Mat 5, 1911] 



SCIENCE 



701 



and the constant effort of the teacher should be 

 to ground the student thoroughly in these funda- 

 mentals, which are too often lost sight of in a 

 mass of details. 



The defects mentioned are universally ad- 

 mitted and complained of, and they are per- 

 haps more apparent in the subject of elemen- 

 tary dynamics than in any branch of pure 

 mathematics, but the writer is of the opinion 

 that they are due rather to the test -books than 

 to the teachers. As the committee says: 

 " What is most needed at the present time is a 

 series of synoptical text-books which shall 

 present in compact form (1) the fundamental 

 principles of the science, and (2) a classified 

 and graded collection of problems." 



The writer heartily commends the two fol- 

 lowing sentences in the report : 



The poimdal is never used in practise, and no 

 instruments are on the market which give readings 

 in poundals; it should be dropped from the text- 

 books. The so-called ' ' engineers ' units of mass, ' ' 

 namely, the G-kilogram, or "metric slugg, " and 

 the g-pound, or "slugg," are never used in prac- 

 tise, and no set of weights is on the market giving 

 readings in terms of these units; they should be 

 dropped from the text-books. 



The chief trouble with the text-books is due 

 to the fact that some early writer, in his at- 

 tempt to overcome what he conceived to be a 

 difficulty in teaching the subjects of weight 

 and mass, invented the " poundal," and others 

 invented the " engineers' unit of' mass," " gee- 

 pound" or "slugg" (32.2 pounds of matter), 

 to overcome the imaginary difficulty, and other 

 test-book writers blindly followed them. 

 These devices did not overcome the difficulty, 

 but on the contrary only created confusion, 

 and students had to spend weary hours on 

 these worse than useless units, which after- 

 wards, when they became engineers, they had 

 to unlearn. 



The idea of the " poundal " is probably a 

 development of the " C.G.S." system, in which 

 1 dyne (force) acting on 1 gram (matter) 

 free to move, for 1 second, gives it a velocity 

 of 1 centimeter per second. In English units, 

 if 1 pound force acts for one second on 1 

 pound of matter, the velocity at the end of the 



time is 32.2 feet per second. If the English 

 system only had either a unit of force ^1/32.2 

 pound, or a unit of quantity of matter ^ 32.2 

 pounds, then the figure 32.2 in the equation 

 of the relation of force to acquired velocity 

 would disappear, and the equation would look 

 simpler; so the two new units, poundal and 

 gee-pound, were invented, to the confusion of 

 the subject, and they now have to be expunged 

 from the language. 



Merely getting rid of the poundal and the 

 gee-pound, however, does not get rid of the 

 whole trouble arising from the use of the 

 same unit, pound, to express both a force and 

 a quantity of matter. The committee (or 

 perhaps only one or two members of it, to 

 whom were assigned the subject of dynamics) 

 have wrestled with the problem, but in the 

 writer's opinion they have failed to find the 

 best solution of it. 



In order to make this criticism as clear as 

 possible, the following extracts are first made 

 from the committee's report, and comments on 

 the several paragraphs follow. 



Extracts from the Report 



(a) The most important mechanical quan- 

 tities for engineering purposes are length, 

 time, angle and force. Derived from these 

 are : area, volume, pressure, linear velocity, etc. 



(b) Less important for the engineer, but of 

 great importance in general scientific work, is 

 the quantity called "mass" (gaUey 39). 



(c) The units of mass are of little impor- 

 tance to the engineer, since the quantity that 

 enters into the equations of engineering is not 

 the mass of the body, in units of mass, but its 

 normal weight, in units of force. 



{d) The mass of a body may be thought of, 

 roughly, as the amount of matter in a body 

 (gaUey 40). 



(e) The ratio W/g for a given body is con- 

 stant in all localities. This quantity, or a 

 quantity proportional to this ratio, is called 

 the mass, m, of the body (gaUey 35). 



(/) The weight, TF, of a body, at a given 

 place, is the force that causes it to fall when 

 unsupported (galley 34). 



(p) If the mass of a body is one pound 



