144 



NA TURE 



[Dec. 16, 1880 



they too have intelligence, and that they exhibit at times 

 a very respectable amount of common sense. The stories 

 about them are strictly true, and from their very nature 

 strictly new. But the volume tells also of many a two- 

 footed friend, and the last few chapters almost e.xclusively 

 treat of the fishes of the coast. There is much in this 

 portion of the volume of interest to the scientific worker ; 

 there is much in every part of it to make it of value to 

 those who care to learn something of the habits of Tas- 

 manian beasts, birds, and fishes. One feature of the 

 volume must be specially noticed — the eight coloured 

 drawings, excellently chromolithographed from the 

 water-colour drawings of the author. From a personal 

 knowledge of the splendid colouring often present in 

 freshly-caught tropical fishes, these plates are, we should 

 say, by no means too brilliant. Four are devoted to 

 some of the strange, wondrously-coloured fishes, and 

 four to flowers, fruits, and insects. 



This volume would be an excellent and not over- 

 expensive Christmas present, which may lie on any table 

 however select, and be read by any person however 

 critical. 



LETTERS TO THE EDITOR 



[ Tlie Editor does not hold himself responsihle for opinions expressed 

 by his corresponds nls. 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 correspondents to keep their letters as 

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

 is impossible othenoise to ensure the appearance even of com- 

 munications containing intei-estinz end nerve! facts.'] 



Mr. Spencer and Prof. Tail 



Prof. Tait's explanation itself shows that the word commonly 

 applied to product.s of imagination, was applicable to his slate- 

 nients; for the only justification he assigns is that he "assumed," 

 that is to say, imagined, that his substitution of "definition " for 

 "formula" must have been the ground of offence. How inade- 

 quate a plea this is, will be seen on re-reading the questions I put, 

 which were these : — 



"He [Prof Tait] says lliat because he has used the word 

 ' definition ' instead of ' formula,' he has incurred my ' sore 

 displeasure and grave censure.' In what place have I expressed 

 or implied displeasure or censure in relation to this substitution 

 of terms ? Alleging that I liave an obvious motive for calling it 

 a 'formula,' he says I am 'indignant at its being called a 

 definition.' I wish to see the words in which I have expressed 

 my indignation ; and shall be glad if Prof. Tait will quote them. 

 He says : — ' It seems I should have called him the discoverer of 

 the formula ! ' instead of 'the inventor of the definition.' Will 

 he oblige me by pointing out where I have used either the one 

 phrase or the other? " 



Every reader would infer that, for these specific statemcn's 

 made by Prof. Tait, there are specific foundation?, which could 

 be named when asked for. He does not name them, for the 

 suflicieut reason that they do not exist. Unable, as he says, to 

 see in the passages I quoted from him, anything else to call for 

 "censure" (a strange inability!), he "of course " assumed that 

 this change of terms was the ground of censure. And the 

 assumption thus made, is the only warrant he assigns for these 

 positive assertions. 



This is not all, however. Prof. Tait says: — "I could not 

 have ventured to suppose that Mr. Spencer ^ did not even hno~w 

 that he was in the habit of saying formula rather than definition! 

 This naive confession cannot but be correct." Of Prof Tait's 

 motive for putting this statement of mine in italics and calling 

 it naive, the reader may judge for himself. Plow entirely correct 

 it is, and how well Prof. Tait might have "ventured to suppose" 

 it, will quickly appear. For there is proof that I am not in the 

 habit of always saying formula rather than definition ; and Prof 

 Tait had the proof before him. In the note on page 565 of the 

 Appendix forming the pamphlet in question — a page which Prof. 

 Tait must have read, since it concerns Mr. Kirkman and himself 

 — I have used the w'ord "definition." So that not only had 

 Prof. Tait no evidence on which to base his distinc.t statements. 



hut there was under his eyes positive evidence which negatived 

 them. 



Very possibly it will be said that the question about my uses 

 of these words is a trivial one. But this is not the question. 

