622 EEPORT— 1887. 



13. On the Nomenclature of Elementary Dynamics. 

 By John Walmslet, B.A. 



The exposition of the subject is in need of a suitable nomenclature and is un- 

 settled in phraseology. 



First felt strongly in regard to ' acceleration,' which is used to denote increase 

 of rate of velocity, and also rate of the same increase. Restricting it to the latter, 

 * addend ' may be used for addition made to velocity, and the same term is useful 

 elsewhere. This removes only part of the risk of cou fusion. 



Matter is the medium of motion, so far as we consider it. The mass of 

 matter may be called the e.vte7mon of motion, while velocity is its inteimty. These 

 are its two real dimensions (not derived dimensions), whence momentum is onv. 

 Momentum is used confusedly, its meaning of quantity of motion being overlooked. 

 ' To charge ' (borrowed from electricity) would be a useful term in speaking of in- 

 fusing motion into mass. 



Unit of mass called ' mass-pound ' might be called libra with advantage, so as 

 to avoid rivalry with ' weight-pound.' The symbol I. would be distinct from lb. 



Force is defined as 'cause' of motion, which can only mean flow of momentum. 

 This may be called ' impulsion,' uniformly with 'impulse,' which Maxwell uses for 

 the result of it. Then rate of impulsion is force according to stricter definition we 

 have to come to later on. The 'cause '-definition evidently very bad preparation 

 for the final one, and not now needed. 



' Force is said to do work when it moves its point of application.' This defini- 

 tion absurd in both clauses. Force is a ' rate ' of charging mass with momentum, 

 and is only one of two main elements of work, the other being linear space. Force 

 cannot, strange as it may seem, move anything, but energy may. These and other 

 points regarding new parts of dynamics should be kept out of influence of old 

 habits of the subject. 



Difficulties glanced at would be relieved by the adoption of unit-names. 

 Numerous proposals on the subject show the want is felt. But some think for- 

 mula in L, M, T would suffice. These would show relationships between units, 

 and would assist in many ways. 



We may get tlie advantage of formulre combined with ease and brevity of articu- 

 lation with a little extra trouble at the outset. 



Represent mass-unit by /. (for libra), length-unit by /or o, but not both at once, 

 time-unit by s or e, the representation of the last two being thus dual. Formula- 

 names follow for the derived units of ordinarj' elementary dynamics without diffi- 

 culty. Thus for units of velocity, acceleration, momentum, force, work, power, we 

 have o/s, ojse, lojs, lojse,^ojse,Jlolsse ; which might be pronounced as if the ' per ' 

 were not there, and with s sharp. The / shows where ' denominator ' elements 

 begin, those on left of it being ' numerator ' ones. 



These names are here presented simply on their merits as formulae, like those of 

 chemistry. Thus, remembering that ' lojse ' is formula of poundal, its elements are 

 kept in mind. W=Mgisalso worth looking at occasionally, as W . /o/.se - M . ^ 

 X g . ojse. Advantage of the formulte in working problems is often great. 



The above remarks apply to the English units. The French are easily obtained 

 on the same plan. 



Thus putting c or i for centimetre, g for gramme, and s or e for second, from 

 the great variety of names which are possible the following set of unit-names seems 

 the best choice : ijs, ijse, gijs, gifse, cailse, caijsse, to be pronounced as French words. 



Both sets of names are to be taken as invariable in grammatical number ; which 

 is no practical inconvenience, but rather otherwise. 



14. Exhibition and Description of Henry Draper Memorial Photographs 

 of Stellar Spectra. By Professor B. C. Pickering. 



The researches which constitute the Henry Draper Memorial are conducted at 

 the Observatory at Harvard College, and consist in the investigation of stellar 

 spectra. For this purpose Mrs. Draper has sent to the observatory the 11-inch 



