4i6 



NATURE 



IMarck I, 1888 



CHAPTER Iir.— THERMOMErER. 



Section I. Coiwersion. 



1. R. 



2. F. 



3- C. 



4. F. 



5. C. 



C. 



c. 



F. 



C. differences 

 F. 



Section II, Reduction of Temperattire to Sea-level. 



X. Metric. 

 2. English. 



CHAPTER IV.— Barometer. 



1. Barometer to 0° C. Metric (o°'i C. and 5 mm.). 



2. ,, ,, 32° F. (o°'5 F. and o"2 ins.). 



3. Gravity ... Latitude ... metric. 

 4- ,, ... ,, ... English. 



5. „ ... Altitude ... metric. 



6. ,, ... ,, ... English. 



7. Barometer to sea-level metric. 



8. „ , English. 



CHAPTER V. — Hygro.metrv, Rain, and Evaporation. 



1. Vapour-tension to o'l C. from - 30° C. to -1- 101° C. 



2. . ,, ,, 0-2 F. „ - 20° F. „ + 214° F. 



3. Boihng-point (from 680 mm. — S03 mm.) ... metric. 

 4- t> . >. „ English. 



5. Vapour-tension about 100' C. metric. 



6. „ ,, 212° F. English. 



7. Weight of water in cubic metre of air ... metric. 

 8- .'<.." f-"^' " '•• English. 

 9. Relative humidity metric. 



10. „ ,, English. 



CHAPTER VI.— Wi.MD. 



1. Lambert's formula. 



2. Natural tangents. 



3. Kilometres per hour to metres per second. 



4. Metres per second ,, kilometres per hour. 



5. Miles per hour ,, metres per second. 



6. Metres per second ,, miles per hour. 



CHAPTER VII. — Magnetism and Electricity. 



1. English mag. units to C.G.S. units. 



2. C.G.S. „ Eng. mag. 



Weight and Mass. 



Ihe review of Kennedy's "Mechanics of Machinery " in 

 Nature, December 29, 1887 (p. 195), strikes at least one respon- 

 sive chord on this side of the world. There are some questions in 

 reference to the nomenclature of dynamics which "will not down " 

 until they are "downed" by a convention or agreement between 

 those who have to do with the theory of mechanics and those 

 who have to do mostly with practice, and in this some conces- 

 sions will doubtless be necessary on both sides. While in hearty 

 sympathy with much that the reviewer sa3's in his discussion of 

 dynamical terms (the book under notice I have not yet seen), I 

 wish to dissent from and to protest against one of his leading 

 propositions. 



It must be admitted that in the " vernacular " the word pound 

 is used in two distinct senses — that is, as a unit of force and a 

 unit of mass. Authors of mathematical treatises have some- 

 times, and perhaps unconsciously, ignored the latter meaning, 

 and at other times have failed to recognize the former. 



The proposition of the reviewer is to eliminate the word mass 

 altogether and to use weight in its stead. To accomplish this he 

 isjobliged to use the word lueight as meaning what is now gener- 

 ally expressed by the word mass. This, it seems to me, would 

 be a grave error. Is it not true that zveight, as understood by both 

 the "learned and the unlearned" always carries with it the idea 

 of force, the force of attraction between the earth and the par- 

 ticular body under consideration ? And is it not also true that 

 there are many problems in the work of the practical engineer 

 in which mass, in the ordinarily accepted sense, is the essential 

 element, rather than weight, in the ordinarily accepted sense ? In 

 short, in my judgment, the engineer does require the word " mass," 

 andhealso needs the word " weight." It is a misfortune when 

 one word must be used to mean two entirely different things (as 

 is the case of the word " pound "), and we ought to congraUilate 

 ourselves that we have the words "mass" and "weight" so 

 commonly and generally used to represent two distinct ideas. 

 To discard one of them and force the other into its place would 

 be to introduce confusion rather than order. To satisfy the re- 

 quirements of both mathematical or theoretical and practical 

 convenience I have been accustomed to use the following : — 



The word pound is used in tv\o senses ; it may mean a unit of 



mass or a unit of force. It is always easy by the context to tell 

 in which sense it is used. 



