January, 1907.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



19 



CORRESPONDENCE. 



Brakes. 



To the Editors of " Knowledge & Scientific News." 



Sirs,— In an article on brakes in the November number 

 of " Knowledge & Scientific News " I notice the sentence, 

 " This argument may serve as an illustration of the danger 

 attending arguments based on the doctrine of energy." 



It seems to me that this stricture on the value of methods 

 based on the doctrine of energy is wholly unwarranted. 

 The fallacv lay in the illogical assumption that two quanti- 

 ties are necessarily greater than one. 



Further, the action of a brake does not necessarily produce 

 sliding motion. 



In the following analysis I use P lo represent the brake 

 couple produced bv friction at the brake itself and at the 

 axle, and O to represent the moment around the axis of 

 the frictional force called into play at the rail. 



Then if P is greater than the maximum value of O, the 

 wheels will lock, and the retarding force of the rails is a 

 maximum. 



If P is less than this value, the wheels will continue to 

 revolve and onlv such a force will be called into play at the 

 rails as to make O equal to P. 



Hence we see that the greatest retarding force is called 

 into plav when the wheels are locked. 



Further, since these couples P and Q are very large, and 

 the vis rira of the wheels small, we see that the motion is 

 (except for minute intervals of time) either pure rolling or 

 pure sliding, the sliding being the more effective. 



If the lock is only just effective, and a rough portion of 

 the rail be reached, then the sliding might be converted into 

 rolling, which would continue until the roughness was 

 passed. Sliding would again intervene, but on a different 

 portion of the rim. 



These interchanges of pure rolling and pure sliding might 

 be very frequent, and so prevent the formation of a " flat." 



In this way only, then, can " mixed " rolling and sliding 

 be more effective than a complete " lock." 



Yours trulv, 



H. S. H. 



[The wrong (but now discarded) conclusion, based on an energy 

 argument, is one which several of my friends came to as well as 

 myself ; that is the only reason I think such an argument to be 

 attended with danger. I am glad that " H. S. H." agrees with me 

 as to the source of error. I am equally glad that he has not 

 attempted to employ an argument from energy in the rest of his 

 letter. I am also in geneial agreement with him (though he does not 

 seem to think this) as to the suggested mode in which mixed 

 rolling and sliding occurs. Into the details of this I did not enter 

 in my note, which was probablv too short except to set others 

 thinking.] '_ A.W.P. 



Basaltic Columns. 



To the Editors of " Knowledge & Scientific News." 

 .Sirs, — Mr. Martin, in his interesting paper on the Giant's 

 Causeway, does not allude to a possible concrcfinnary origin 

 of the blocks. In a " Grotto " I once saw in Germany, but 

 cannot recall the place, the basaltic pillars were composed of 

 balls in vertical rows. The blocks had weathered into that 

 form. Mr. Martin alludes to the " cup and ball " joints. 

 If, as a hypothesis only, we assume centres of contraction, 

 due to a concretionary process, to arise along a vertical 

 line, such would account for hexagonal pillars with cup 

 and ball joints, the blocks being resolvable into balls by 

 weathering. 



If one taps a thin biscuit in the middle on a table, it 

 almost invariably breaks along three lines approximately at 

 120° apart; so contraction round spheres in contact would 

 produce the hexagon, as equal spheres touch at six points on 

 the same plane. 



Concretionary processes are, of course, extremely common. 

 Thus they used to be dredged ofT Felixstowe for making 

 Roman cement, being derived from the London clay. 

 Pisolitic limestone of the Oolites and botryoidal (magnesian) 

 limestone of the Permians are other instances. I have found 

 them in a metamorphic st.-ite in Jersey, so that they mav 

 equally well occur in basalt. 



As to the caiises, I should much like to hear what present- 

 dav geologists have to say about the origin of concretions. 

 In the pisolite there is usually a fragment of some organism 

 in the middle, but why the molecules of carbonate of lime 

 a:e attracted to it, I do not know. In basalt and meta- 

 morphic rocks some other centres of attraction must exist. 



In the basalt two causes would have to be discovered — 

 (1) Whv there should be centres of aggregation producing 

 contraction at all ; and (2) \\'hy they should lie on one and 

 the same vertical line. 



There is a basaltic quarn,' a few miles from Coblentz on 

 the east side of the Rhine. Short pillars (about 6 inches 

 diameter) are extracted and used as posts to carr>- a chain 

 bv the side of the road running by the Rhine. In these it 

 looks as if the "centre-; of concretion " had become a strainhi 

 line, so that the basalt never broke up into blocks at all. 



Faithfully yours, 



Leamington. George Henslow. 



Lightning Flashes from Earth to Cloud.' 



To the Editors of " Knowledge I'i: .S< ientifk News." 



Dear Sirs,- Apropos of Dr. Lockyer's interesting article 

 upon Lightning, I might mention that during the summer 

 of 1005 a short storm passed over the south-west of London 

 about mid-day. 



While watching this from Clapham Common I saw a 

 flash which, apparentlv, started from earth, travelled verti- 

 callv upwards for a short distance, then turned at a right 

 angle and went due west, finally the streak " swelled out " 

 into a large ball of fire and vanished. 



The thunder which followed in about 10 seconds was 

 quite consistent with the character of flash, viz., a short 

 peal, slight, momentary- modulation, a long peal ending in 

 a loud report like that of a heavy gun. 

 Yours faithfully, 



66, Lillieshall Road, Herbert Wilkins, L.R.C.S.I. 



Clapham Common, S.W. 



REVIEWS OF BOOKS. 



ASTRONOMY. 

 An Atlas of Hindu Astronomy with Key and Notes, by 



Kalinath Mukherji, B..\., B.L. ; price 3 rupees; Popular 

 Hindu Astronomy, Part I., by the same author (Calcutta : 

 Han Press). — The author, a former undergraduate of 

 the Krishnagar College, was inspired among others to take 

 an interest in astronomy by Sir \V. J. Herschel, of the 

 Indian Judicature. The atlas supplies the native student 

 with maps modelled on those with which English readers 

 are familiar, but with the more or less conventional con- 

 stellation figures replaced either by the equivalent from 

 Indian mythology, or by difl^erent arrangements which ap- 

 pear to be commonly accepted in India. Some of the 

 changes are interesting, thus, the well-known square in 

 Pegasus becomes a sort of bedstead with turned legs, Orion 

 a deer transfixed with an arrow (the Belt), and Argo an 

 ineffective-looking paddle-steamer. There arc also dia- 

 grams of the planetary system, comets, and eclipses. The 

 letterpress being in Oriental character does not appeal to 

 English readers, only the names of constellations and bright 

 stars being translated. The other work, however, can be 

 used in conjunction with the maps, being to a great extent 

 in English, though bristling with quotations from Indian 

 epics. There are m.any interesting legends connecting the 

 stars with Indian mythology, but very little astronomv as 

 generally understood. As is only to be expected, there are 

 numerous typographical errors, more than a hundred of 

 which are given as errata at the beginning of the little 

 book, but the list is still incomplete. The Slilkv \Yav is 

 likened to a piece of white cloth, worn through in places, 

 twelve feet hroail. The author accepts the translation of 

 \'edic scholars, but not their interpretation, which he con- 

 siders is refuted by looking at the sky. We are not suflfi- 

 ciently familiar with Vedic literature to take sides on this 

 question. He also makes considerable use, for purposes of 

 parallelism, of the constellation studies of Mr. Robert Brown, 

 jun., to whom, as also to Professor Henry Stephen, he 

 owns his indebtedness as to the interpretations, other than 

 Indian, of the star names and figures. He uses numbers 



