260 



SCIENCE 



[N. S. Vol. XLV. No. 1159 



call the force exerted by the cord, because it 

 produces centripetal acceleration, an " unbal- 

 anced force." 



What is the average student to make of it 

 when he is told in one of our best texts that 

 " force is exerted only while the motion is 

 changing," and yet on the next page reads " a 

 locomotive pulling a train with uniform veloc- 

 ity along a level track exerts force" on the 

 train ? 



Or when in another text he is told that to 

 every action there is always an equal and con- 

 trary action, and is then informed that an 

 unbalanced force acting on a mass produces 

 acceleration ? 



Or when he reads in one of the very best of 

 our first-year texts that " forces always occur 

 in pairs, one of the pair being equal and oppo- 

 site to the other," and yet is told a little 

 farther on that " by an unbalanced force we 

 mean more push or pull in one direction than 

 the other"? 



Why can not we frankly admit that inertia- 

 reaction acts in one respect like a force, and 

 is actually a kind of force, even if we con- 

 tinue to use the term " unbalanced force " in 

 the sense of a force opposed only hy inertia- 

 reaction? A porter pushing a heavily laden 

 truck at uniform speed feels the reaction due 

 to friction; if the friction suddenly vanished, 

 he would feel the reaction due to the inertia 

 of the truck. He might not know the differ- 

 ence, except that in the latter case he would 

 succeed in giving the truck a small accelera- 

 tion. But he would doubtless be greatly aston- 

 ished to learn that in the first case his push 

 was balanced by an equal counter-force, while 

 in the second case his push was an " un- 

 balanced force " I 



The writer finds that the clearest (if some- 

 what tautological) definition of force for the 

 average student is that which produces motion, 

 change of motion, compression and tension. 

 Under this definition the inertia-reaction of 

 the ball revolving at the end of the rubber 

 cord is a force, because it produces tension 

 in the cord. 



Inertia-reaction can oppose other forces, it 

 can in that sense balance them, but it can not 



hold them in equilibrium, because a force op- 

 posed only by inertia-reaction always produces 

 acceleration, positive or negative, and may for 

 that reason be called an unbalanced force. 



If the drawbar pull of a locomotive is 1,000 

 pounds, and the sum of the opposing forces due 

 to the friction of the wheels, journals, wind, 

 etc., is 600 pounds, we may say that the un- 

 balanced force exerted by the engine on the 

 train is 400 pounds, and this produces accelera- 

 tion. But the pull on the drawhar is the same 

 in hoth directions — it is manifestly impossible 

 for it to be otherwise — and the backward pull 

 is made up of 600 pounds of frictional forces 

 and 400 pounds of inertia-reaction. 



Andrew H. Patterson 



University of North Carolina, 

 Chapel Hill 



SCIENTIFIC BOOKS 



Diseases of Occupation and Vocational Hy- 

 giene. Edited by G. M. Kober and W. C. 

 Hanson. P. Blakiston's Son & Company. 

 Philadelphia, 1916. Octavo. Pp. xxi + 918. 

 $8.00. 



Ten years ago there was no such thing as a 

 science of industrial hygiene in the United 

 States. During the last half of the decade 

 Dr. Alice Hamilton, Dr. G. M. Price, Dr. 

 E. E. Hayhurst, Mr. F. L. Hoffman and 

 others have conducted fundamental and im- 

 portant investigations in this field; the Amer- 

 ican Association for Labor Legislation has 

 organized an educational campaign which has 

 resulted in unparalleled legislative advances; 

 and during the past two years three good text- 

 books have appeared dealing with the subject 

 —Dr. G. M. Price's "The Modern Factory," 

 Dr. W. Oilman Thompson's " The Occupa- 

 tional Diseases," and the volume under dis- 

 cussion — besides a wealth of monographs on 

 accident prevention and other special phases 

 of the subject. 



" Diseases of Occupation and Vocational 

 Hygiene" is the most ambitious of these 

 works, having been prepared under the editor- 

 ship of Drs. Kober and Hanson by thirty-one 

 American and foreign specialists in various 

 branches. Many of the topics are so treated 



