6o4 



NATURE 



{April 2%, 1887 



hints are given for tlie management of the battery. In 

 an appendix is a description of the arrangement of his 

 accumulator house, a photograph of the interior of which 

 forms the frontispiece to his bool<. He also gives a brief 

 account of the history of the installation, from which we 

 learn that the total cost of buildings and installation has 

 been about 6000/., that the number of lamps is about 500 

 of 20-candle power, but that the greatest number used at 

 any one time has rarely exceeded 200, and that an arc 

 lamp taking 40 or 50 amperes and one or two motors have 

 been used together with them. The expenses — including 

 wages, coal, oil, waste, washers, repairs, lamp renewals, 

 &c. — were, in 1886, 2 10/., or at the rate of Id. per 20-candle 

 lamp per hour. He does not, however, say anything as 

 to the cost of the accumulators ; and as to the length of 

 time which they may be expected to last he only says 

 vaguely that "the cells may last for years in perfect order 

 if all the instructions here given are properly attended 

 to." His present accumulators were only put up in August 

 1885, those previously erected having proved to be 

 unsatisfactory. 



It is a pity that the useful information in this work is 

 not conveyed in better language. In what is intended as 

 a workshop hand-book we do not look for the elegance 

 of an Addison ; but we do want plain English. As an 

 example of the language we may quote the following 

 sentence from the preface : " Only cells of the Electrical 

 Power Storage Company, of Messrs. Elwell-Parker, and 

 their type, are dealt with, because at this moment there 

 is probably no other better kind, or largely in use." The 

 meaning of this is clear, though even that is more than 

 may be said of other sentences in the book. 



Among minor faults we protest against the coinage of 

 such a hideous word as " acidometer," by which, appar- 

 ently, is meant the instrument commonly known as a 

 hydrometer ; we do not think " s.g." is an improvement on 

 the ordinarily accepted abbreviation for specific gravity ; 

 and we object to the plural "dynamos" as against the 

 ordinary rule of our language which gives us " potatoes " 

 and " echoes." 



A more serious error is found in the " Index of Terms " 

 on the back of the title-page — 



"Watt = volt X ampere = measure of force or energy." 



We should recommend the author to get the work 

 revised by some onewho understands the rudiments of 

 science as v^ell as those of the English language. 



School Hyf;ieiie: The Laws of Health in Relation to 

 School Life. By Arthur Newsholme, M.D. (London: 

 Swan Sonnenschein, Lowrey, & Co., 1887.) 



About the importance of the matters dealt with in this 

 little book there can, of course, be no dispute ; and, as 

 Dr. Newsholme points out in his preface, they have 

 engaged the serious attention of many School Boards and 

 Committees, and been made the subject of a good deal of 

 useful legislation. Most school-managers still have some- 

 thing to learn about the principles of school hygiene, and 

 many of them will, no doubt, find in Dr. Newsholme's 

 volume exactly the kind of information they want. He 

 discusses the subject under two heads, " Schools " and 

 "Scholars." Under the first head he presents his ideas 

 on questions connected with the choice of sites for 

 schools, the construction of school buildings, school fur- 

 niture, lighting of school-rooms, general principles of 

 ventilation, natural ventilation, ventilation and warming, 

 and draining arrangements. In the part relating to 

 " Scholars " he has chapters on mental exercise, excessive 

 ment d exercise, age and sex in relation to school work, 

 nvi-c'ilar exercise and recreation, re^t and sleep, chil- 

 dren's diet, children's dress, baths aid bathing, eyesight in 

 rel ition to school life, communicable diseases in schools, 

 md school accidents. Dr. Newsholme has studied his 



subject thoroughly, and his conclusions are all the more 

 valuable because they have been to a large extent sug- 

 gested by his experience as a medical officer of health, 

 and-as a medical referee for various schools and training 

 colleges. 



LETTERS TO THE EDITOR 



{The Editor Joes not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake to 

 return, or to correspond with the writers of, rejected manii- 

 srripts. No notice is taken of anonymous communications . 



[ The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of lOinmunicationscontaining interesting and novel facts.} 



Miss, Weight, and Dynamical Units 



If the laws of dynamics were made solely for the use of 

 engineers, as a celebrated engineer declared of rivers that they 

 were intended to feed canals, it might be conceded to Prof. 

 Greenhill that it would not be necessary that tlie more abstract 

 notion of mass should be distinguished from that of weight, and 

 that the fundamental equation of dynamics might conveniently be 



taken as /" = U - ■ where fKis the weight (or mass) of the body 



moved expressed in pounds or tons, &c., and /'the force produc- 

 ing in it the acceleration /i and reckoned in the same units. But 

 a pound weight as a force is a variable unit, unless it is taken at 

 some particular place (as Greenwich), and then the corresponding 

 value of g must be employed (though the variation of g on the 

 surface of the earth is not so great as often to need to be taken 

 account of by the engineer); and so the astronomer and physicist, 

 as well as the student of abstract dynamics, are right in de- 

 manding a less arbitrary measure of force than one founded on 

 the force acting vertically downwards on a body at the earth's 

 surface, as well as an absolute constant belonging to each body (its 

 mass) independent of time and place. I conceive, therefore, in 

 spite of Prof. Greenhill's arguments, that, in the interest of 

 clearness of thought, as well as to avoid the practical inconveni- 

 ence of a variable unit of force, the notions of mass and weight 

 must be kept distinct, and the equation IV = A/j^, as a special 

 case of the general equation P = Jiff still insisted on by all 

 teachers of dynamics, at any rate to non-engineering students ; 

 while it would be well for engineers also {mejudice) to have their 

 dynamics cast in the same mould as the rest of the scientific 

 world, 



I should not have troubled you with the above very obvious 

 remarks had I not wished to observe, a propos of Mr. Geoghegan's 

 suggestion of names for units of velocity and acceleration in 

 Nature of April 7 (p. 534), that it is highly desirable that a 

 convenient and consistent notation, as well as nomenclature, should 

 be adopted for the several dynamical units. 



With respect to the particular suggestion that vel and eel (or 

 veto and celo, as used by Mr. Lock in his forthcoming book, 

 " Dynamics for Beginners ") should be employed for the units of 

 speed and acceleration in the foot-second system, I regard it as 

 an almost fatal objection that the accepted C.G.S. system having 

 appropriated the words dyne and erg for its units of force and 

 work has a prior right to vcl and eel for its units of speed and 

 acceleration ; namely, the centimetre per second, and the centi- 

 metre per second per second. If so, the units of the foot-second 

 system might be called the footr'cl and the footed. 



Names for the dynamical units are, however, I think, of less 

 importance than a convenient and suggestive notation for them. 

 I have endeavoured to devise such a notation (or, rather, to fill up 

 and complete notations which have more or less come into use; 

 in a table, which is now under the consideration of a Committee 

 of the Association for the Improvement of Geometrical Teach- 

 ing, and which is substantially as follows. 



In a perfectly general system, let |_, |V|, "J" stand respectively 

 for the fundamental units of length, mass, and time, and let 

 Vi Ai U" A> E stand for the units thence derived of speed, 

 tjuickening (acceleration), momentum or impulse, force, and 

 energy or work respectively ; then the statement that the unit of 

 speed is the speed of the unit of length per unit of time is 



