56o 



NATURE 



[January 21, 19 15 



apparatus was a "vacuum spectrograph" having 

 prisms and lenses of fluorite, but Prof. Lyman has 

 employed a concave grating of one metre radius, 

 which is much simpler in adjustment and has the 

 great advantage of permitting the determination 

 of wave-lengths. A full description of this instru- 

 ment is given, together with many practical details 

 which have contributed to its successful manipula- 

 tion. The absorption of various gases and 

 materials in the region of short wave-lengths is 

 fully discussed, and the spectra of the various 

 elements which have hitherto been investigated 

 are described in detail. Complete tables of these 

 •spectra, and a bibliography of the whole subject, 

 are also included. 



It is interesting to note that the author has 

 lately succeeded in photographing the spectrum 

 to wave-length 905, thus extending it as far 

 beyond Schumann's limit as this was beyond the 

 limit reached by Stokes, about wave-length 1850, 

 if counted on the scale of oscillation-frequencies. 

 Further extension by the direct spectroscopic 

 method is beset by many difficulties, but the 

 author does not consider it hopeless to make 

 further attempts to reduce the gap between the 

 shortest Schumann waves and the waves consti- 

 tuting X-rays, which have a wave-length of about 

 one Angstrom unit. In this gap the relation 

 between " light " and matter undergoes a profound 

 change, and further exploration would doubtless 

 lead to results of great value. 



The book is a valuable record of successful 

 work in an important field of research, and will 

 be indispensable to those wishing to undertake 

 investigations in the Schumann region, or having 

 occasion to make use of the available data of 

 observation. It may, however, be confidently 

 recommended to the larger circle of readers who 

 are interested in the progress of physical science. 



OUR BOOKSHELF. 



The City of Dancing Dervishes and other Sketches 

 and Studies from the Near East. By H. C. 

 Lukach. Pp. xi + 257. (London: Macmillan 

 and Co., Ltd., 1914.) Price 75. 6d. net. 

 This collection of papers, the work of a careful 

 student of the Nearer East, most of which have 

 already appeared in periodicals, is of value at the 

 present time. The treatment, however, is sketchy, 

 and some of the questions discussed do not easily 

 lend themselves to popular writing. The chapter 

 on the jests of the Turkish humorist, Khoja 

 Nasr-ed Din, scarcely deserved re-publication, 

 being a collection of "chestnuts," unless an at- 

 tempt could have been made to trace their ana- 

 logues in Western folklore. An interesting paper, 

 based on personal knowledge, describes an inter- 

 view with the Chelebi Effendi, the leader of the 



NO. 2.q6o, VOL. 94I 



Dancing Dervishes, a Persian by origin, a scholar 

 and theologian, intimately associated with the 

 Turkish Court, his function being to gird each new 

 Sultan with the historic sword of Osman at the 

 Eyub mosque at Constantinople, a ceremony 

 which no Christian is permitted to witness. The 

 best result to be expected from the sketch of the 

 Sunni and Shiah sects and of the origin and influ- 

 ence of the Caliphate may be to attract the reader 

 to the standard authorities, from Gibbon to the 

 Encyclopaedia of Islam. 



The net result of the book is to show the weak- 

 ness of the Turkish Sultans' claim to the Cali- 

 phate, which will probably now be transferred to 

 the new Sultan of Egypt, and to illustrate the 

 hopelessness of the prospect of a Jehad or holy 

 war preached from Constantinople. The Allies are 

 quite aware of the weakness of the Turkish posi^ 

 tion, and one result of the war must be that Islam 

 will be revolutionised, much to their advantage. 



The Principles of Irrigation Practice. By Dr. J. A. 



Widtsoe. Pp. XXV + 496. (New York : The 



Macmillan Co.; London: Macmillan and Co., 



Ltd., 1914.) Price 75. 6d. net 

 As President of the Utah Agricultural College, 

 Dr. Widtsoe has had almost unique opportunities 

 of learning at first hand the results obtained by 

 irrigation in the dry States of America, and in 

 this book he gives a summary of his own investi- 

 gations and those of others in neighbouring 

 States. The general importance of the subject 

 becomes manifest when it is realised that about 

 25 per cent, of the earth's surface receives ten 

 inches or less of rainfall annually and can only 

 be reclaimed by irrigation, while another 30 per 

 cent, receives between ten and twenty Inches of 

 rainfall and requires irrigation for all intensive 

 cultivation. 



The first few chapters deal with theoretical 

 considerations. The soil is likened to a mass of 

 mineral particles, on which hangs a film of water ; 

 calculations are given to show its thickness, and 

 some of Its properties are sketched out This view 

 is necessarily only approximately correct, because 

 It is now known that the soil has not this simple 

 constitution, but is essentially a colloidal mass. 

 Many of the theoretical considerations founded on 

 the older and simpler Idea therefore require 

 modification. Similar remarks also apply to some 

 of the coefficients and constants applied to the soil 

 moisture : their theoretical Interest Is not great, 

 although they are very Important as guides to the 

 practical man in irrigation practice. The chapters, 

 however, give as good a summary as our present 

 knowledge permits, and they clearly show how 

 necessary it is to overhaul the whole subject in 

 the light of modern conceptions. 



By far the greatest part of the book Is taken 

 up with an account of the effect of irrigation on 

 the crop and on the land, and illustrations and 

 diagrams are given In profusion. The work is 

 admirably done, and gives one of the best sum- 

 maries of the subject that we have seen for some 

 time. 



