79 6 THE POPULAR SCIENCE MONTHLY. 



perceived forces, save as experimentally justified, some validity can be 

 claimed for our judgments respecting unperceived forces, where no 

 experimental justification is possible. 



The peculiarity thus exhibited in Professor Tait's general thinking 

 is exhibited also in some of his thinking on those special topics with 

 which he is directly concerned as a Professor of Physics. An instance 

 was given by Professor Clerk-Maxwell when reviewing, in " Nature," 

 for July 3, 1879, the new edition (1879) of Thomson and Tait's " Treatise 

 on Natural Philosophy." Professor Clerk-Maxwell writes : " Again, 

 at page 222, the capacity of the student is called upon to accept the 

 following statement : ' Matter has an innate power of resisting external 

 influences, so that every body, as far as it can, remains at rest or moves 

 uniformly in a straight line.' Is it a fact that ' matter ' has any power, 

 either innate or acquired, of resisting external influences ? " And, to 

 Professor Clerk-Maxwell's question thus put, the answer of one not hav- 

 ing a like mental peculiarity with Professor Tait must surely be No. 



But the most remarkable example of Professor Tait's mode of 

 thought, as exhibited in his own department, is contained in a lecture 

 which he gave at Glasgow when the British Association last met there 

 (see "Nature," September 21, 1876) a lecture given for the purpose of 

 dispelling certain erroneous conceptions of force commonly entertained. 

 Asking how the word force " is to be correctly used," he says : " Here 

 we can not but consult Newton. The sense in which he uses the word 

 'force,' and therefore the sense in which v,e must continue to use it 

 if we desire to avoid intellectual confusion, will appear clearly from a 

 brief consideration of his simple statement of the laws of motion. 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 stated Thus Professor Tait quotes, 

 and fully approves, that conception of force which regards it as sor~r>- 

 thing 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 right- 

 ly 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 tbese 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 relation (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 producer of phenomena. Whether Professor Tait succeeded 



