May 14, 1903 J 



NATURE 



29 



iEOLOGY FOR AGRldULTURAL STUDENTS. 

 iiricultural Geology. By J. E. Marr. Pp. xi + 318. 

 (London : Methuen and Co., 1903.) Price 6s. 



IN the teaching of any technical subject, like engin- 

 eering or agriculture, which touches and depends 

 upon several of the pure sciences, there has always 

 been dispute about the amount and nature of the pure 

 science to be exacted from the technical student, the 

 present controversy over mathematics for engineers 

 being a notable example. In the past, as a rule, the 

 pure science man has ruled the roast, secure in a 

 plausible logical position which regards the technical 

 as " applications " of the principles laid down in the 

 pure science, as "riders" in fact; now, however, he 

 has, thanks to the spread of truer conceptions of edu- 

 cation, to justify his teaching and discard those in- 

 tellectual gymnastics which leave the student " as 

 he was," and confine himself to a development of the 

 subject to the given end. 



In the book before us, Mr. Marr has put together 

 that portion of geology with which a serious agricul- 

 tural student ought to be equipped as a basis for his 

 study of soils ; more particularly the book is intended 

 tor candidates preparing for the examination for the 

 National Diploma in Agriculture. 



The earlier part of the book seems to us to be 

 admirably suited to the agricultural student ; he will 

 get from it just the introductory knowledge of minerals 

 and rocks, rock structures, and the work of geo- 

 logical agencies that he requires for an intelligent 

 appreciation of the structure of the country. There is 

 nothing superfluous, and, on the other hand, the proper 

 point of view is obtained, the subject is developed as 

 a whole, and not allowed to become a series of scraps 

 ot useful knowledge. 



fwo excellent chapters follow on the construction 

 iiiid interpretation of geological maps and sections, 

 but we should have liked to see the later chapter on 

 " water supply " brought into connection with this 

 section, and treated in much more detail. To the 

 agriculturist, structural geology is in the main im- 

 portant only as bearing upon water supply; it is 

 fundamental that he should be able to read a geo- 

 logical map so as to gauge the probabilities of obtain- 

 ing either surface or deep-seated water at a practic- 

 able depth, or to trace the origin of landsprings and 

 decide upon a plan for tapping them or otherwise 

 drying the land. We trust Mr. Marr will see his way 

 in another edition to work out for the student some 

 examples of the varying conditions of water supply, 

 not by generalised diagrams, but from the actual survey 

 maps. 



The weakest part of the book is the last, the chapters 

 dealing with stratigraphical geology; the economic 

 products are but lightly touched upon, and the agri- 

 cultural character of each formation is dismissed in 

 a very sketchy and generalised fashion. If we com- 

 pare the two pages or so devoted to the structure of 

 Graptolites — the chitinous rod, the periderm, the hydro- 

 thccae, &c. — with the amusing reference to the clay- 

 with-flints, " Little will grow on it, though in places 

 it has been made to yield good root crops," we see 

 the differertce between Mr. Marr the geologist, writ- 

 NO. 1750, VOL. 68] 



ing of what he likes and understands, and Mr. Marr 



" getting up " things for the agriculturist. Lastly, 

 we should have liked a little more about the " drift " 

 and the superficial deposits generally, for the farmer 

 is more concerned with them than with the solid 

 geology. In this connection we should like to know 

 Mr. Marr's evidence for the following statement 

 (p. 128):— 



" One very important character of glacial drift 

 from the point of view of soil formation is due 

 to the fact that the disintegrating action of ice is purely 

 mechanical, and, consequently, the soluble constituents 

 of the rocks from which the drift has been derived 

 have not been removed. These soluble constituents 

 may be taken up by the plants but slowly, and ac- 

 cordingly the drift soils may not yield such abundant 

 crops as other soils at the outset, but, on the other 

 hand, they may continue to furnish supplies of these 

 soluble materials long after those of other soils have 

 been exhausted." 



We are not sure we understand the meaning 

 of this paragraph, but at any rate we demur 

 to the apparent implication that soils become exhausted 

 by cultivation as practised in this country. 



APPLIED MECHANICS. 

 Elementary Applied Mechanics. By Profs. T. Alex- 

 ander, C.E., and A. W. Thomson, D.Sc. Pp. xx + 

 575 ; 281 illustrations. (London : Macmillan and 

 Co., Ltd., 1903.) Price 425. 



THE title of this book is misleading. It is really a 

 large and fairly advanced work dealing with 

 certain engineering problems usually, now, classed 

 under the headings " Strength of Materials " or 

 "Theory of Structures." Simple problems in connec- 

 tion with stress and strain are taken up in chapter i., 

 useful numerical examples being given by way of 

 illustration and enforcement. Such examples, in 

 fact, form a valuable feature of the work throughout. 

 The authors — professors at Trinity College, Dublin, 

 and Poona, India, respectively^ — dedicate the book to 

 the memory of their late teacher, Prof. Rankine. 

 Their study of that great authority has not, however, 

 produced that terseness and lucidity of expression now 

 so much prized. Thus the lengthening of a strut is 

 called "augmentation," and shortening, we are led 

 to infer, is "negative augmentation." Again, we 

 read, 



" The Proof Load is the stress of greatest intensity 

 which will just produce a strain having the same ratio 

 to itself which the strains bear constantly to the stresses 

 producing them for all stresses of less intensity. 

 If a stress be applied of very much greater intensity, 

 the piece will break at once, &c." 



One notices circumlocutions of this kind in various 

 places. 



Internal stresses and strains, simple and compound, 

 are next taken up, and a picture of a model for 

 illustrating Rankine's " ellipse of stress " is given and 

 explained. 



The stability of earthwork is dealt with in chapter iv. 

 — as usual in such investigatiotis, all depends on a 

 knowledge of .the "angle of repose," a very variable 



