April 14, 1887] 



NA rURE 



557 



it. The style of the whole book leads one to doubt the 

 author's claims as a botanist to write it, and though it 

 it may be a suitable guide to those who have to acquire a 

 knowledge of botany in the course of their studies, it is 

 practically useless for the rearing of botanists. Though 

 one is reluctant to attribute a wrongly-spelt word to other 

 than the conveniently necessary printer, the occurrence of 

 Fi'licineic, not once, but regularly, and, moreover, in the 

 boldest and most conspicuous type of the headings of 

 sections, does tempt one to think that the printer's fault 

 lay in not having corrected it. A detailed criticism of the 

 book would exhibit the author's imperfect acquaintance 

 with the types discussed and his errors in description. 

 Such, however, is beyond the scope of this notice. 



LETTERS TO THE EDITOR 

 [Tkt Editor dots not hold himselj responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake to 

 return, or to correspond ^uith thewriters of, rejected manu- 

 scripts. 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 olhenuise to insure the appearance even 

 of communications containing interesting and novel facts.] 



A Plant which destroys the Taste of Sweetness 



Du RING his tenure of office as Governor of Madras, Sir 

 Mountstuart Grant Duff found time, in a way at which I 

 never ceased to man'el, to correspond with this establishment 

 about every iiind of detail connected with the botanical pro- 

 ductions of Southern India. In one of the last letters which I 

 received from him at the close of last year, before his departure 

 from India, he writes : — " I send you inclosed in this a portion 

 of that delightful plant Gymnema sylvestre, an Asclepiad. I 

 shall be curious to know whether when it gets to you it retains 

 the very interesting properly that, if you chew carefully two or 

 three leaves of it, it absolutely abolishes for the time the power 

 of tasting sugar. This is no fable, for three of us, I being one, 

 tried it this morning at breakfast with the most complete success. 

 I ate pounded sugar after it without the faintest perception of 

 its saccharine character. I also drank coftee without any sugar 

 in it, and ta>ted it just .is well as I ever did. 



" General Elles has just been up to my room to tell me that 

 he also found it abolish the power of enjoying a cigar. Do try 

 it, and report to me, when we meet, whether it stands the 

 long journey. This Gymnema might conceivably be important 

 medically." 



We found that the leaves sent by Sir Mountstuart Grant Duff 

 did retain the property he described in a marked way. I 

 immediately wrote to Mr. Lawson, the Director of Public 

 Gardens and Plantations, O jtacamund, to endeavour to procure 

 some seed which we might grow at Kew, so as to obtain 

 material for future experiment. In a letter received from him 

 this morning he promises to do this when the fruit is ripe. He 

 has, in the meantime, been so good as to inclose in his letter a 

 paper by Mr. Hooper, the Government Quinologist, which 

 appears to me to well deserve the wider publicity of the pages of 

 Nature. 



The whole matter is a good illustration of the useful work 

 which can be done by scientific men in distant parts of the 

 Empire, which indeed could hardly be done in any other way. 

 W. T. Thiselton Dyer 



Royal Gardens, Kew, April II 



[Mr. Hooper's paper will be found on pp. 5^5~67] 



Units of Weight, Mass, and Force 

 It is not easy tofoUow Mr. Greenhill in his letter which appeared 

 in Nature of March 24 under the above heading. His main 

 contention appears to be that "weight" connotes not "force" 