 The question is whether it is allowable to make an opponent 

 look absurd by ascribing to him, in a quite positive way, things 

 which he has neither said nor implied ; and that, too, when he 

 has implied the contrary. Herbert Spencer 



Criterion of Reality 



Will you kindly allow a learner to ask for the criterion 

 according to which Kinetic Energy and Work are real things, 

 while Momentum and Force are unreal ? Prof. Tait says J^/w" 

 and wh express real things, but mv and %ut unrealities (Nature, 

 vol xxiii. p. 82). 



\iwt be "as unreal as is the product of a quart into an acre," 

 how is it that wh is real ? The illustration of quart and acre is 

 as applicable or inapplicable to the one as to the other. In 

 both cases we take the product of two numbers, not two con- 

 crete magnitudes, which of course it would be absurd to speak 

 of multiplying together. In one ca=e the product is the number 

 of units of Momentum, in the other case it is the number of 

 units of Kinetic Energy. If it be said tliat a thing is real if its 

 quantity cannot be altered, and vice versa, A\hy is mi^ said to 

 be real, and mv unreal ? They vanish together. When Prof. 

 Tait asserts " there is no such thing as Force," " it is merely a 

 convenient expression for a certain rate'" (Nature, vol. xiv. 

 p. 459), he seems, if I may venture to say so, to confound the 

 measure of Force with Force itself, and to lay himself open to 

 Mr. Spencer's comment that " a relation changes the state of a 

 body." Certainly niv is not a thing, but neither is mv" a thing : 

 yet the latter is the measure of something which Prof. Tait 

 asserts to be " as real as matter itself " : why is not that of 

 which the former is the measure equally real ? E. G. 



Bardsea 



[What Prof Tait asserts may be correct or not, but it is self- 

 consistent. He asserts in his lecture on " Force " (Nature, 

 vol. xiv. p. 462) that matter and energy must be looked on as 

 real things, because we cannot change the amount of either. Such 

 expressions as hnv'^, and wh, are to be considered as wholes, not 

 as products of two or more factors. This separation into factors 

 (where one is mv, or w, for instance) he asserts to be a relic of 

 the old erroneous belief in the trustworthiness of the impressions 

 made on the " muscular " sense. — Ed.] 



Landslips 

 In Nature, vol. xxii. p. 560, I pointed out that landslips 

 often occurred in the Salt Districts. I did not then expect that 

 I should so soon be able to refer again to the subject ; but on 

 December 6, at an early hour in the morning, one of the largest 

 subsidences and landslips ever known in Cheshire occurred. I 

 pointed out that whenever fresh water reaches the rock salt it 

 dissolves it. In certain districts in the immediate neighbourhood 

 of Northwich the ground is completely honeycombed with rock- 

 salt mines that had been worked out and abandoned. Into many 

 of these fresh water had penetrated, and had become by solution 

 strong brine. This brine has of late been extensively pumped 

 up, and many of these extensive cavities had become nearly 

 empty. The thin crust of rock salt forming the roof of these 

 old mines had become gradually thinner, owing to its solution by 

 water, and on Monday morning the roof of one pit gave way, 

 and let the superincumbent earth down into the mine, rifting and 

 opening the ground to the surface. The surface rift passed 

 across the bed of a large brook, and the water of the brook ran 

 through the crevice into the mines below. In a short time the 

 water made a more extensive cavity, and as the brook was cut in 

 two about 200 yards above its entrance into a large lake that was 

 drained by the Weaver River, the water in the lower portion of 

 the brook and of the lake, as well as of the Weaver, commenced 

 to return and ran down the enlarged cavity. For four or five 

 hours this return stream increased in velocity, pouring down the 

 crater-like hole. Notwithstanding the water of the brook and 

 the return water, as well as a large body of water from another 

 small lake entering this cavity, the water standing in the funnel- 

 shaped hole gradually lowered. The velocity of both portions 

 of the brook increased, and such was the force of the water that 

 the bottom of the brook for 100 yards was scooped out from 

 2 feet in depth to 10 feet, and the banks were washed away, 



