458 



NA TURE 



[September 14, 1893 



for the most delicate phenomena of the physical labora- 

 tory, such as Capillarity ; and numbers run very high in 

 ordinary dynamical problems. 



Millions of i^i'/fej' of impulse would be required to flick 

 a sixpence across the counter ; and the answer " millions," 

 which Albert Smith said he received from the stoker when 

 he asked how many degrees of temperature there were in 

 the stoke-hold, would not be wrong if he had asked what 

 pressure the boilers carried ; " fifteen millions '' might be 

 the answer of the scientific stoker of to-day, trained in 

 the use of the C.G.S. system. 



Another banishment from this treatise to be grateful 

 for, is that of " the whole pressure of a fluid on a curved 

 surface.'' 



If, however, this whole pressure is divided by the sur- 

 face, we obtain the average pressure over the surface, a 

 distinct mechanical motion, sometimes useful ; with this 

 resetting the " visionary problems of pure mathematics " 

 on whole pressure might be allowed to survive, as some 

 of them embody elegant geometrical applications. 



Generally throughout the work Mr. Minchin has 

 secured the assistance of his colleague Mr. Stocker, the 

 Professor of Physics, for the experimental illustrations 

 and diagrams, and we meet with many novel and in- 

 genious experiments, for instance in the illustration of 

 Boyle's Law in Fig. 57. 



This gives a flavour of the Physical Laboratory 

 to the book, and not that of the Engineering Theatre, 

 except for the elegant geometrical treatment of the 

 ine of Thrust in a Reservoir Dam. The Hydraulic 

 Press of Fig. 7 could hardly serve to lift a girder of 

 the Britannia Bridge, or squeeze a steel forging with 

 a thrust of thousands of tons. 



The equilibrium and stability of a floating body is 

 illustrated in Fig. 49 by what looks like a champagne 

 cork, and not by the cross-section of an ironclad or 

 Atlantic steamer, with compartments bilged and full of 

 water to illustrate the effect of petroleum or liquid 

 cargo, or the unfortunate capsizing of the Victoria. 



The diagram of a floating body in the ordinary mathe- 

 matical treatise, where it is not like a cinder or a potato, 

 but a vague idea of the cross-section of a ship, has the 

 metacentre placed somewhere up the mast. 



Prof Minchin reduces this metacentric height to more 

 reasonable figures, 5 or 6 feet ; but even this is excessive, 

 as H.M.S. Prince Consort, with a metacentric height of 

 6 feet, was a notorious bad roller ; vessels of the greatest 

 size are plying successfully with a metacentric height of 

 under i foot ; and we read a day or two ago of one of 

 the largest modern steamers becoming unstable when 

 being undocked. 



The question of the stability of a ship involves the two 

 antagonistic qualities of " stiffness " and " steadiness." 



A " steady " vessel has a small initial metacentric 

 height, and " stifi"ness " under sail is secured by making 

 the metacentre rise rapidly as the ship heels. 



The whole theory of the geometry of the ship is one of 

 great mathematical interest ; and the valuable compila- 

 tion of all the best recent work on this subject, made by 

 Sir E. J. Reed in his " Stability of Ships," deserves to be 

 better known among mathematicians. 



Chapter vi., on Gases, is one which will excite great 

 admiration, from the way in which the leading parts of 

 NO. 1246, VOL. 48I 



Thermodynamics are introduced ; the most recent 

 theories have been incorporated and illustrated numeri- 

 cally and experimentally ; here the valuable assistance 

 of Prof Stocker is acknowledged. In this part of 

 the subject we think that a simplification would be 

 effected by pointing out that with the gravitation units 

 employed in §48, the quantity X' in the equation / = ^p 

 is the " height of the homogeneous atmosphere." 



Hydraulic and Pneunatic Machines are carefully 

 described and illustrated in Chapter vii. Fig. 71 of the 

 Fire Engine is curious as illustrating the continuity of 

 mathematical diagrams, as it might have been copied 

 from the one given in Hero's Pneumatics B.C. 120, as in- 

 vented by Ctesibius. 



The hydraulic ram {bMier hydraulique), Fig. 73, is 

 here attributed to Whitehurst, of Derby (1772). This 

 will raise a protest in France, where Montgolfier is 

 considered the inventor ; but, on the other hand, Mr. 

 Minchin gives Mariotte a half share in the discovery 

 of Boyle's law. 



Chapter viii , on " Molecular Forces and Capillarity,'' is 

 very complete but rather formidable, as it does not: shirk 

 the difficult theories of Laplace on Molecular Pressure. 

 The author must utilise in the next edition the scale 

 invented by Mr. C. V. Boys, for drawing with accuracy 

 the various capillary curves. 



In the two hydrodynamical Chapters, ix. and x., there 

 may appear some need for the use of the absolute units j 

 but considering that the motion discussed is due to 

 gravity, the only effect of a change from gravitation to 

 absolute units is to remove g from the denominator of 

 certain terms to the numerator of the remainder in the 

 equations. 



The use of hyperbolic functions would simplify the 

 expressions on the last page of the book, in the discussion 

 of Kelland's state of wave motion. 



Judiciously selected examples are introduced in small 

 sets, to illustrate the principles at easy stages ; these 

 are printed in smaller type, and the book is thereby 

 kept within a handy size ; at the expense, however, of the 

 eyesight of some readers. 



A. G. Greenhilu 



LETTERS TO THE EDITOR. 



[ Ihi Editor does not hold himself responsible for opinions «• 

 pressed by his correspondents. Neither can he uvdertaki 

 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.] 



PalsEozoic Glaciation in the Southern Hemisphere. 



The interest evinced in the above subject in so many quar- 

 ters, and the evident ignorance of what has been done in the 

 matter, is my excuse for asking space for some notes on my per- 

 sonal researches. 



South Africa. — In July, 1872, while journeying ihrough 

 Bushmanland, at Mr. Niekerk's farm " Welgevonden," ne»r 

 Prieska, on the Orange River, I observed extensive accumuU- 

 tions of pebbles and boulders loosely piled, many of ihem 

 striated, scored, and facetted — in fact, unmistakably ice- 

 marked. One of the boulders I took to Cape Town, and de- 

 posited it in the South African Museum. This was the fii»t 

 discovery of glaciation in Cape Colony, and it attracted some 

 attention at the time {vide Cape RIonthly Magazine, &c.). While 

 crossing Bushmanland, the boundaries of this conglomerate 

 were jotted down, and they were delineated on my Sketch 



