<62 THE SUBMARINE LEAKAGE, ETC. 



But, says Mr. Thomson, "When it is taken into consideration 

 that the chmate of the Bourke district is one of the hottest and 

 driest in the country, it will not, I presume, be difficult to 

 imagine that an enormous volume of the somewhat scanty 

 rainfall must be lost by evaporation as compared with the 

 rather small quantity discharged by the river ... In the 

 case of our rocks and soils there is rapid evaporation under 

 favourable atmospheric conditions, owing to high temperature 

 produced by internal as well as external heat." What, in this 

 case, evaporation through the agency of " internal " heat 

 means, is that water penetrating the soil may reach a portion 

 ■of the earth's crust where the heat is great enough to convert it 

 into vapour, and that the vapour may rise through superincum- 

 bent strata, and escape at the surface. There could, however, 

 be no such escape, as the vapour would be condensed on 

 reaching the cooler strata near the surface, and, in the form of 

 water, would gravitate inward, retracing the steps of its 

 outward course. 



Mr. Thomson does not, however, seriously dwell on the 

 possibility of evaporation by means of internal heat, although 

 from subsequent remarks he seems to see an analogy between 

 the crust of the earth and the moleskins which form the outer 

 integument of many Australians, but passes on to the subject of 

 evaporation by means of solar heat. He asserts that at the 

 Enoggera Reservoir the water evaporates at the rate of a quarter 

 of an inch per day, "and in the central regions of Australia 

 it has been estimated, by a series of experiments and reliable 

 observations elsewhere, that the process of evaporation goes on, 

 under favourable conditions, at the rate of 1 inch per day."* 

 " Just imagine," he adds, " what this means in regions where 

 the annual rainfall is not greater than from 10 inches to 

 12 inches ! " It means, no doubt, that a- rainfall spread out in a 

 thin sheet on a non-absorbent clay soil, which does not slope 

 sufficiently to permit of rapid drainage, will be dissipated by 

 evaporation in a few days or weeks, and nobody can deny that 



* In a letter dated 22nd July, 1896, Mr. Russell says : " I have, for a numbpr of 

 years, measured the actual evaporation in the Western districts. The greatest recorded 

 evaporation in a year in our hottest country is oft. 5in., and this from a tank 4ft. In 

 diamerer and 3ft. deep, kept full or nearly so— a condition in which sun and wind have 

 more effect than they have in ordinary reservoirs. 