As a unit of force it has not yet been accurately defined, but it 

 means, in general, a force equal to the attraction between the 

 earth and a mass of one pound. As this attraction varies 

 slightly, the pound as a unit force cannot be regarded as abso- 

 lutely constant, but is sufficiently so for practical purposes. 



When, by a convention of authorities, the conditions under 

 which this attraction is accepted as equal to one pound are pre- 

 scribed, it will become an invariable unit. 



There are in the English system two units of force, the poundal 

 and the pound. There are also two units of work, the foot- 

 poundal and the foot-pound ; each is the work done by the 

 corresponding unit of force working through a distance of one 

 foot. 



The ordinary equations of dynamics, when the foot-pound- 

 second units are used, give results in poundals or fo5t-poundaIs, 

 which may at once be reduced to pounds or foot-pounds. 



The above is open to the objection that the pound as a unit 

 of force is not constant, but the remedy for this is indicated, and , 

 the errors introduced are of no moment in " practice." 



To lessen the confusion somewhat, I have often used, in writ- 

 ing, the symbol lb. to represent the unit of mass, and the word 

 pound that of force. In my own experience the adoption of 

 these definitions has greatly facilitated the work of students. 



I entirely agree with the criticisms made upon the equation so 

 constantly appearing, zv — mg. To the learner it is generally 

 "confusion confounded," and I would cheerfully join in a 

 " boycott" against it. T. C. Mendenhall. 



Rose Polytechnic Institute, Terre Haute, Indiana, 

 U.S.A., January 26. 



Once more Prof. Greenhill devotes a large portion of a review 

 to emphasizing and insisting on his paculiar, and I may say 

 extraordinary, mode of regarding the meaning of elementary 

 terms (see Nature, February 16, p. 361 ; also December 29, 

 1887, p. 195). 



One must assume, therefore, that these views are regarded by 

 him as useful and conducive to clearness. 



I find it difficult to express strongly enough my entire disse.it 

 from such a proposition without being apparently impolite. 



That engineers are entitled if they see fit to employ as their 

 third fundamental standard a standard of force rather than one of 

 mass, I admit. I do not think the plan satisfactory or clear, bat 

 there are temptations towards it, and perhaps no very serious 

 objections. My own experience of engineering students is, how- 

 ever, that they are beautifully uncertain whether to put g into 

 the numerator or the denominator of a new expression, or 

 whether to leave it out altogether ; and that they generally get 

 over the difficulty either by asking where it must go, or by 

 seeing which plan will give an answer of most reasonable 

 magnitude. The real rule on engineers' principles would be to 

 put g somewhere into the expression for any quantity with which 

 gravity has nothing to do, and to leave g out whenever gravity is 

 primarily concerned. 



But, irrespective of this standing and well-known controversy, 

 Prof. Greenhill's attempt to simplify matters does indeed make 

 confusion worse confounded. He says that in the vernaculat 

 the term "weight" does not mean the force with which the 

 earth pulls a body, but does mean the body's mass or inertia. 



What kind of " vernacular" can he be thinking of? 



Ask any ordinary member of the British public what he or sh^ 

 means by the " weight " of a thing, and you will get answers 

 such as "its heaviness," or " its heft," or the " force reqi'.red to 

 lift it," or "the difficulty of raising it," or "the pull up you 

 must give it," or any number of such replies ; but if he ever got 

 the answer, " I mean the mass of the body, in other words its 

 inertia, a measure of the quantity of matter i\\2 body contains," 

 surely he would not be satisfied with this as a fair specimen of 

 the vernacular, but would rather regard it as one of those answers 

 so frequently given to examiners — the product oi a mind so 

 tortured by instructors that its com.non-sense and vernacular are 

 completely atrophied. Oliver J, Lodge. 



The Composition of Watar 



Two days after the publication of my letter in Nature (p. 

 390), on the composition of water, I received the Manchester 



