144 



SCIENCE 



[N. S. Vol. XXXII. No. 813 



Professor Webster (note 61) says: "I 

 feel that it is a pity to argue so much about 

 the dyne and erg. I would teach them, 

 and give credit to those who can do them 

 well, but not make this a sine qua non." 

 That is, the practical question before us 

 here is one of proportion and proper em- 

 phasis. I do not intend to deny or ques- 

 tion the statement that a boy of average 

 high-school intelligence can at the age of 

 seventeen grasp the principle of the for- 

 mula / = wi X a or learn and understand 

 the definitions of dyne and erg. But to 

 undei-stand and learn definitions is one 

 thing; to remember them is another thing. 

 Initially, by their mere sound or form, 

 these two words mean little or nothing to 

 the boy. So far as he can see, the names 

 dyne and erg might perfectly well be ex- 

 changed. Moreover, and this is the really 

 significant fact here, he practically never 

 hears or sees these words outside the phys- 

 ics class room. Volt and ampere are much 

 harder M'ords to define than dyne and erg, 

 but they are in common speech ; they are 

 in the newspapers. It is true that com- 

 mon speech and the newspapers show a 

 tendency to dispense with amperes and 

 reckon current strength in volts; but the 

 boy knows, when he is studying the mean- 

 ing of these words, that he is getting hold 

 of terms that men use familiarly in busi- 

 ness, that he is making acquaintances for 

 life. He sees voltmeters and ammeters, 

 and he knows that they are indispensable 

 instruments of applied science. But even 

 in the physical laboratory he never sees an 

 instrument measuring force in d,ynes or 

 work in ergs. Of course, we could make 

 such instruments. We could, for example, 

 take any spring balance and mark its scale 

 iia dynes. But how could we justify such 

 an operation? We should have to say, It 

 is important to make the boy familiar with 

 this kind of an instrument in his physics 



course, because he will never see it any- 

 where else. 



Even the word poundal, which because 

 of its relation to pound is more easily as- 

 similated than dyne and erg, has never 

 come into much use outside physics 

 courses. Engineers will have none of it, 

 and mathematicians in their dynamical 

 writings are serenely independent of any 

 units to which they need give names. Ac- 

 cordingly, when we ask the youngsters in 

 school to remember and distinguish the 

 "absolute" units of force and work by 

 name, we should not take them or their 

 teachers very seriously to task if in the 

 stress of examinations they get these terms 

 a little mixed. I would suggest that the 

 examiner who does not feel free to leave 

 out all mention of dynes and ergs can use 

 them rather helpfully than otherwise by 

 framing his questions in such a way as to 

 test the candidate's knowledge of principle 

 and fact rather than his memory of words. 

 For example. How great a force (dynes), 

 acting on a 50-gram mass for 10 seconds, 

 will impart to it a velocity of 100 centi- 

 meters per second, and how much work 

 (ergs) will the force do in this time? 



But such a question, little as it taxes the 

 verbal memory of the candidate, seems al- 

 most too academic for a college entrance 

 examination. I am sure that the boy 

 would feel himself much nearer the im- 

 portant realities of life in dealing with a 

 question like the following: If a shell 

 iveighing 800 pounds acquires in 0.04 sec- 

 ond a velocity of 2,000 feet per second in 

 the hore of a gun, how great (reckoned in 

 pounds) is the accelerating force (supposed 

 uniform), and how many foot-pounds (or 

 foot tons) of work does this force do in 

 giving this velocity f 



I am not here advocating English units 

 as against the units of the metric system. 

 I am merely illustrating the greater «of- 



