Africa, owing to the dry climate, most of the indigenous plants have special apparatus for stopping transpiration, and the cultivated area is practically negligible, so that whereas in normal countries only 30 per cent of the rainfall on land is derived from the sea, and the rest supplied by the land itself, in South Africa we have to rely principally on the sea for our rains. In other words, we could in South Africa increase our rainfall by something like 70 per cent if the waste lands were cultivated.
In regard to the solid particles in the atmosphere, they exist in enormous numbers. A special apparatus for counting them was devised by Dr. Aitken and used in Melbourne. The volume of air taken was a cubic inch, and the lowest number recorded, on a wet day, was 128,000 particles; the highest number was two million. In the eruption of Krakatoa, in 1883, the island, some 6 ml. long by 3 wide, with mountains 2500 ft. high, was blown up into the air, and soundings of 164 fathoms are obtained where it once was a little crescent-shaped islet, a mile long, is all that remains. The dust thrown into the atmosphere by this explosion remained suspended for many months and floated completely round the world. But besides such paroxysmal upheavals, the atmosphere is constantly being invaded by dust; winds, especially in South Africa, carry the sand for long distances, but the finer particles remain suspended and float about for months. Where ponds and vleys dry up, the weeds perish and the microscopic life which frequently swarms in them is killed, and the dead bodies of these and particles of the dried-up plants may be carried over wide areas. In 1902 a fall of black rain was recorded in Kimberley. Some of the residue from the water
