March 24, 1887] 



NA TURE 



487 



ing to this definition, the answer is "Zero," but ordinary people 

 would calculate the result in millions of tons, from the data of 

 the mean radius and the mean density. 



Take again a question of a similar nature : " Prove that 288 

 pounds at the pole weigh the same as 289 pounds at the equator." 

 To realise this question we must imagine a balance constructed 

 of which the arm is curved into a quadrant of the earth, reach- 

 ing along a meridian from the pole to the equator, and supported 

 by a fulcrum in latitude 45' ; then 288 pounds at the pole will 

 equilibrate 289 pounds at the other end of the balance at the 

 equator. Without requiring a balance with so long an arm, we 

 can have 289 pounds at the bottom of the shaft of a mine weigh- 

 ing the same as 288 pounds at the surface, provided the shaft is 

 of sufficient depth. 



Some years ago, being troubled myself with this confusion of 

 language, I wrote to Prof. Ma.xwellto ask him for a good illus- 

 trative example of the correct and incorrect use of the word 

 "weight," and received the following characteristic reply on a 

 postcard: — "Compare .St. John xix. 39, io-e! Alrpas kxwT&v, 

 with the -X.V. (authorised version), and keep to the original 

 Greek. ' The translation in the authorised version is "about a 

 hundred pounds ■weight." 



Here we see that Maxwell recognised the ambiguous nature of 

 the word "weight," and advised its omission wherever possible ; 

 but the exigencies of language compel us to use it ; and in fact 

 we shall generally find writei^, even after the above incorrect 

 definition of weight, proceed subsequently to use the word in its 

 ordinary meaning of daily life. 



I wish to repeat that writers on dynamics only create confusion 

 in appropriating the word " weight '' to the sense of the force of 

 attraction of the earth on a body, as we never speak of "a force 

 -•feigning io many pounds " ; and I wish to support the language 

 in ordinary use by engineers and practical men as perfectly 

 correct in using the words "pound" or "ton" side by side in 

 two senses, first as meaning the weight (or mass) of a body, and 

 secondly as meaning the force with which the body is attracted 

 by the earth ; one being sometimes distinguished as a pound 

 weight, and the other as a pound force. 



If we use Prof. James Thomson's admirable word " poundal " 

 for the British absolute unit of force, this slight confusion of 

 terms will disappear, although engineers will still continue to 

 think in gravitation units of force, as gravity is the one universal 

 force from which there is no escape ; and I fear it will be im- 

 possible ever to persuade them to think in C.G.S. units like the 

 centimetre, gramme, dyne, erg, &c., which, though admirably 

 adapted for the minute measurements of experiments in physics, 

 arc unsuitable for large magnitudes. 



In conclusion, let the equation W = .li^ be dismissed from the 

 text-books, as leading to statements such as " The mass of a body 



IV 

 weighing fK pounds is — ;" the true equivalent equation being 



I'f = ''/, and therefore unnecessary ; and with it let the con- 

 fusing "astronomical unit of mass" disappear, and introduce 

 instead the "constant of gravitation" in our equations. Let 

 us also recognise that the primary idea of " weight " is the same 

 as " mass," and fomi our dynamical definitions on the usages 

 of ordinary language. A. G. Greenhill 



Woolwich, February 28 



Mr. Herbert Spencer's Definition of Life 

 I HAVE read with much interest the report in Nature of 

 Prof. Judd's address to the Geological Society, in which he 

 attempts to show that Mr. Herbert Spencer's definition of life is 

 not restricted to those cases only which display the ordinarily 

 acknowledged characteristics of vitality ; a certain correspond- 

 ence between internal and external changes being displayed l)y 

 minerals. 



I write to draw attention to what I think tends to show 

 :hat the mass of evidence brought forward really tells /« favour 

 of the definition ; bearing in mind th.at the hypothesis of evolu- 

 tioii " implies insensible modifications and gradual transitions, 

 which render definition difficult— which make it impossible 

 to separate absolutely the phases of organisation from one 

 another" ("Principles of Biology," vol. ii. p. 10), and that 

 consequently there can be no "absolute" commencement of 

 life. 



