September 2, 1920] 



NATURE 



in the pre-Cambrian, Devonian, Permian, and 

 Jurassic periods. But this succession of cold 

 spells only braced Northern Man to greater efforts 

 and greater triumphs, and sent Southern Man to 

 scrapple with the tropics, and to digest and partly 

 i.vercome their germ diseases. Now the tropics, 

 and above all the sub-tropical regions, are being 

 threatened by drought. The desert is spreading 

 in sub-tropical North America, in tropical South 

 America, in temperate and sub-tropical Asia and 

 i;i stern Europe, in northern and north-central 

 Africa, and in that prolongation of the African 

 continent which lies beyond the Zambezi and 

 Kunene Rivers. 



Prof. Schwarz theorises on the earlier theories 

 of others — British, South .\frican, French, and 

 German — and propounds explanations and sug- 

 gests remedies derived from his own geographical 

 and geological investigations in South and South- 

 west Africa ; and the result is an exceedingly in- 

 teresting little book, well illustrated and well 

 worth the modest price asked for it — though in- 

 sufficiently served by its maps. He points out 

 that the main cause of the creation — the recerit 

 creation- -of the Sahara, Libyan, and Kalahari 

 deserts in Africa has been the diversion of river 

 courses, most of which ran (more or less) from 

 south to north and north to south, but now reach 

 the sea by shorter courses almost at right angles 

 to their former direction. 



At no great distance ih time it is highly prob- 

 able that the Kunene River of southern Angola 

 had made no breach in the western rock rampart 

 of the Ovambo plain; it flowed instead by several 

 dried-up water-courses into the Etosha lake, and 

 thence along the Ovambo River to the Omatako 

 and the Kalahari plain. Joined further by the 

 waters of the Kwito, Okavango, and Kwando, 

 this accumulated drainage of the lofty plateau of 

 eastern Angola emptied itself, not — as now — into 

 I hi: Atlantic or the Zambezi, but into a huge 

 expanse of water of which Lake Ngami and the 

 Makarikari "depression " and salt pans are the 

 shrunken remnants. Finally, in all probability 

 this great lake of SoiUh-west Africa found an out- 

 l<-t through the Molopo River of western Bechu- 

 analand into the Orange, and so at last reached 

 the sea. But its moisture, through the soil and 

 the atmosphere, rendered the western half of 

 South Africa a fertile land endowed with exuber- 

 ant vegetation and animal life, and able to support 

 a large human population. 



The author shows in this book ht)W by engineer- 

 ing operations less difficult, probably, than those 

 vhich have tripled the value of the Nile waters, 

 t!.e Kunene and Okavango and their tributaries 

 might once more be diverted into the old channels 

 NO. 2653, VOL. 106] 



and restore to human use and benefit an area of 

 more than a million square miles. His schemes and 

 plans will, of course, be riddled by the same 

 "expert " criticism which declared the Suez Canal 

 an impossibility, or be side-tracked through some 

 political jealousy. But, all the same, if something 

 more or less in the nature of his proposals is not 

 soon put in hand, the habitable area of trans- 

 Zambezian Africa will shrink considerably. 



Man must give up internecine warfare and 

 unite all his forces to defeat his arch-enemy, 

 Nature. He must melt the ice at the North and 

 South Poles, and put a stop to the spread of 

 desert conditions in Asia, Africa, Australia, and 

 the Americas. H. H. Johnston. 



Cement Manufacture and Testing-. 

 Cement. By Bertram Blount. Assisted by 

 William H. Woodcock and Henry J. Gillett. 

 (Monographs on Industrial Chemistry.) 

 Pp. xii -f- 284. (London : Longmans, Green, 

 and Co., 1920.) Price i8s. net. 



THE need for a handy text-book on cement, 

 which should include the modern processes 

 of manufacture and also a review of the chemical 

 aspects of the subject, has been felt for some 

 time, and this work of Mr. Blount should go far 

 towards meeting the requirements. The author 

 has an extensive practical 'experience of cement 

 manufacture and testing, and is accustomed to 

 present accounts of the industrial processes in a 

 readable form. With the exception of a few pas- 

 sages, the work is devoted to Portland cement, 

 and the processes of manufacture selected for de- 

 scription are almost exclusively those which are 

 adopted in England. Rather fuller references to 

 Continental and American methods would have 

 been welcome, since those methods, almost similar 

 in principle, present great differences of practical 

 detail. 



The use of blaSt-furnace slag as a raw material 

 for the manufacture of Portland cement receives 

 only the briefest mention, but this is now exten- 

 sively practised, and the product is likely to play 

 an important part in building and engineering 

 operations in such regions as the North of 

 England and Scotland, where suitable clays or 

 marls are rare, whilst slag is a waste by-product 

 of the iron industry. The further use of finely 

 ground slag as a pozzolanic material is not 

 noticed, and, indeed, the whole subject of pozzo- 

 lanas deserves more attention. It is not suffi- 

 ciently recognised that the lime set free during 

 the setting of Portland cement, like the slaked 

 lime of an ordinary mortar, is capable of combin- 

 ing with silica when presented to it in a sufli- 



