August 3. 1893] 



NA TURE 



115 



on the steam engine is legion, but few are of any use to 

 the engineer as distinguished from the student. 



The book appears to largely consist of notes accumu- 

 lated both in the drawing office and in the works. These 

 are of great value, and particularly so because all 

 dimensions have been reduced to British units, thus 

 rendering possible a comparison between Continental 

 and British practice. 



A careful study of the contents of this book and the 

 arrangement of the sections, leads to the conclusion 

 that there is probably no other book like it in this 

 country. The volume aims at showing the results of 

 practical experience, and it certainly may claim a com- 

 plete achievement of this idea. 



It must not be imagined from these remarks that the 

 steam engine has not been treated in any other manner 

 than that of rule of thumb, a term often used by those 

 who would place theory before practice in the trainingof an 

 engineer. Take, for instance, the diagrams intending to 

 illustrate the defects in valve gears, which may often be 

 met with in practice ; these make the different defects per- 

 fectly clear, and one can see at a glance where the mistake 

 is to be found. 



Section x. deals with the calculations for power and 

 steam consumption, and section xi. explains the effect 

 of the inertia of the reciprocating parts of a steam engine ; 

 with an ordinary amount of mathematics all these can 

 be easily followed. Section xiv. is on boilers. This 

 section is the weak part of the book, and in future 

 editions should be considerably augmented with in- 

 formation having reference to the design and strength 

 of boilers. 



The book is fully illustrated,'>in fact, we are told in the 

 preface that the letter-press has been reduced as much as 

 possible to allow of the introduction of the numerous 

 tables and drawings ; among the latter there is an 

 excellent illustration of a compound Willan's central 

 valve engine with two cranks — probably the best engine 

 of the kind to be had. Some of these illustrations have 

 evidently been especially prepared with the intention of 

 giving an idea of principles of construction to the reader, 

 particularly those having reference to types of steam 

 engines, various ways of arranging cylinders and cranks 

 in double and three-cylinders, compound, and triple 

 expansion engine. These outline diagrams are exceed- 

 ingly clear. Other illustrations are sectioned and finished 

 in such a way so as to render the details evident. All 

 these points add considerably to the value of the work 

 as a text-book for senior students in our technical 

 colleges ; for draughtsmen engaged in stationary engine 

 work, and for mechanic engineers generally. 



N. J. LOCKYER. 



Heat. By Mark R. Wright. (London : Longmans, 



Green, and Co., 1893.) 

 "Of making many books there is no end, and much 

 study is a weariness of the flesh." Truer words than 

 these were never written, and they are specially appli- 

 cable at the present day. Mr. Wright's addition to the 

 literature of science is avowedly " written specially to meet 

 the requirements of the Advanced Stage of Heat as laid 

 down in the Syllabus of the Directory of the Science 

 and Art Department." To say that the author has satis- 

 factorily accomplished his design is, therefore, to give 

 him praise. In an examinational text-book there is 

 little, if any, scope for originality, and all the author can 

 do is to develop new methods of treatment. This Mr. 

 Wright has done to a small extent, and he seems to be 

 in touch with the work that has been done in connection 

 with his subject during the last few years. Of the 136 

 illustrations only thirty-five have been drawn for the book : 

 the majority of the others being of the well-known stock 

 character, which have " had their day " and should have 

 " ceased to be " long ago. 



NO. 1240, VOL. 48] 



LETTERS TO THE EDITOR. 



[ Tht Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.}^ 



Slickensides. 



In the account of M. Daubree's experiments on the geological 

 work of high-pressure gas (Nature, July 6, p. 228), the follow- 

 ing sentence occurs : — " In any case it is perhaps a little diffi- 

 cult to understand how a single movement of one rock surface 

 over another . . . could produce anything like a perfect 

 polish." 



This recalls to my mind a freshly-made fault I examined in 

 1890, in a pit at Longcliff, Derbyshire. The rock was a moist, 

 sandy fireclay or gannister ; an area of about 80 feet square, 

 lying on a slope of 35°, had slid down some 3 or 4 feet. The 

 operations at the foot of the slope removed the support of the 

 mass of rock above the sliding plane, and shortly afterwards it 

 split across the middle, and the lower portion moved about 



:^^ 



Diagram of fault at Longcliff Clay Pit. — A, Mass that slid down 4 ft. and 

 then stopped : u, portion of A that slid 4 ft. further ; c, Slickenside surface ; 

 D, fault or sliding plane. 



3 feet further down, disclosing in the gap thus made the surface 

 of the stationary rock. This surface exhibited every appearance 

 of a typical slickenside ; it was highly polished, striated, and 

 even blackened, though the clay itself is cream-coloured. The 

 striations corresponded with the direction of the movement, 

 which had been a simple downward one. 



Some slickensides may possibly be explained by reference to 

 the action of high-pressure gas, but here at Longcliff was an 

 unmistakable example of one caused by a "single movement of 

 one rock surface over another," and it is very probable that the 

 majority of ordinary slickensides have had a similar origin. 



Mile End Road, London E., July I2. J. Allen Howe. 



Potstonea found near Seaford. 



Paramoudra or potstones are known to geologists as exist- 

 ing in the chalk strata around Norwich and Belfast, but till 

 lately I had supposed they were confined to those districts. 



Last Whitsuntide, whilst enjoying a ramble along the chalk 

 cliffs, east of Seaford, I was surprised to come across what 

 seemed a real, but unusual potstone, lying among the stones 

 below high-water mark, but which must, presumably, have origi- 

 nally fallen from the chalk above. Although consisting of a mass 

 of chert, instead of pure flint like those near Norwich, in every 

 other respect it resembles them. In form it is a large irregular 

 cylinder and lies on its side, so that the sea water, when the 

 tide rises, flows freely through it. It measures roughly between 

 four and five feet in each direction, and the aperture has a dia- 

 meter of twelve inches. 



The enclosure of several large black flint nodules indicates 

 that this peculiar shaped mass of chert has been formed since 

 the flint itself segregated. 



