March 27, 1S85.] 



KNOWLEDGE 



255 



that the same regions of greatest heat should also be those 

 ia which the rainfall should be in general heaviost. For 

 it might appear that the same boat which iiroduceil the 

 evaporation would maintain the water in a state of vapour. 

 The fact, however, that aqueous vapour is lighter than air, 

 operates to produce ascending currents over the region of 

 evaporation, currents strengthened by the expansive etl'ects 

 of the heat. Accordingly, the vapour rises rapidly, and 

 when it has thus risen, n>any circumstances operate to pro- 

 duce precipitation. First the upper regions are rarer ; 

 secondly, they are colder ; thirdly, radiation of heat takes 

 place rapidly from the upper surface of clouds, brought 

 here, as Tyndall e.xpresses it, into the presence of pure 

 space (dri/ air having scarcely any ai)preciable effect in 

 checking radiation). The result is that the uplifting of 

 clouds under the sun's influence is followed regularly over 

 the equatorial regions by the precipitation of heavy rain- 

 showers. Ai d Cdt'Tis pai-ihus, the fall of rain deceases 

 ■with distance from the equator of heat, though not so regu- 

 larly as the amount of moisture decreases. 



The next great law which presents itself to our con- 

 sideration is this, that winds blowing towards the equator 

 are, in general, dry winds, and winds blowing from the 

 equator, rainy. This law is the direct consequence of the 

 former, but it is necessary, for several reasons, to present 

 it as a separate law. There is an erroneous method of 

 accounting for this law which is very commonly met with 

 ia works on meteorology. It is argued, that as winds 

 blowing towards the equator are carrying masses of air 

 from colder to warmer regions, they are necessarily dry 

 winds, since, if the air is saturated, or nearly so, at 

 starting, it cannot be saturated when it has become warmer. 

 And vice versd, winds blowing towards the poles are 

 carrying masses of air to colder regions. The air accord- 

 ingly grows colder, and if not far from being saturated at 

 starting, cannot fail to become unable to keep its whole 

 burden of moisture in the state of vapour, and must 

 accordingly precipitate a portion as rain. This explanation 

 is insufficient. It would, indeed, be just as reasonable to 

 reverse the argucaent thus: a wind blowing towards the 

 equator must bring rain ; for, as it brings cold air into 

 ■warm regions, if the air in these regions is nearly saturated, 

 the introduction of cold air must lead to the precipitation 

 of a part of the moisture, and vice versd, a ■wind blowing 

 towards the poles must be a dry, because it is a heat-bearing, 

 •wind. The simple explanation of the law is, that winds 

 blowing towards the equator are dry, because they are 

 blowing from regions over which moisture is less, to regions 

 over which moisture is more abundant, and vice versd. Of 

 course we must superadd to this the facts mentioned above, 

 because a moist wind blowing towards a heated region 

 ■would not Vjiing rain with it, and a comparatively dry 

 wind, blowing towards a cold region might bring rain. But 

 it must not be forgotten that the main question to be 

 considered is the relative moistness of the transported 

 masses of air. 



We meet with corresponding laws affecting the 'rain- 

 producing powers of winds travelling over continents and 

 oceans. A wind blowing over an ocean towards a conti- 

 nent bring? rain to the continent, unlfss the heat over the 

 latter exceeds slightly, or at the least, does not fall sboit 

 of, the heat over the neighbouring ocean. Such a wind is 

 certain to bring rain to an elevated continental region not 

 protected by a mountain barrier yet more elevated. On 

 the other hand, a wind blowing over a continent towards 

 the ocean in general brings no rain. 



Lakes, marshes, and rivers act in a small way a similar 

 part towards the adjoiniog lands as eceans towards neigh- 

 bouring con'.inf nt . 



There are circumstances alsi to be contidei ed as affecting 

 the rainfall in a different manner, viz., not hy supplying u 

 greater or less auioimt of moisture to the atino-phore, but, 

 by afhcting the jiowcr of the atmosphere to ki ep the mois- 

 ture it s\ip))orts in the vaiiorous i-tate. Siuh are the contour 

 and ehvalion of a country, the nature of its .soil, the 

 quantity of forest land, or, wanting this, the relative 

 abundance or paucity of trees, and so on. 



A moist and warm current of air impinging on a moun- 

 tain range, or even on any well-dtlined rising slope, so as to 

 be carried with sullicient suddenness to colder and rarer 

 regions, is compelled to part with a large portion of its 

 moisture in the form of rain ; and convcr.^ely a wind which 

 has passed over a mountain range or an elevated plateau, 

 and desccjids to a lower region, appears as a dry wind, un- 

 less that region is one over which a continual process of 

 evaporation sullicient to maintain the air nearly in a state 

 of satuiation is going on. In this latter case the effects of 

 the descendiiig wind will vary with circuuutances. It will 

 in general appear as a dry wind, but may produce local 

 showers, since it may act, through the sudden addition of 

 cold air, the part of a condenser. 



Forests are great generators of rain. This is mainly due 

 to the peculiar radiative power of trees and vegetables. The 

 soil, wheie it is covered with vegetation, receives no heat 

 directly from the sun, and but little through contact with 

 the heated air. It may seem like a confusion of cause ati<l 

 effect to speak of vegetation-covered countries as rain- 

 generators, since abundant rain is so important a requisite 

 for the abuudant growth of vegetables. This is, however, 

 a case in which cause and effect are interchangeable, liaiii 

 encourages vegetation, and vegetation in turn aids in pro 

 ducing a state of the superincumbent atmosphere which 

 encourages the precipitation of rain. The result is that, 

 apart from external agencies, regions covered with abundant 

 vegetation, and especially with hi^h trees, present year 

 after year, and century after century, a ranker yet ranker 

 luxuriance of vegetable growth. 



On the contrary, arid regions prevent, by their very 

 aridity, and consequently by the intense teat of the soil 

 and superincumbent air, the downfall of the showers which 

 would nourish vegetation. The result is, that even when 

 the soil itself is favourable, it is exceedingly difficult to 

 convert an arid into a vegetation-covered district, the want 

 of moisture being destructive to trees planted in such soils 

 with the object of encouraging rainfall. The process of 

 change must be a gradual one. On the other hand, the 

 improvement of a region over which rain falls too heavily 

 through overabundant vegetation is a comparatively simple 

 process, a judicious system of clearing invariably leading to 

 the desired result. 



(To he continued.) 



THE BRIGHTON P:LECTRIC RAILWAY. 



By W. Slingo. 



rilHE astonishment which was experienced by those of H^ 

 X or our fathers who beheld the progress of a railway 

 train in the early days of Stephenson's public career, w. s 

 great indeed. ' It was cert.inly a fresh start in life's way^', 

 and fairly de?frved all the attention that was, and still is, 

 being bestowed upon it. The fteam locomotive has, how 

 ever, become so inimen.sely useful to us, that it may be fairly 

 regarded as having earned for itself a place amongst the 

 necessities of life. Nevertheless, strange and impressive as 

 was the idea of a string of carriages being hurried alcijg 

 the iron road at the behest of the engine, still mere 



