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NATURE 



\Feb. 7, 18^89 



yet ofinertio, are — in this country, at any rate — already accus- 

 tomed to pounds or ounces as the practical units of force. . My 

 suggestion is simply, " Don't s.vap horses while you are crossing 

 the streatri." 



Dr. Lodge appears to object to my using the word inertia in 

 the sense of the coefficient m. But he does exactly the same in 

 his own book on "Mechanics'' (p. 49); and the usage is, I 

 think, quite common. 



A. M. Worth i.NGTOX. 



R.N.E. College, Devonport, January 26. 



As a student and teacher of physics who has come much into 

 contact with engineers and other artisans, I venture to say a few 

 words on the vexed question of dynamical units now tmder dis- 

 cus ion in your pages. It seems to me that it would be a 

 distinctly retrograde step to adopt the proposals which Mr. 

 Wprthington makes in a recent number of Naturk. It w.iuld 

 amount virtually to a return to the cumbrous and discredited 

 system of units in use in British text-books of dynamics before 

 the appearance of " Thomson and Tait,"and the introduction of 

 the Gaussian units of mass, force, iSic. 



It is certain that, whether the word "pound " be properly used 

 to denote a unit of force or not, a comuion usage < f the ;erm is 

 to denote a certain quantity of matter — that which has the same 

 gravity at the same place as the so-called standard of weight. 

 This is a standard quantity of matter and is a constant. Ts^iw 

 in dynamics the primary property of matter is inertia, and inertia 

 alone. When we compare the masses of bodies dynamically, 

 we compare only their inertias ; and that the forces of gravity 

 on different bodies are proportional to their masses we have from 

 JNewton's pendulum experiment, &c. It seems natural and con- 

 venient, therefore, >tarting from this primary property of matter, 

 to take the unit of mass as we find it defined, and give to that 

 unit the unit of inertia. Then if the numeric of mass of a particle 

 be m, of its acceleration dvldt, the numeric of the inertia-reaction 

 is VI . dvjdt simply. Thei plan proposed by Mr. Worthington 

 Wfuld introduce quite gratuitously the relation of his "unit of 

 inertia" to the unit of mass, a relation which has been in the 

 past— and would, I fear, be again — a great source of confusion 

 to the student. 



It is to be remembered, further, that the Gaussian system of 

 units has been adopted by most civilized nations for practical 

 electrical work. Certain units are constantly used in electrical 

 engineering, which are simple multiples or submultiples of the 

 .various .derived units in this system. It is too late in the day to 

 change all thi«, and thereby run the risk of throwing things 

 intp the state of chaos from which with great labour and trouble 

 they have been rescued. Hence the engineer, whatever units 

 .' he u-es for steam-pressures, &n., must, if he is taught dynamics 

 at all, be taught how to express results given in gravitational 

 units in terms of units independent of locality, or any other 

 ' varying circumstance. It seems to me de^irable, therefore, on 

 the ground that the Gaussian system of units is in use in a great 

 and growing department of engineering, to adopt it in our teach- 

 ing at the outset. The true i-elations of other units is then got 

 at once, and unfailingly. 



My experience as a student and as a teacher is all in favour of 

 the system and nomenclature f llowed by the persons whom 

 Prof. Greenhill (I think) called " precisionists." Words are, 

 of course, used in more .<^enses than one in popular language ; 

 but if a popular word, such as "pound " or " weight," is to be 

 adopted for scientific u?e, a restriction of its meaning to one 

 sense is absolutely necessary if confusion is not to result. This, 

 at any rate, is the principle on which scientific nomenclature 

 has proceeded hitherto. This precision in the use of terms is 

 absolutely necessary in teaching, and confusion of thought 

 cannot be avoided without it. Of course there is want of 

 consistency — no teacher can be perfectly precise ; but that is 

 hardly an argument for throwing precision overboard altogether. 

 Methods of teaching, after all, must stand or fall by their 

 results, and I s-hould like to join my testimony to that of those 

 who .'ay from experience that the Newtonian method in its 

 original simplicity, with the system of units which Gauss gave, 

 and which has produced so great and far-reaching scientific 

 results, is the best way of approaching the study of dynamics. 

 Students properly taught in this way have no difficulties beyond 

 those inherent in a confessedly difficult subject. 



Andrew Gr.w. 