The fact, treated by Mr. Spencer when seeking a definition 

 of life, that there is a correspondence between life and its cir- 



cumstances gives the clue showing us that the "vitality of 

 minerals " is a misnomer ; a fallacy he himself exposes when he 

 treats of the internal actions — the feathery crystallisation — dis- 

 played by the misnamed storm glass in correspondence with 

 external changes. Using his own words, we see that : — 



"Subtle as is the dependence of each internal upon each 

 external change, the connection between them does not, in the 

 abstract, differ from the connection between the motion of a straw 

 and the motion of the wind that disturbs it. In either case a 

 change produces a change, and there it ends. The alteration 

 wrought by some environing agency on an inanimate object, does 

 not tend to induce in it a secondary alteration, that anticipates 

 some secondary alteration in the environment. But in every 

 living body [in a living body, mark !] there is a tendency 

 towards secondary alterations of this nature ; and it is in their 

 production that the correspondence consists. The diliference 

 may be best expressed by symbols. Let A be a change in the 

 environment ; and B some resulting change in an inorganic 

 mass. Then A having produced B, the action ceases. Though 

 the change A in the environment, is followed by some consequent 

 change a in it ; no parallel sequence in the inorganic mass 

 simultaneously generates in it some change b that has reference 

 to the change a. But if we take a living body of the requisite 

 organisation, and let the change A impress on it some change 

 C ; then, while in the environment A is occasionally «, in the 

 living body C will be occasioning c : of which a and c will show 



a certain concord in time, place, or intensity " (vol. i. 



p. 78). 



" That the word correspondence will not include, without 

 straining, the various relations to be expressed by it," is best 

 met by the reply " that we have no word sufficiently general to 

 comprehend all forms of this relation between the organism and 

 its medium, and yet sufficiently specific to convey an adequate 

 idea of the relation ; . . . The fact to be expressed in all cases, 

 \', that certain changes, continuous or discontinuous, in the 

 organism, are connected after such a manner that, in their 

 amounts, or variations, or periods of occurrence, or modes of 

 succession, they have a reference to external actions, constant or 

 serial, actual or potential — a reference such that a definite rela- 

 tion among any members of the one group, implies a definite 

 relation among certain members of the other group ; and the word 

 correspondence appears the best fitted to express this fact." (vol. 

 i. p. 79). 



In deer-stalking we see a realisation of these symbols. In the 

 deer the primary internal change — the perception of odour, or, 

 as I believe it is called, " winding " — is followed by that second- 

 ary internal change which induces a desire to increase the 

 distance between the living organism and the inferred source of 

 danger, a change differing not only in degree, but in kind, 

 differing toto civlo from any of those actions which take place 

 in minerals and crystals. 



That the address contains many valuable facts furthering not 

 only Mr. Spencer's view of life, but also his views of evolution, 

 becomes apparent when we consider how it carries out and 

 develops these ideas to an extent which would have been im- 

 possible at the time when the "Principles of Biology" were 

 first published, now twenty years since. I say "furthering," 

 for I wish now to touch upon a very important point, which I 

 cannot but think has been much enlarged and amplified by Prof. 

 Judd. It is to the much more expanded meaning which can 

 now be attached to the fact that the degree of life varies as the 

 degree of correspondence betioeen internal and external relations. 



For the correspondence displayed by a crystal or mineral 

 is shown to be of a very much lower degree than that 

 displayed by the simplest plant or animal. These latter 

 present correspondences of greater complexity, greater rapidity, 

 and greater length in the series of them than the former, 

 which, during its long "millions of years," can respond only 

 to the two or three forms of molar and molecular forces 

 ■alluded to. The changes in the mineral simply respond to 

 changes in the environment ; whereas in an organism it is a 

 relation between changes in it which responds to a relation 

 between changes in the environment. 



Churchfield, Edgb.oston F. Howard Collins 



An Equatorial Zone of almost Perpttual Electrical 

 Discharge 



The recent reference in your columns to Edlung's theory of 

 the aurora borealis, recalls a very curious observation that I 



