NATURE 



[November 5, 1908 



are produced simultaneously; the Gradatas, in which 

 there is a definite succession in time and space; and 

 the Mixtse, in which there is a succession in time, but 

 no regular succession in space. 



" These three tj'pes appeared successively in fjeo- 

 logical time : the Simplices were the characteristic 

 ferns of the primary rocks, though many of that 

 type still survive; the Mixt» are the dominant ferns of 

 the present day, while the Gradatce take a middle 

 place." 



The scheme on p. 653, showing the approximate 

 relations of the several families of ferns, will be of 

 great service to students of this class (now more 

 important than ever to the morphologist). Altogether, 

 the author's account of the filicales is no doubt the 

 best yet published. 



The concluding part of the book gives a full and 

 final statement of the author's theoretical position, 

 and is the part which will most appeal to the reader 

 whose interest lies mainly in the theory rather than 

 the details. Enough, however, has been said on the 

 points in dispute; any attempt at a full discussion 

 would far exceed the limits of a review. 



The book is excellently got up, with abundant and 

 admirable illustrations throughout. It is almost free 

 from misprints. One, however, occurs in an 

 important passage on p. 237, where " Riccia cell " 

 appears to be a printer's error for " Ricciaceae." 



Nothing can be better for English botany than the 

 appearance of such a book as this, a full and most 

 original treatise on an important branch of the science 

 by one who is an acknowledged master of his sub- 

 ject. Prof. Bower is to be warmly congratulated on 

 this, the latest product of his energy and devotion to 

 research. D. yi. S. 



WINDMILLS AND WATER-WHEELS. 

 Natural Sources of Power. By R. S. Ball. Pp. xvi+ 



348. (London : A. Constable and Co., Ltd., igo8.) 



Price 6s. net. 

 ' I "HE classification of a source of power as a 

 -»- " natural " one is purely arbitrary. The dis- 

 tinction would imply that a source of power could 

 be "artificial," which would, of course, contradict 

 the first law of thermodynamics. The author of the 

 present volume simply uses the word to describe those 

 sources of power which provide us directly with 

 mechanical energy without any intermediate trans- 

 formation, such as combustion or the like; .and the 

 two particular supplies of energy to which attention 

 is directed are wind-power and water-power. 



As is natural, the author commences his book with 

 a reference to the, said to be, not distant day when 

 all the coal, and all the oil, in the world will have 

 been used up, and mankind, in order to sustain itself, 

 will have to rely wholly upon the water-wheel and the 

 windmill for that tremendous amount of energy which 

 will be necessary to keep the immense population of 

 the earth in the state of comfort to which it has, with 

 the progress of civilisation, attained. It is an in- 

 teresting speculation to picture to oneself what the 

 state of the world will be when this prophesied day 

 arrives, and the coal-measures of the world have 

 NO. 2036, VOL. 79] 



disappeared. Will the great manufactures migrate 

 from Lancashire and Northumberland to Norway, 

 Italy, and the West of Ireland, or will, ere that day 

 arrives, our cotton mills and blast furnaces be run by 

 radium engines, utilising sources of energy which 

 are at present wholly unexploited? Certainly, nobody 

 who has studied the development within the last few 

 years of the science of radio-activity will be prepared, 

 out of hand, to deny the possibility. 



It is rather surprising to be told that the demand 

 for windmills was never so great as it is to-day, or 

 the trade of the manufacturer of such motors never 

 so brisk. On the other hand, evidences of the utilisa- 

 tion of the water-powers of the world are everywhere 

 abundant, the chief agent in this being the develop- 

 ment of electrical technology. A book, therefore, such 

 as the one under review, dealing with these subjects 

 in an easily understandable manner, is to be accorded 

 a welcome. The style of the book, while being simple, 

 is yet not entirely popular. It is not a complete 

 treatise, a certain amount of elementary mathematics 

 is necessary, but the calculus is not used, the author 

 giving a general review of his subject, with the object 

 of showing the desirability of not allowing the many 

 small sources of wind- and water-power which exist 

 to run to waste. The book can be specially recom- 

 mended to those readers who, while not being special- 

 ists in the particular branch dealt with, desire to 

 obtain a general survey of the subject. 



The first chapter deals with general principles, such 

 as the distinction between " power " and " energy," 

 efficiency of machines, units, &c. The discussion of 

 the electrical units of energy on p. 7 is hardly 

 happy. This, we think, is due to the author placing 

 in juxtaposition the " foot-pound " and the " watt," 

 which latter, he says, is " allied to a power unit." 

 The confusion in electrical units of power, which 

 the author mentions, is, we think, entirely of his 

 own creation. The watt is not " allied " to a power 

 unit, but is actually the electrical unit of power, there 

 being really no confusion in the matter ;it all. 



Chapter ii. is concerned with " water power and 

 methods of measuring." As is only fit, the funda- 

 mental theorem of Bernouilli, which says that the 

 sum of the pressure head, the velocity head, and the 

 height above datum level is the same at all points in 

 a pipe running full of water, is stated and discussed, 

 as are also weirs and the general principles of survey- 

 ing as called for in the lay-out of a water development 

 scheme. 



Subsequent chapters deal with the different kinds of 

 water-wheels and hydraulic turbines, their general 

 design, theory and regulation. The construction of 

 water-power plants and the fundamental principles 

 of dams are also referred to, while descriptions of 

 several typical installations working under such widely 

 different conditions as heads of 2 feet and 2000 feet 

 are given. 



The last 120 pages of the volume discuss windmills 

 and wind-motors. It is stated that there is a rapid 

 extension and enormous trade done in small windmills. 

 These are used chiefly in the great agricultural coun- 

 tries for pumping purposes, and the attempt made 

 to utilise such motors for driving electrical generators 



