APPENDIX. 577 



The first of these laws is : Every body continues in its state of rest or of 

 uniform motion in a straight line, except in so far as it is compelled by 

 impressed forces to change that state" 



Thus Professor Tait quotes, and fully approves, that concep- 

 tion of force which regards it as something which changes 

 the state of a body. Later on in the course of his lecture, after 

 variously setting forth his views of how force is rightly to be 

 conceived, he says " force is the rate at which an agent does 

 work per unit of length." Now let us compare these two 

 definitions of force. It is first, on the authority of Newton 

 emphatically endorsed, said to be that which changes the state 

 of a body. Then it is said to be the rate at which an agent 

 does work (doing work being equivalent to changing a body's 

 state). In the one case, therefore, force itself is the agent 

 which does the work or changes the state; in the other case, 

 force is the rate at which some other agent does the work or 

 changes the state. How are these statements to be reconciled? 

 Otherwise put the difficulty stands thus: force is that which 

 changes the state of a body; force is a rate, and a rate is a re- 

 lation (as between time and distance, interest and capital); 

 therefore a relation changes the state of a body. A relation 

 is no longer a nexus among phenomena, but becomes a pro- 

 ducer of phenomena. Whether Professor Tait succeeded in 

 dispelling " the wide-spread ignorance as to some of the most 

 important elementary principles of physics" whether his 

 audience went away with clear ideas of the " much abused 

 and misunderstood term " force, the report does not tell us. 



Let us pass now from these illustrations of Professor Tait's 

 judgment as exhibited in his special department, to the con- 

 sideration of his judgment on a wider question here before 

 us the formula of Evolution. In Nature for July 17, 1879, 

 while reviewing Sir Edmund Beckett's Origin of the Laws of 

 Nature and praising it, he says of the author: 



" He follows in fact, in his own way. the hint given by a great mathe- 

 matician (Kirkman) who made the following exquisite translation of a 

 well-known definition : Evolution is a change from an indefinite, inco- 

 herent, homogeneity to a definite, coherent, heterogeneity, through con- 

 tinuous differentiations and integrations.* 



[Translation into plain English.'] Evolution is a change from a no- 

 howish, untalkaboutable, all-alikeness, to a soraehowish and in-general- 

 talkaboutable not-all-alikeness, by continuous somethingelseifications, 

 and sticktogetherations." 



* A conscientious critic usually consults the latest edition of the work 

 he criticizes, so that the author may have the benefit of any corrections 

 or alterations he has made. Apparently Mr. Kirkman does not think 