 but "mass " m engineering formula;. Surely it would be more 

 correct to say that the primary idea among engineers is that of 

 force, mass being of secondary consideration and being measured 

 by means of force : the force most commonly referred to being 

 that of gravitation, which is the force, par e.x.ellcnce, with which 

 the engineer has to deal. And I think it would be impossible 

 to find any ordinary engineering formula involving W (which is 

 g enerally supposed to stand for weight) in which IV does not 



mean gravitation force. Also, in formula- which have nothing 

 to do with gravitation, and in which .^f (or mass) would 

 naturally appear, the engineer puts IV^g instead of J/, so as 

 to enable him to e.xpress it in terms of his unit of force, the 

 weight of a pound. Thus, the kinetic energy of a moving body 

 is iil/;'-' (where -I/is its mass and z' its velocity), and is quite 

 independent of its position in space. Engineers, however, who 

 only caie about bodies near the earth's surface, express the 

 energy in terms of the merely local phenomenon, the weight or 

 gravitation force acting on the body, which is sufficiently constant 

 for their purposes, and write i.H'v'-i-g. There is consequently 

 a struggle between engineers and phy. icists as to whether 

 ''pound," "/.«,'' &c., shall connote the fundamental engin- 

 eering quantity, namely, weight, or the fundamental physical 

 quantity, namely, mass; and, n.aturally, neither side is very 

 willing to give way. The easiest way perhaps would be for 

 the physicists to give another name to the mass-unit, and leave 

 engineers to the enjoyment of their use of the word "pound" ; 

 though meanwhile the word might very well connote either 

 mass or weight [i.e. gta\i\.3.tion foice) according to the context, 

 the terms pound-mass and pound-weight being used when special 

 clearness is desired. But do not let us, as Mr. Greenhill seems 

 to desire, use weight and mass as synonyms, so losing the 

 advantage of a good w"ord for no good reason. 



But Mr. Greenhill's most incomprehensible attack is on the 

 formula IV = Mg. 



The equation means fundamentally neither more nor less 

 than that the force of gravitation on any mass near -the earth's 

 surface gives, or tends to give, to that m,ass a constant accel- 

 eration called " g," and is to be measured by mass and 

 acceleration conjointly, in accordance with Newton's second 

 law, the fundamental law connecting force and motion. The 

 symbol = means " equivalent to," as it often does. 



From this fundamental equation can be deduced special 

 numerical equations by means of definitions of arbitrary stand- 

 ards. Thus a " poundal " is the force which will produce in a 

 pound-mass an acceleration of a foot-per-second per second ; 

 . '. Jr(in poundals) = i]/(in pounds) x ^(in ft.-per-sec. per sec.) 

 = J/ (in pounds) x 32, approximately, 



this equation being merely a numerical equation deduced from 

 the fundamental physical equation above. For W (in poundals) 

 means the ratio of the weight of a body to the force called a poundal, 



or weight per poundal, or 



weight 



and so is a mere 



one poundal 



number depending on the particular mode of measuring W: and 

 similarly with the other quantities. 



Again, a pound-weight is the force which produces in a 

 pound-mass the acceleration g ; 



.'. JK(in pound- weights) — /I/ (in pound-masses), 

 or ambiguously 



W (in pounds) = M (in pounds), 

 which is another merely numerical equation, and of course also 

 only an approximate one ; as Mr. Greenhill incidentally shows 

 by means of his hypothetical balance at the coal-pit. 



Too much importance can hardly be laid on the r.adical dis- 

 tinction between a physical equ.ition and the various numerical 

 equations which by choice of special units can be deduced from 

 it. This must be my excuse for dwelling so much on the above 

 example. It throws light on the way in which the error cited 

 by Mr. Greenhill in his last paragraph can creep in. Thus, if 

 the mass of a body of weight IV is IV -h- g, it really follows that 

 the mass of a body whose weight is W pounds (or, less am- 

 biguously, IV pounds- weight) = W pounds-weight -^ ^ ; but 

 by definition one pound-weight -— g = one pound-mass, .'. the 

 mass = /F pound-masses. In Mr. Greenhill's example IVisa. 

 mere number, and he shows the error caused by trying to insert 

 it in a formula where IV means a weight. 



In conclusion, if Mr. Greenhill insists on the abolition of the 

 equation IV = Mg, will he kindly say how he would symbolise 

 the connexion between the force of gravitation on a freely fall- 

 ing body and the induced acceleration^? .'Vi.fred Lodge 



Cooper's Hill, March 30 



The Association's "Geometry" 



.\s the President of the Association for the Improvement 01 

 Geometrical Teaching did me the high honour to mention with 

 special approval my work on geometry in his remarks before the 



