144 



NATURE 



[November 30, 191 1 



specimen it could easily be tranif erred to the clear Latin 



italics. 



" Thus the transference of (^plnoglotsuih grnnululuin, 

 lIv'T, to Vuxus granulata (Hwr) would indicate that an 

 exiv.'cdingly doubtful determination had been replaced by 

 one with some scientific basis. Any worker in another 

 branch of science, seeing (S^. griuinhitum, in tiuthic, would 

 be warned at least to look into the grounds for the 

 determination for himself before he — let us imagine — used 

 the record for his stratigraphic work in correlating horizons, 

 or in writing up the early history of the Ophioglossacci-, 

 when he would otherwise assume that the living genus w.is 

 represented in Cretaceous times in the Ambay Cinvs nf 

 North America. This is merely an illustration of ul 

 very widely spread in fossil botany ; but It may st-i \ 

 give i>oint to the general proposition that the time has come 

 when it would be of real service to the science to attempt 

 a conscientious distinction between valuable and doubtful 

 determinations, and that Gothic lettering might give us an 

 easy indicator. 



" The need for this is all the greater, because the results 

 of palaeobotany touch so many other fields of research, in 

 animal palaeontology, geology, and palseogeography, as 

 well as botany itself. Workers from these other fields are 

 seldom able to estimate the evidence that they are taking 

 to build into their own work, even had they the time to 

 go into the details ; and thus a single error gets widely 

 disseminated. Often it is not entirely the fault of the 

 one who originally described the fossil, for he may say in 

 his text that the nature of the specimen is doubtful, and 

 that, in default of better evidence, he gives a certain name 

 with hesitation. That name, however, once given, is 

 quoted and put into lists without being in any way dis- 

 tinguished from the rest ; and the results are detrimental 

 to the advanc of true knowledge in every way. It is no 

 solution to call every leaf, as some conscientious workers 

 do, ' Phyllites ' : different things, however doubtful, must 

 have distinct names, and the use of Gothic characters for 

 the very doubtful ones would greatly tend to ' create con- 

 fidence ' in the science of pala?obotany." 



Marie C. Stopes. 



The Unit of Momentum. 



Doubtless all teachers of experiencfe are agreed that in 

 dealing with the measurable quantities considered in 

 mathematics and physics there is great difficulty in giving 

 to students a clear idea of the quantities measured unless 

 a definite name be given to the unit in terms of which 

 measurement is made. 



Until the word " radian " was introduced, circular 

 measure was a stumbling-block to all beginners in trigono- 

 metry ; the sentences " the circular measure of this angle 

 is 1-7 " and " arc over radius equals 1-7 " make little 

 impression on the untutored mind ; but the sentence " this 

 angle contains 17 radians — the radian being rather more 

 than 57° "is as easy of comprehension as " the value of 

 this is 2 J guineas — the guinea being 21 shillings." So 

 in electricity, without the names ampere, volt, ohm, &c., 

 the learner would be — as he was thirty years ago — lost in 

 hopeless vagueness. 



In mechanics we have given names to almost all the 

 units; why not give one to the unit of momentum? I 

 venture to suggest for general use the names which I have 

 myself made use of for many years, and have found 

 helpful in inducing clear thinking on the part of mv 

 students ; they are analogous to the names of the units of 

 work. 



Thus the work done by a force i lb. acting for i ft. is 

 one ft-lb. 



The momentum produced by a force 1 lb. acting for 

 I sec. is one sec-lb. 



The work done by a force i poundal acting for 1 ft. is 

 one ft-poundal. 



The momentum produced by a force i poundal acting 

 for I sec. is one sec-poundal. 



The work done by a force i dyne acting for i cm. is 

 one cm-dyne (ergV 



The momentum produc d hy n force i dyne actiticj for 

 I sec. is one sec-dyne. 



NO.- 2iq6, VOL. 881 



Similarlv we may apeak of a secHon and a sec-tondal. 



Th< ' ■ -»-^- of the above names i« that th'-' '••■"■'^ -s 

 the < >tinction between momentum 



energy, ...ai one measures what I may t; 1 



effect of a force and the other its space-effect ; or, to pu; 

 differently, when we know the momentum of a mo\ 

 IxKly we know how long a given force i 

 duce the motion, and when we know ) 

 we know how far the force must act. 1 

 seems to me far more desirable than t 

 the beginner that momentum is mv, or " ;- 

 such a suggestion he must either receive in deadly apa 

 or he must worry his brain with the impossible task 

 tying to conceive how mass can be multiplied by veloci: 

 lis teacher may try to slur over the difficulty for him 

 a word-juggling substitution, and say momentum is : 

 product of mass and velocity ; but then, what is a produ 

 In the mathematical sense of the word it is the result 

 multiplication; in the non-mathematical sense it is 

 thing resulting from or produced hertn certain others ; . 

 in this latter sense kinetic energy is just as much the j' 

 duct of mass and velocity as momentum is. So, also, : 

 time-honoured but vague phrase " quantity of motion 

 could be used to express kinetic energy with quite as mu' 

 aptness as to express momentum. 



Using these names, we would teach — 



" The momentum of m lb. moving at v f.s. is > 

 poundals, or mvjg sec-lbs. ; its K.E. is \mv* ft-poundals. 

 imv'/g ft-lbs." " The momentum of m grams moving 

 V c.s. is mv secdynes ; its K.E. is ^mv* ergs (cm-dynei- 



F. R. Barreli 



The University, Bristol, November 7. 



i 



I AM in the habit of saying, " the amount of momenti 

 is 12 in C.G.S. units " or " the amount of momentun. 

 12 in engineers' units." The use of such a complex n.'r 

 as "gram-centimetre per second " would be absurd. Nu 

 Mr. Barreli has made an excellent suggestion; the nam^- 

 sec-dyne with all students, and sec-pound with suuii 

 students as use engineers' units, are not only short and 

 easy to remember, but they keep before the student the 

 fundamental fact that force is time rate of change 

 momentum. It seems to me, however, that at the s' 

 we must define momentum as mv, and boys must fi 

 the start learn that they are no longer multiplying n- 

 numbers. I ne/er found that a boy had any difficulty ,. 

 his master did not create one. A boy knows at once that 

 if a distance of 100 yards is passed over in 20 seconds 

 there is an average speed of 5 yards per second ; he "' 

 no difficulty in dividing space by time, but his teacher r 

 create great confusion in his mind if he philosoph' 

 about it. John PERR-i 



Fish and Drought. 



In my letter on " Fish and Drought " published 

 Nature of November 23 there is an error, due, withe _: 

 doubt, to a mistake in my MS., which may confuse the 

 reader. In the tenth line from the bottom of the right- 

 hand column of p. 108 the word " north " occurs ; it should 

 read " south." 



It has been pointed out to me by friends who have r 

 the letter that the final paragraph is rather difTicult 

 follow. With your permission I beg to take this opj 

 tunity of rectifying its expression, as follows : — 



In conclusion, I think that the observations above 

 corded show that the material of geological format! 

 need not necessarily have been " laid down ": it may h. 

 been produced in situ like the mud in the ditch round thp 

 Park of Marchais, and that the enclosure in it of animal 

 remains may have been in some cases due to a voluntary 

 act of self-inhumation, undertaken, perhaps usually, with 

 a view to self-protection. They also show that two neigh- 

 bouring strata, the one carrying abundance of life and the 

 other being destitute of it, may nevertheless be contem- 

 poraneous in date and conterminous in locality of forma- 

 tion. J. V. FrCIIANAN 



No%-ember 26. 



