922 



SCIENCE 



[N. S. Vol. XXX. No. 782 



transformations, nearly all problems in dy- 

 namics (except those relating to impact) may 

 be solved. The sign = is to be considered 

 as meaning only " is numerically equivalent 

 to," and never as meaning " is," as in the 

 inaccurate statement " force is the time rate 

 of momentum." 



The main ideas in the above syllabus are: 



1. That words should be used as far as pos- 

 sible in their most common meaning; thus, 

 " weight," and not " mass," is the word that 

 means, in ordinary language, the quantity of 

 matter expressed in pounds. 



2. That the English system of weights and 

 measures is used until the subject has been 

 carried as far as this syllabus extends. The 

 metric system can now be used with the same 

 equations, and the C.G.S. system with its 

 " dimensions " postponed to the end of the 

 course. 



3. An attempt has been made to use no more 

 technical terms and definitions than are abso- 

 lutely necessary. The poundal and the gee- 

 pound are ignored and the dyne and the erg 

 postponed. 



4. Force is treated as an entity, capable of 

 being measured directly as matter and space 

 are, and not as a mere mathematical function 

 derived from length, mass and time. 



5. The word " mass," which gives most diffi- 

 culty to younger students, is postponed till 

 near the end of the syllabus, and is then intro- 

 duced merely as a convenient substitute for 

 W/g. The word weight is explained to mean 

 quantity of matter as determined by weighing 

 it on an even balance scale (not on a spring 

 balance). When W is so weighed, the London 

 value of g is always used in finding M = W/g. 



6. In teaching a class, after the preliminary 

 study of the derivation of the equations is 

 finished, I would put them on the blackboard 

 and let them remain there for handy reference 

 when solving problems. A great number of 

 problems should be solved by the class. 



I am aware that much of what I have 

 written above wiU be considered rank heresy 

 by many teachers of physics, but I submit 

 that since in the past they have failed to 

 teach dynamics in such a manner that the 



students can grasp the subject and use it in 

 college, it is high time they changed their 

 methods and try the method that was success- 

 fully used fifty years ago. 



Wm. Kent 



influence op oxygen on the value of coal 



To THE Editor op Science: Dr. David 

 White, in Bulletin No. 382 of the United 

 States Geological Survey, under the title of 

 " Oxygen in Coal," has presented some inter- 

 esting and valuable matter. There are certain 

 features, however, which may profitably have 

 more exact examinations. 



Dr. White shows that the presence of oxygen 

 in coal has an effect equal to that of ash in 

 reducing the heating power value of the fuel; 

 it is, of course, true that it displaces an 

 amount of combustible matter equal to that 

 of ash. There is some question, however, as 

 to what is implied by the statement. As far 

 as the combustion process is concerned, oxygen 

 is present as a gas and aside from displacing 

 a corresponding weight of combustible matter 

 in the coal, its only harmful influence is in 

 carrying away a small amount of heat in its 

 exit to the chimney; while the presence of 

 ash, on the other hand, has a very harmful 

 influence on the result produced in combus- 

 tion, because ash makes clinker and other- 

 wise obstructs the fire. 



W. L. Abbott' has shown in a well-conducted 

 series of experiments that the value of fuel, 

 for the purpose of making steam, drops to 

 zero when the ash content is equal to 40 per 

 cent., as shown by the accompanying diagram. 

 Therefore, it appears that ash is enormously 

 more harmful than oxygen, because if the 

 latter was present to the extent of 40 per cent, 

 the loss of heat from this cause would not 

 exceed 6 or 7 per cent. 



While particular emphasis has been placed 

 upon the presence of oxygen as an element by 

 Dr. White, I would prefer considering the 

 matter from the standpoint of the total inert 

 volatile matter instead of simply that of oxygen 



' Journal of the Western Society of Engineers, 

 Vol. XI., p. 529. 



