Maech 16, 1917] 



SCIENCE 



259 



memorial to men fallen in the war. The cost 

 of the scheme will be £150,000. 



We learn from Nature that Mr. D. M. 

 Forbes, who died on December 13 last, has 

 bequeathed to the University of Edinburgh his 

 books relating to the Philippine Islands, and 

 the residue of his property, which, with the 

 property abroad, will amount, it is under- 

 stood, to about £100,000, for the purposes of 

 education. 



The council of the University of Liverpool 

 has recently received from a donor who de- 

 sires to remain anonymous a sum of money 

 sufficient partially to endow a chair of geog- 

 raphy. The council has felt justified, under 

 the circumstances, in establishing the chair, 

 and a professor will be appointed in a few 

 weeks. 



Walter A. Patrick, Ph.D. (Gottingen), of 

 Syracuse University, has been appointed asso- 

 ciate in chemistry at the Johns Hopkins Uni- 

 versity. After two years spent in physical 

 chemical research at the Massachusetts Insti- 

 tute of Technology, Dr. Patrick spent a year 

 with Preundlich, at Braunschweig, a year 

 with Zsigmondy at Gottingen and a year as 

 private assistant to Professor Donnan, at Uni- 

 versity College, London. 



Dr. Howard T. Karsner, professor of 

 pathology, has been elected secretary of the 

 school of medicine. Western Eeserve Univer- 

 sity. Dr. Eussell J. Collins, demonstrator of 

 pharmacology, has resigned because of ill 

 health. 



The University of Cambridge wiU here- 

 after grant the degrees of master of letters 

 and master of science for somewhat the same 

 qualifications as the doctorate of philosophy 

 is awarded by German and American univer- 

 sities. A proposal that the degree of doctor of 

 philosophy be awarded was rejected. 



DISCUSSION AND CORRESPONDENCE 



WHEN IS A FORCE NOT A FORCE? 



The article by Mr. Gordon S. Fulcher in 

 Science for ISTovember 24, 1916, calls atten- 

 tion in a most timely way to the vagueness 

 which characterizes the discussion of the idea 



of force in most of our modern text-books of 

 physics, but does not make clear just how he 

 would " use force only in the single definite 

 sense implied in the laws of motion." Let us 

 take the following simple case: a ball is at- 

 tached to a rubber cord, say three feet in 

 length. A person grasps the ball and pulls it 

 with a force F, stretching the rubber cord to 

 a length of five feet. The strain in the cord 

 is produced by the two forces +F and — F 

 acting at the ends of the cord. The third law 

 of motion covers the case. 



ISTow suppose the person swings the ball 

 around his head at the end of the rubber cord 

 until its velocity is great enough to stretch 

 the cord again to a length of five feet. The 

 stress in the cord is the same as before. The 

 question is, what is the nature of the " reac- 

 tion " which the ball is exerting on the cord 

 to stretch it ? It is certainly a " force " F 

 (otherwise the cord would not be stretched as 

 it is), and it is in one sense balancing the 

 equal " action " of the cord on the revolving 

 ball, which we know as centripetal force. Is 

 the " centrifugal force " (inertia-reaction of 

 the ball) in this case a force in the " single 

 definite sense implied in the laws of motion " ? 

 Does the third law also cover this case? 



We usually define force as that which pro- 

 duces (or tends to produce) a change in the con- 

 dition of motion of a mass, either in magni- 

 tude or in direction. Certainly inertia-reaction 

 might not come under this definition, but un- 

 doubtedly our definitions of force are intended 

 to describe ordinary forces — mechanical, mag- 

 netic, electrical, etc. — which can do three 

 things : (1) oppose other forces, (2) produce 

 acceleration, and (3) produce deceleration. 

 The force called friction can do only the first 

 and third of these things; it can not produce 

 acceleration (except in indirect ways). Is 

 friction a force in good and regular standing 

 in the "single definite sense implied in the 

 laws of motion " ? 



Inertia-reaction can do only the first of these 

 three things; it can not, by its very nature, 

 produce either acceleration or deceleration. 

 And yet even while it is opposing the restoring 

 stress in the rubber cord mentioned above, we 



