906 



SCIENCE 



[N. S. Vol. XLII. No. 1095 



Force. — The cord is stretched when the stone is 

 hung on it. Measure the stretch per foot of length. 

 . . . But the cord may be stretched by pulling jt 

 between the two hands horizontally. . . . The 

 pull of the earth upon the stone . . . the pull of 

 the hands, . . . each is called by the name force, F. 



Space. Time. — Let the cord be suddenly de- 

 tached from the stone. The stone falls to the 

 ground. It traverses a certain distance, 8 feet, 

 in a certain time, T seconds. 



Here are four elementary, fimdamental and 

 independent concepts. Neither one of tliem is 

 deriyed from any function or ratio of tlie other 

 three. 



(P. 161) In regard to the equation V^FTg/W, 

 which has been proposed by Mr. Kent, my feeling 

 agrees with that expressed by Professor Hoskins, 

 namely that no equation which covers only the 

 special case of a body starting from rest . . . 

 (can be considered as) a fundamental equation in 

 mechanics. Mr. Kent's paper, however, is not 

 ■without interest on the pedagogical side. 



Not without interest! I have been told by 

 those who have used my method that it is ped- 

 agogically admirable. The editor of Science 

 has sent me a letter from an engineer in Cali- 

 fornia enclosing 15 cents for a copy of Science 

 containing my article, to be sent to his son in 

 college, saying that ia 25 years' experience it 

 was the best presentation of the subject he had 

 seen. I presented it on the blackboard at the 

 Princeton meeting of the Society for the Pro- 

 motion of Engineering Education, June, 1914, 

 where I challenged the professors present that 

 if they did not like my method they write out 

 a better one. Thus far no one has accepted 

 my challenge. The problem is not one of me- 

 chanics; it is one of a method of teaching; it 

 is one of pedagogy and the English language, 

 how to find a form of words to be put into a 

 text-book to explain the fundamental prin- 

 ciples of dynamics in a way that will appeal 

 to the young student and get these principles 

 into his head in the easiest way possible. 

 After over a year of " watchful waiting " I 

 have put a condensed summary of the method 

 as given in my article in Science of March 19, 

 1915, into the chapter on Mechanics in the 

 ninth edition of my " Mechanical Engineers' 

 Pocket-book," which will be off the press about 



December 10. The young engineers who use 

 the book wiU there find an antidote to what 

 they have been taught in the past about 

 poundals, slugs, gee-poimds, engineer's unit 

 of mass, derived " concepts," " force is the 

 time-rate of the increase of momentum," 

 " mass is the ratio of force to acceleration," 

 " the unit of mass is 32.2 pounds, the unit of 

 force is 1/32.2 of a pound," and the like. I 

 even have hopes that the Committee on Teach- 

 ing of Mechanics, of the Society for the Pro- 

 motion of Engineering Education, of which 

 committee I am a disturbing member, disturb- 

 ing the slumbers of the committee about once 

 a year, will in two or three years more get over 

 its negative acceleration or minus inertia and 

 adopt my method in its final report. 

 Professor Hoskins, August 27: 



That "the result of weighing a body on a bal- 

 ance scale" is a proper measure of "amount of 

 material ' ' certainly requires explanation to the 

 beginner. 



Not to a boy who understands the English 

 language and has ever seen a grocer's scale 

 used to weigh sugar. 



I see no reason why the unit which has been 

 called the slug should be regarded with ridicule or 

 even semi-ridicule. The convenience of the slug ig 

 due to two facts (1) that the pound force is cus- 

 tomarily employed in a great deal of practical 

 work, and (2) that the dynamical formulas almost 

 universally employed are based on a relation of 

 units such that unit force acting on unit mass 

 causes unit acceleration. 



The dynamical formulas universally used 

 by engineers are based on no such relation. 

 They are (1) FT = MV, (2) F8 = lMy^, 

 (3) y={FT/W) X 9> and in each case where 

 M is used it means simply W /g. 



In order to make the equation FT =^MV, 

 in anything but the C.G.S. system, harmonize 

 with the statement that " unit force acting on 

 unit mass causes unit acceleration," we must 

 do violence to the English language and cus- 

 tom and use an artificial expedient not sanc- 

 tioned in literature outside of the text-books, 

 or in commerce, or in engineering practise. 



Thus we may say 



F X T = M X V; 



