June 29, 1900.] 



SCIENCE 



1029 



to time, but not to change or to substitute those 

 already in use. There was a time when the word 

 ' energy ' (not the idea) was new ; wlien force 

 was confounded with worlc ; when power was 

 not generally so completely distinguished from 

 both force and work as it now is. There was 

 then justification for Grove to write of ' The 

 Correlation and Conservation of Forces,' and 

 for Helmholtz to discuss the ' Conservation of 

 Force' (Erhaltung der Kraft). But from the 

 somewhat turbid and confused use of terms 

 fifty years ago, there crystallized out a clear 

 and well-defined meaning to go with each one 

 of several names, and these have become so 

 well established that there is little excuse for 

 misuse of them. Of these force, work, energy, 

 and power are or should be among the most 

 familiar, and a scientific writer who confounds 

 one of them with another is as gravely at fault 

 as a literary critic who confounds will and shall, 

 .or lie and lay. 



It was a little surprising, therefore, not very 

 long ago to read in a leading educational jour- 

 nal, an article by the physical director of a 

 leading university, discussing such exercises as 

 jumping, running, and the like, seriously com- 

 paring the force necessary to raise the performer 

 six feet with that required to lift him one foot, 

 and implying that the force increased with the 

 height. Just when the laws of gravitation were 

 so changed as to make a man heavier at two 

 feet above the earth than at a height of one foot 

 it would be interesting to learn. That the im- 

 pulse needed to give him the requisite velocity 

 to rise a definite height, might be acquired by 

 means of any force in excess of his weight ac- 

 cording to the time during which it is exerted 

 was overlooked or at least not taken into ac- 

 count. In other portions of the article, also 

 appeared misconceptions of the relations of 

 force exerted, time of its action, work expended, 

 and energy acquired, as witnessed by such ex- 

 pressions as 'number of foot pounds lifted,' 

 ' every pound of energy,' etc. It would be un- 

 gracious to doubt that the author knew what 

 he meant, but he used technical terms in mis- 

 taken senses. 



Still more awry are the statements of a writer 

 in a recent number of one of the best and most 

 popular engineering magazines, in expatiating 



upon the power in a pound of coal. On their 

 face these statements are striking, so much so 

 as to be reprinted by newspapers culling the best 

 extracts from the current literary and scientific 

 magazines. Examined critically they are as- 

 tonishing. The writer is at some pains in the 

 beginning to explain the meaning of work and 

 to point out that ' in all mechanical work we 

 must consider the element of time,' and then 

 follows with statements that ignore it, by con- 

 trasting the energy of the pound of coal with a 

 man-power, or a horse-power. He recurs fre- 

 quently to the idea ' that this pound of coal 

 contains within it the power of 236 horses.' Its 

 power might as well have been declared that of 

 472 horses, or of only 118 horses, or of any 

 other number. 'It is interesting,' he con- 

 tinues, ' to follow this out in order to find how 

 much of the power which nature has given us 

 in this pound of coal we are able to get out of 

 it.' Nature has given us not 236 horse-power 

 in the pound of coal, but an indefinite power, 

 and if we can get any work out of it the power 

 is only limited by the rate at which the work 

 can be obtained ; for example, the number of 

 foot-pounds per minute. 



Apparently the reason in some of these in- 

 stances for using the word power instead of en- 

 ergy is the idea that possibly the latter term 

 may be less familiar to the reader than the 

 former ; but if both are to be used in a strictly 

 technical sense, one is probably as generally 

 understood as the other, and certainly if the 

 author had taken as much pains to define en- 

 ergy In the beginning as he did to define power, 

 he would have cleared the way for the use of 

 the term in its proper sense instead of continu- 

 ally using the other word in a sense at variance 

 with his own explanation. It is true that we 

 do sometimes come upon the word power for 

 force, as where the power and power arm, are 

 compared with the resistance or the weight and 

 its arm. The term activity has been introduced 

 to get rid of the tendency to misuse the word 

 power, but even if we admit the untechnical 

 meaning of the word as it appears in the title 

 of the article referred to, the moment it appears 

 in the compound ' horse-power ' it is inevitably 

 technical in form and in meaning. As such it 

 is not force, nor work, but rate of working. 



