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NATURE 



[July 5, 1877 



geologists of the country as schoolboys, whom he had to 

 drill in the beggarly elements, and divided them into 

 classes according to their acquirements or their aptitude 

 to receive his lessons. He began by enforcing his views 

 as those of Hutton and Playfair, and gradually so identi- 

 fied himself with them that he regarded them and the 

 very words expressive of them as his own property, which 

 no one should claim or touch except in the way he chose 

 to sanction. Peace be with his memory ! He did a good 

 work in his time. Men gladly overlooked his personal 

 failings for tint sound sense so often underlying his self- 

 asserting remarks about geological forces which had not 

 been adequately understood in this country when he 

 began his crusade of " Rain and Rivers." The present 

 volume is a reprint of his letters on all manner of subjects, 

 written at different times from 1859 to 1875. But surely its 

 publication was not needed for the scientific reputation of 

 the author. The letters are given as they originally 

 appeared, full of references to passing incidents, and to 

 letters by other writers, which of course are not inserted, 

 but without which Col. Greenwood's diatribes are often 

 unintelligible. There is no attempt at editing. The title 

 of the book also is misleading. Instead of a treatise on 

 river terraces, it is a medley of clippings from the 

 columns of various periodicals relating to such varied 

 subjects as Spelling, the Possessive Augment, Source of 

 the Nile, Glen Roy, a Horse-Chestnut Tree, Rain and 

 Rivers, Sirloin, Pronunciation of Latin, Lakes with Two 

 Outfalls, a Beech pierced by a Thorn Plant, Origin of the 

 Chesil Bank, &c., &c 



LETTERS TO THE EDITOR 



[ The Editor does not hold hirnself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



The Editor urgently requests correspotidents to keep their letters as 

 short as possible. The pessure on his space is so great that it 

 is impossible otherwise to ensure the app.'arance even of com- 

 7?!unications containing interesting and novel facts.\ 



Tail on Force 



In Prof. Tail's lecture oa "force," which its writer seems to 

 have intended as a model of perspicuity and accuracy, we are 

 told that "we must measure a force by the rate at which it 

 produces change of momentum." Nothing could be clearer or 

 more satisfactory than this statement. Then Prof. Tait proceeds 

 to tell us what force " is," and we read — " Force is the riite of 

 change of momentum " — giving to this word "is" the meaning 

 which the so-called metaphysicians give to it ; and it seems to 

 me that we might jangle over it for ever, without ever knowing 

 whether this latter statement be true or not ; for ahhough we 

 may all agree as to tlie proper measure of a force, it seems to be 

 more difficult to tell what force "is." Po?sibIy we might 

 roughly measure the hunger of a man unjer different circum- 

 stances, by determining the number of pounds of beef he would 

 consume, but it would be hardly warranted to s.iy that hunger 

 " is " a certain rumber of pounds of beef. 



Perhaps it may be advantageous to apply the name force to 

 the thing which we have heretofore called rate of cliange of 

 mamentum due to force, but I cannot imagine how any one can 

 think that a certain "rate of change of momentum" can pro- 

 duce a unit of momentum m a unit of time. Until this shadowy 

 " phantom " called force can be brought a little more sharply 

 into locus, it seems to me that considerations as to what it " is " 

 may profitably be left to those who appear to delight in the 

 obscurity of obscure things — the metaphysicians. 



St. Louis, June 4 Francis E. Nipheu 



P.S. — On showing this note to a friend, my attention was 

 drawn to a note by Robert Napier in Engineering, which makes 

 the present one seem almost superfluous. Remembering, how- 

 ever, the difficulty which I had in understanding these very 

 points on account of the loose way in which they are put in 

 many textdiooks, I feel that too much cannot be done to pre- 

 vent such things from going into text-books in the future. — 

 F. E. N. I 



[Prof Nipher's censure does not apply to my lecture, simply 

 because he fails to remark that I had two objects in view, (i) to 

 point out the sense in which the word force must bi used if we 

 desire to avoid confudon ; (2) to point out that, in all probability, 

 there is no such thin^ as force. Under the first head I of course 

 referred to Newton's "Laws," and in them language is used 

 which at least suggests the objective reality of force as the cause 

 of change of motion. We must take Newton as we find him. 

