blow obliquely 

 across the ocean until they reach the coast of 

 Brazil. By this time they are heavily laden with 

 vapour, which they continue to bear across the 

 continent, depositing it as they go, and supplying 

 with it the sources of the Rio de la Plata, and the 

 southern tributaries of the Amazon. Finally they 

 reach the snow-capped Andes, and here is wrung 

 from them the last particle of water that that very 

 low temperature can extract." 



Reaching the summit of that range, they now 

 tumble down as cool, and dry winds on the Pacific 

 slopes beyond. Meeting with no evaporating sur- 

 face, and with no temperature colder than that to 

 which they were subjected on the mountain-tops, 

 they reach the ocean before they become again 

 charged with fresh vapour, and therefore before 

 they have any which the Peruvian climate can ex- 

 tract. The last they had to spare was deposited as 

 snow on the tops of the Cordilleras to feed moun- 

 tain streams, imder the heat of the sun, and to 

 irrigate the valleys on its western slopes. Thus we 

 see how the top of the Andes becomes the reservoir 

 from which are supplied the rivers of Chili and 

 Peru. 



Supposing these observations to be correct, we 

 might reasonably expect that up to a certain eleva- 

 tion on a mountain side, the amount of rain would 

 steadily increase. Careful observations with rain 

 gauges thus placed, show that such indeed is the 

 case. 



This remark does not indeed apply to gauges 

 placed in level districts at only a few feet from the 

 ground — since, for some cause not yet clearly ex- 

 plained — the rain gauges placed near the earth 

 collect more rain than those more elevated. This 

 was long since noticed in the cases of some gauges 

 placed on York jMinster and other high buildings ; 

 and this was supposed to have arisen from eddies 



