92 



DISCOVERY 



from economy in power supply, increased capacity of 

 the line for traffic, and so forth, are not outweighed 

 by the capital cost of the conversion. An investigation 

 indicates that at a certain traffic density electrical 

 w^orking will show sufficient advantage to justify the 

 capital expenditure, and the higher the traffic density 

 above this critical value the greater will be the advan- 

 tage. The effect is to some extent cumulative in that 

 the improvement of the service resulting from the 

 change usually creates new traffic, so that the actual 

 results may be even more favourable than were 

 anticipated. Because of this it may be predicted that, 

 given the requisite financial conditions for the raising 

 of the capital, the future will see great developments 

 in the electrification of railways which will be amply 

 justified by their results. 



REFERENCES 

 Books and papers which may be referred to for full technical 



treatment of the subject of this article : 



Jiailway Electric Traction. By F. W. Carter. (E. Arnold & Co.) 



Jilectrical Traction. By E. Wilson and F. Lydall. (E. Arnold 

 & Co.) 



" Electric Railway Contact Systems," Journal of the Institu- 

 tion of Electrical Engineers, vol. Iviii, p. 838. 



" Application of the Electric Locomotive to Main-line Trac- 

 tion on Railways." By H. E. O'Brien. Journal of the 

 Institution of Electrical Engineers, vol. Iviii, p. 858. 



Rainfall and Civilisation 



By Colonel H. de H. Haig, late R.E. 



It is about one hundred years since Malthus startled 

 and shocked the world with his theory, that the 

 natural increase of the population must overtake the 

 land surface of the earth on which it lives, because 

 one is growing and the other is fixed and limited. 

 The population is still growing, but it has not yet 

 overstepped the numbers which can be supported. 

 There are still large empty spaces, though unfortu- 

 atcly most of them are almost, if not quite, deserts. 



Ghographical Position of Deserts 

 If we take an atlas, the iirst thing that strikes us 

 is that the deserts are not where they might be 

 expected to be, that is, in the hottest part of the earth, 

 on the Equator. The Sahara, for instance, is far to 

 the north of the Equator. There, on the contrary, we 

 find hot and steamy climates like Stanley's Rain 

 Forest of the Congo, the Amazon Region, and the 

 East Indian archipelago. The true deserts lie to the 

 north and south of the Equatorial lands, forming 

 roughly two rings round the earth, north and south of 



the tropics of Cancer and Capricorn. This points to 

 some general and world-wide cause, which may be 

 the circulation of the atmosphere at the Equator 

 under the sun and the descent of the currents which 

 form the trade winds. 



The sun heats the air in the belt between the tropics 

 and in consequence it expands and rises. When it 

 reaches the higher atmosphere it rises above the 

 surrounding air and has to overflow to the north or 

 south. This rise and consequent expansion chills 

 it, and the moisture condenses and falls as the well- 

 known tropical rains. 



The air thus chilled, dried, and pushed forward, 

 flows over the warmer air beneath, gradually sinking 

 on account of its greater density, until it reaches the 

 surface of the earth in the regions just outside the 

 tropics. 



When it meets the surface the air is very dry, so 

 that it greedily absorbs what moisture it encounters. 

 If the surface be the sea it becomes a wet wind like 

 the rainy south-west winds of Europe, but if it first 

 impinges on the land, it dries up the surfaces and 

 causes the deserts which extend roughly in two rings 

 round the earth, and which seem to be continually 

 tending to extend their boundaries. 



Beginning with the Sahara, we have Arabia, Syria, 

 Mesopotamia, Persia, Baluchistan, the Indian desert 

 of Bikanir, and the Gobi. The belt continues in the 

 same latitudes in North America in South California, 

 Arizona, and New Mexico. Similarly in the Southern 

 Hemisphere we have the Australian desert. North 

 Chile, part of Peru, the Atacama Desert, and the 

 Kalahari in South Africa. 



Deserts as Centres of Past Empires 



It is a significant fact that it is in these regions, 

 obviously the very best parts of the earth's surface 

 in ancient days, that the great nations of antiquity 

 appeared — Assyria, Babylonia, Persia, Phoenicia, the 

 Hittites, Crete, Egypt, Carthage, India, and China, 

 and in the new world the Aztecs and Incas. All of 

 these flourished in lands now suffering from insufficient 

 rainfall, but which with abundant water easily yield 

 two crops a year. 



Effects of Clim.\tic Change on Civilisations 



There seem to be three stages through which 

 countries pass ; 



(i) The rich and productive one, when the rainfall 

 is abimdant at all seasons, crops are certain, grazing 

 is ample throughout the year, and in warm climates 

 several crops can be raised. 



(2) The irrigation stage, when the rainfall is insuffi- 

 cient or badly distributed. This necessitates the 