 But there is no inconsistency in afterwards proceeding to give 

 reasons which appear conclusive against the objective reality of 

 force, 



With some of Prof Nipher's other remarks I can cordially 

 agree. Since my lecture was published I have been in almost 

 daily receipt of passages containing errors amounting often to 

 the wildest al)surdities, due to misuse of the term force. The 

 latest to which my attention has thus been called is in the Corn- 

 hill Alagazine for June. Here the non-scientific public is 

 gravely told that "what mathem^ilicians call the moving force 

 extrted by the earth on the moon is eighty-one times greater than 

 the corresponding force exerted by the tnoon on the earth." — 

 P. G. T.] 



On Time 



If I understand V. K. Julius's letter in NATURE, vol. xiv. 

 p. 122, on the measurement of time, it may be thus summa- 

 rised : — 



As equal times, unlike equal linear magnitudes, cannot be 

 brought alongside of each other, their equality can be ascertained 

 only by means of velocities. (This wiU not be disputed.) We 

 define those times as equal during which the same space is tra- 

 versed by equal velocities ; but the postulate that a velocity, e.g. 

 that of the earth's rotation, contmues unchanged, is arbitrary, 

 incapable of proof, and juitified only by practical convenience. 



It seems to me, on the contrary, that the postulate is not 

 necessarily arbitrary, but may bs absolutely justified by facts. 

 The best case to put is that of the pendulum, which, according 

 to Sir William Thomson, is probably capable of measuring time 

 with greater accuracy than the motion of the earth itself. If we 

 assert that equal forces acting through equal spaces produce 

 equal velocities (and this is rather a definition than an axiom), 

 then the assumption of the equal velocity of all the pendulum's 

 strokes postulates nothing except that the force of gravitation 

 continues unchanged. I admit that I see no way of proving 

 this, but it may be safely assumed in the absence of any knov/u 

 or probable cause of change. Joseph John Murphy 



Old Forge, Dunmurry, Co. Antrim, June 19 



The Antiquity of Man 



I HAVE no desire to enter into the controversy respecting the 

 age of the paL-eolithic implements found in brick earth near 

 Brandon, by Mr. Skertchly. I had the [great pleasure of going 

 over some part of the ground with him and Mr. Belt in Novem- 

 ber last. But what I saw then was not sufiicient for me to 

 make up my mind upon the question. Of course Mr. Skertchly, 

 with his immense experience, has far more evidence m his 

 repertorium than a cursory visit could alford to me. 



My object in writing this letter is to point out that, if it 

 should eventually be proved that a glaciation of the surface has 

 occurred in East Anglia subsequent to its human occupation, 

 but one which is not identical with, but posterior to, that glacia- 

 tion (whether land or marine) which deposited the great chalky 

 boulder-clay, then this is no more than I put forward many years 

 ago in my papers on the "warp " (Geol. Journal, 1866) ; and 

 on the "glacial origin of denudation" (Geol. Mag., 1866) ; and 

 on the " denudations of Norfolk " (Gcol. Mag., 1S6S). 



I think this is the direction to which the course of opinion 

 appears to be tending, and I ask you kindly to bring under the 

 notice of the younger generation of geologists the speculations of 

 an elder brother. 



I call the product of this supposed glaciation "trail." The 

 more orthodox, I believe, consider it to be " rain wash," and I 

 had hoped th.at some competent writer would have thought me 

 worth confuting. But none has done so. I have reason to think 

 that one of your correspondents did actually put down as boulder 

 clay this very deposir, at one of the most important sections 

 which I saw near Brandon. O. Fisher 



Harlton, Cambridge, June 28 



