September 23, 1922] 



NA TURE 



4i3 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his cortespondents. 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.] 



The Primitive Crust of the Earth. 



Prof. Cole's letter in Nature of July 8, p. 39, con- 

 cerning the earliest known rocks — a group of sedi- 

 ments — and their relation to the ortho-gneisses, will, 

 I think, call forth the sympathy of students of 

 African geology. In 1904-1905 I came to the con- 

 clusion that the para-schists of Southern Nigeria were 

 older than at least the majority of the ortho-gneisses, 

 and searched — without success — for the real founda- 

 tion-stones which received upon their surface these 

 earliest sediments. 



Later I have suggested that the Turoka Series of 

 para-schists of Kenya Colony may prove the oldest 

 rocks of that part of Africa and have thought that, 

 were the main directions of foliation of the ortho- 

 gneiss mapped throughout that continent, we should 

 find, not directions produced by dynamic metamor- 

 phism, but directions indicating the upwelling of 

 granitic bathyliths along zones of failure of the 

 primitive crust, i.e. foliation swirls on a huge scale 

 akin to those produced bv the flood of the Laurentian 

 magma around the disrupted blocks of Grenville 

 sediments. It might be possible to tell what section 

 of the eroded complex was being studied by observa- 

 tions on the frequencv of the occurrence of syn- 

 tectics, the degree of admixture, the proportion of 

 reconstituted sediments to ortho-gneiss, or even by a 

 passage from complete to less complete metamorphism 

 in the invaded rocks. 



The newly established surveys of Uganda, Tan- 

 ganyika Territory and Nyasaland will, one hopes, 

 throw a flood of light upon this fascinating problem 

 of the constitution of the African Archaean. 



John Parkinson. 



Athenaeum Club, S.W. 



Action of Cutting Tools. 



Mr. Mallock's theory of cutting tools (Nature, 

 August 26, p. 277) is extremely illuminating, but the 

 term " Coefficient of Friction " in his final paragraph 

 seems scarcely justified, as it would imply that the 

 shearing force is always in the same direction and 

 thus independent of the material being cut, of the 

 depth of the cut, rate of feed and tool angles. 



In practice even among skilled workers there is 

 considerable variation in the tool angles used, and 

 there is no decided preference by the workman for a 

 tool ground on a fine-grained emery wheel as against 

 one ground on a coarse grindstone ; this on very 

 heavy work where the friction might be important. 

 Moreover, the general variation in practical tool 

 angles seems more related to what may be termed 

 the pliability of the material. Thus, for mild steel 

 and wrought iron, angles of 50° to 55° are common, 

 for cast-steel and cast-iron 6o° to 65 or 70° are the 

 rule, whereas the brass-finisher's tools are almost flat 

 topped with an angle of 8o° to 85°. Copper and 

 aluminium turnings bend very nicely, and thus the 

 sharp tool angle required for them agrees with the 

 pliability theory. 



This idea of pliability is not antagonistic to Mr. 

 Mallock's main argument but strongly in support of 

 it, for when a metal yields pliable turnings, these 



NO. 2760, VOL. I 10] 



turnings slide on the upper face of the tool a great 

 deal more than when they break off short as in a 

 friable material. Thus the curly turnings of a pliable 

 material may exert more frictional force on the upper 

 face of the tool, but not necessarily because the 

 coefficient of friction is higher. Pliable turnings can 

 slide and thus cause friction : brittle turnings break 

 off with very little sliding. This may be seen very 

 clearly in the rough turning of gun barrels. With 

 certain tool angles and not too heavy a cut, the 

 turnings curl off and are hot, whereas with a more 

 obtuse tool and even a heavier cut the metal crumbles 

 off and is probably not so hot. In the latter case the 

 surface turned has a corrugated periphery showing 

 the periodic impulsive friction on the tool face. The 

 friction theory thus leads to a plausible explanation 

 of certain forms of " chatter." If the friction on the 

 tool face fluctuates on account of either vibration or 

 crumbling of the cutting, and if the system has a 

 natural period in tune with what may be termed the 

 crumbling wave-length, resonance occurs. 



But for practical difficulties Mr. Mallock's theory 

 might be of value to investigators of friction, for in 

 no set of actual conditions is it likely that metal slides 

 on metal with more intimate contact thair near the 

 point of a cutting tool. Even with cutting lubricants 

 it is doubtful if any liquid reaches the point of the 

 tool unless there is chattering. 



In attempting any conception of coefficients of 

 friction between the tool and the cuttings, a further 

 difficulty arises which renders the comparison with 

 clean dry surfaces almost impossible. In some 

 circumstances the cutting of metals produces, in 

 addition to the obvious turnings, a fine smooth 

 powder. This is presumably produced by the abrasion 

 of the cutting on the upper face of the tool, and it 

 may be that this smooth powdered metal acts as a 

 lubricant or ball-bearing for the escaping turning. 

 If so, it would be another of Nature's modes of auto- 

 matic alleviation — as tears allay the irritation' of dust 

 in the eyes, and as the skin is cooled by evaporating 

 sweat. H. S. Rowell, 



Director of Research, 



Research Association of British Motor 



and Allied Manufacturers. 



15 Bolton Road, W.4, August 27. 



The Smoke of Cities. 



With reference to Prof. Cohen's article on smoke 

 abatement in Nature of August 26, p. 269, I 

 should be much interested to know why Man- 

 chester smoke is qualitatively so much worse than 

 London smoke. Comparing Guy's Hospital and 

 Gower Street with the University of Manchester — ■ 

 the three places of which I have had sufficient experi- 

 ence to judge — I should judge that the quantity of 

 dirt in one's laboratory is about the same ; at any 

 rate it is not obviously less in London and, so far as 

 I remember, the published measures of atmospheric 

 pollution confirm this impression. But the Man- 

 chester dirt is far more unpleasant and destructive 

 to one's hands, papers, and apparatus. It seems to 

 contain more very fine sticky particles, which get in 

 everywhere and are difficult to clear off : the London 

 dirt is more gritty and granular, makes things dirty 

 enough but is comparatively easily removed. Any 

 one who has spring-cleaned laboratory cupboards in 

 the two places and essayed afterwards to clean them- 

 selves will have realised that the dirts are of quite 

 diverse characters. From what Prof. Cohen says I 

 should judge that London smoke is relatively less 

 domestic in origin than the Manchester product, but 

 it seems difficult to reconcile this with what one 



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