.June 25, 1915] 



SCIENCE 



939 



Dr. Paul A. Schule to be lecturer in bac- 

 teriology. 



Dr. Florence Peebles, of Bryn Mawr, has 

 been appointed professor of biology in ISTew- 

 eomb College, Tulane University, New Orleans. 



Assistant Professor Harold A. Everett, of 

 the department of naval architecture and ma- 

 rine engineering of the Massachusetts Insti- 

 tute of Technology, has been appointed to the 

 position of professor of marine engineering in 

 the post-graduate department of the United 

 States Naval Academy at Annapolis. 



PIenry Josef Quayle has been promoted to 

 a full professorship of entomology in the citrus 

 experiment station and graduate school of trop- 

 ical agriculture of the University of California. 



Mr. pi. Scott, of Trinity College, Cam- 

 bridge, has been appointed curator in entomol- 

 ogy in the university. 



' M'A' 



DISCUSSION AND COBBESPONDENCE 



THE FUNDAMENTAL EQUATION OF MECHANICS AGAIN 



To THE Editor of Science : I have followed 

 with the utmost interest the correspondence in 

 Science on the proper method of teaching the 

 relation between force, mass and acceleration, 

 and have heretofore refrained from adding to 

 the discussion. I am mindful that there must 

 be many readers of Science who have not had 

 any advanced courses in mechanics, but who 

 are trying to present to their students this 

 equation, adaptable to any system of units, in 

 a way that does not seem artificial. The diffi- 

 culty of teaching this properly to students 

 who have had little practical experience, and 

 no occasion to do any amount of computing', 

 and who will probably not go beyond their first 

 course in physics, must be apparent to all. At 

 the risk of being hissed out, I beg your leave 

 to state a method often used, and which I 

 have always found very successful with my 

 classes at Wells College. 



I follow a program very much like that out- 

 lined by Gordon S. Fulcher in your issue of 

 April 30. I teach the dependence of accelera- 

 tion on force, using the same mass, etc. ; and 

 set up equations in the form of proportions. 

 The combined equation would then be 



Then following a method similar to that used 

 in books on geometry in the treatment of the 

 area of a rectangle, I say that we may take as 

 our unit of force that force which gives to 

 unit mass a unit acceleration. Then numer- 

 ically F=.MA; if the unit of mass be the 

 gram, and of acceleration, the cm. per sec. per 

 sec, then the unit of force is called the dyne. 

 All equations in which a force is computed by 

 multiplying a mass by its acceleration, would 

 give the answer in dynes. But the dyne is 

 inconveniently small. A more natural unit is 

 the weight of a kilo. To obtain the force ex- 

 pressed in the larger unit, " we divide the an- 

 swer expressed in the smaller units by the 

 number of the smaller units contained in the 

 larger unit." Then I go on to explain that this 

 procedure of taking natural units is very com- 

 mon: one person is head and shoulders above 

 another, a certain type of tree is about twice 

 the height of a man, etc. If you multiply the 

 number of square yards in a floor by the price 

 (in cents) of one square yard of carpet, and 

 you want the answer in dollars, divide by the 

 number of cents in a dollar. 



It is easy, then, to go over to the British 

 system, in which we have but an artificial 

 analogue of the dyne. Let us fetch that back- 

 ward baby, the poundal, into the room for an 

 inspection, at least long enough to learn that 

 the weight of a pound is 32 poundals. Then 

 remembering that, for instance, centripetal 

 force, ^mv-/r, is in absolute units, we get 

 that force in pounds' weight by dividing by 32. 



I have little sympathy for those who " in- 

 flict " on their students such absurd ideas as 

 that we measure sugar, stones or anything 

 else in one kind of pounds, but use a different 

 poixnd when we find the force necessary to 

 accelerate that mass. And the idea of a grav- 

 ital is equally bizarre. 



As for the metric system, I am almost dis- 

 couraged at the conservatism of this progres- 

 sive ( ?) nation. It is perfectly true that it 

 would involve a large expense to change our 

 manufacturing machinery to conform to the 