 University College, Bangor, January 28. 



Use of Sucker-Fishes in Fishing. 



With reference to Mr. Sclater's note in Nature of 

 January 24 (p. 295), on the use of the Rimora in fishing, I would 

 like to call attention to the use of sucker- fishes by the aboriginal 

 inhabitants of Cuba. Ferdinand Columl.us ("Churchill's 

 Voyages," 1704, vol. ii. p. 616) says these people used the 

 sucker-fish to catch both other fish^and turtles. These fishes 

 when tied " by the tail run themselves against other fish, and by 

 a certain roughness from the head to the middle of the back, they 

 stick so fast to the next fish they meet, that, when the Indians 

 perceive it, drawing their line, they draw them both together." 



Lightcliffe, Yorkshire, January 26. H. Ling Roth. 



Remarkable Rime and Mist. 



The extraordinary rime described by your correspondents 

 was also experienced here (at 425 feet above sea- level) in January. 



Though not an unusual occurrence in severe weather, this has 

 never been equalled in my recollection. 



The freezing fog lasted three days, each succeeding one 

 appearing to add to the thickness of the rime, which culminated 

 on the 6ih, when it was difficult to believe that the trees were 

 not covered with snow. On that date I measured one of the 

 sheaves of spiculse attached to a terminal shoot of a beech-tree, 

 and found it very nearly 2\ inches in length. This, of course, 

 was rather exceptional. - E, Brown. 



Further Barton, Cirencester, February i. 



It seemed to me scarcely necessary to mention the amount of 

 what may be called, for the sake of brevity, " sooty matter," in 

 the rime referred to by Mr. Maw (p. 295). Some of the pro- 

 ducts of combustion are frequently restored to the ground 

 without contact with water particles ; but many are carried 

 about in the atmosphere for a considerable time, and are 

 returned to the earth through aqueous precipitation. I am not 

 sure that the subject of the varying results of analyses of rain- 

 water, obtained under various conditions of weather, has received 

 the amount of attention which it deserves. The heavy rains of 

 our summer thunderstorms seem to contain less sooty matter 

 than is brought down in drizzling rain, when we have made the 

 necessary allowances for direction and force of wind, hygro- 

 metrical and thermal conditions, type and quantity of previous 

 rainfall, &c. This is probably due to the fact that rain-drops of 

 the thunderstorm fall from the greater altitude and fall more 

 vertically through the lower strata of the atmosphere. I .should, 

 however, like to learn from some readers of Nature whether 

 the larger rain drops may not also, from the motion of air which 

 they produce, treat some of the particles of sooty matter with 

 the kindly neglect shown by them to the midges. Snow (as, I 

 suppose, most people have observed by the sense of taste, with- 

 out chemical analysis) contains, when melted, more sooty matter 

 than rain, and I should have expected the inhabitants (including, 

 of course, the tobacconists) of certain localities on our globe <o 

 feel, rational gratitude to those slanting flakes which, in their 

 voyage through our air, cleanse it of its sooty particles at those 

 seasons when we are most fertile in producing the latter. But 

 the drifting fogs which traverse a considerable area of land where 

 there are factories, chimneys, &c. (their wa'er-particles moving 

 in lines nearly concentric with the earth's surface, and at no 

 great height above it), should give the air a more thorough 

 washing than is provided by the more common forms of pre- 

 cipitation. The ice-crystals produced by such fogs necessarily 

 furnish, when melted, a maximum of sooty matter. 



The letter from Mr. Lowe (p. 319) confirms what was antici- 

 pated, that the fig and rime were considerably less in the west 

 of England than' in the Midlands. It is perhaps contrary to 

 the rules of good taste for me to criticize the words of so great 

 an observer as Mr. Lowe, but is it not a contradiction of the 

 laws which govern atmospheric phenomena, considering the 

 great distance between true cirri and the fog described, to sup- 

 pose that the upper surface of the latter " rapidly changed to 

 cirri clouds " ? 



Speaking of mist, it is almost impossible not to refer to the 

 very interesting article by Prof. J. H. Poynting, F.R.S. (p. 

 323). May it not be possible that the quivering so often seen 

 in a summer haze is, after all, the result of evaporation, as 

 Wordsworth, the poet of Nature, himself seems to have thought 

 in his use of the word "steam," in " The Excursion." 



Annie Ley. 



