May 24, 1888] 



NATURE 



77 



examination for commercial certificates, a chapter on 

 exchange and foreign money has been added (in a worked- 

 out example on p. 151 there is an error of some pecuniary 

 magnitude), and the chapter "On Recurring Decim ils, 

 not required by Commercials," finds a place at the close 

 of the text. Mr. Lock is generally so careful in his 

 explanations that we are surprised at his omitting all 

 reference to brokerage in his account of the transferment 

 of stock. Numerous examples are given in the text, 

 and six examination-papers and answers to all questions 

 complete a capital hand-book. 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take to return, or to correspond with the writers of, 

 rejected manuscripts intended for this or any other part 

 of Nature. No notice is taken of anonymous communi- 

 cations. .] 



Weight and Mass. 

 Prof. Greenhill, in his letter which appears in Nature 

 of May 17 last (p. 54), has a^ain repeated his views on the use 

 of the word weight. He has not, however, replied to the 

 criticisms of those who differ from him (see Nature, vol. xxxvi. 

 pp. 221, 317). 



His opponents wish to know how practical engineers who use 

 the word weight as synonymous with the physicists' mass, treat 

 a problem involving inertia. Prof. Greenhill has not yet 

 given us an example of such a problem taken from some modern 

 text-book of the practical engineer ; nor has he yet given us in 

 simple language a definition of weight. Prof. Greenhill some 

 time ago referred me to Kennedy's " Mechanics of Machinery " 

 for such a definition, but I venture to say that there is no such 

 definition to be found in that standard work. 



My own idea is as follows : Matter has many properties — 

 inertia, weight (the force with which the earth pulls it), volume, 

 &c. — and Newton's great discovery consisted partly in seeing 

 clearly that the universal property of matter by which it must 

 be measured is its inertia, defined as its capacity for resisting 

 change of velocity. 



The mass of a body is that which can be ascertained by the 

 operation of massing ; such an operation, that is, as the follow- 

 ing : To a given lump of matter apply some strain or force, and 

 observe the acceleration produced in the matter by that force ; 

 then ascertain by experiment to how many lumps of matter called 

 pounds this same force will communicate an equal acceleration. 

 The weight of a body is that which is ascertained by the 

 operation of weighing. To weigh a body it is placed on a 

 spring balance, and the force of the earth's attraction is ob- 

 served by showing the compression of the steel spring of the 

 machine. 



It happens, however, that the mass of a body is proportional 

 to its weight ; consequently it is sufficient to ascertain whether 

 the weights of two masses are equal in order to ascertain that 

 their masses are equal. The weights of two masses are ascer- 

 tained to be equal by putting them each on one side of a 

 balance, and observing that the force of the earth's attraction 

 on each is the same. Hence the very difficult operation of 

 massing as described above is replaced by the easy operation of 

 weighing. 



Prof. Greenhill tells us that " now the invariable unit, the 

 mass, is measured in terms of a variable unit." Is this so ? Is it 

 not a fact that those who use exclusively the force of the earth's 

 attraction as the measure of matter, rarely if ever have any 

 conception of the idea of inertia ? When the practical engineer 

 has to do with inertia, as in cases of " centrifugal force," he 

 works by formulae or rule of thumb. 



Prof. Greenhill's sentences, "a force equal to the weight of 

 the mass of 10 pound weight*," and "the weight of 32 pound 

 weights on the Earth is at the surface of Jupiter a force of 71 

 pounds' weight," are entirely original. 



I believe he means to express " the weight of 10 pounds," and 

 the weight of 32 pounds on the earth is a force equal to the 

 weight of 71 pounds on the surface of Jupiter. 

 Caius College, May 21. John B. Lock. 



Work and Energy. 



While a discussion of the nomenclature of mechanics is going 



on in Nature, I would venture to sugge: t that an effort should 



be made to get rid of the practice of expressing energy in foot- 

 pounds or foot-poundals. There are certain quantities of work, 

 not of energy. To speak of a foot-pound of energy is quite as 

 incorrect as it would be to speak of a pint of velocity, a yard of 

 acceleration, an acre of momentum, or a pound of duration. 

 There is great need of a short name for the unit of \mv l . 

 Bardsea, May 21. Edward Geoghegan. 



On the Reappearance of Pallas's Sand Grouse 

 (Syrrhaples paradoxus) in Europe. 

 I beg to add the following statements to my communication 

 of May 12 concerning Pallas's sand grouse in Central Europe 

 (see Nature, May 17, p. 53): — 

 April 22, Cernozitz, Bohemia. 



,, 26, Portitz, near Leipzig, Saxony. 

 ,, 27, Guttmannsdorf, near Reichenbach, Silesia. 

 ,, 27 ? near Hanover. 



,, 27-28, near Hermannstadt, Transylvania. 

 ,, 29, Marmarosch-Comitate, Hungary. 

 Last days of April : Alsofeher-Comitate, Transylvania. 

 Gebhardsdorf, Silesia. 

 Brod, Bohemia. 

 First days of May: Tullner- field, near Vienna. 

 Moravia. 

 Hungary. 



Enzersdorf, near Vienna. 

 Anclam, Pomerania, Prussia. 

 May 6, Haida, Bohemia. 



,, 6, Eidelstedt, near Hamburg. 



,, 7 ? near Schweinitz, Silesia. 



,, 7, Oederan, Saxony. 



,, 7, 6.30 a.m., near Oederan, Saxony. 



,, 8, Wiener Neustadt, Austria. 



,, 8? Dalmatia. 



,, 8 ? Grossvoigtsberg, Saxony. 



,, 8 ? near Leipzig, Saxony. 



,, 8? near Herrenhut, Saxony. 



,, 9, Oederan, Saxony, and nearly" every following day 



there. 

 ,, 13, Selb, Saxony. 

 ,, 13 ? Gro? svoigtsberg, Saxony. 

 ,, 13, Schluckenau, Bohemia. 

 ,, 16, 5 p.m. Oederan, Saxony. 



A. B. Meyer. 

 Royal Zoological Museum, Dresden, May 20. 



A farm in this neighbourhood was visited yesterday by a 

 flight of about forty sand-grouse (pin-tailed). They were first 

 seen about 6 p.m. feeding on a ploughed field. On rising 

 they took a north-westerly course. A pair which were shot by a 

 gamekeeper are in my posse- sion. The presence of these birds 

 in our country is, I believe, of sufficiently rare occurrence to 

 justify me in asking whether they have been noticed in other 

 districts during the last few days. F. M. CAMPBELL. 



Rose Hill, Hoddesdon, Herts, May 21. 



Tables of Reciprocals. 



In investigating spectral phenomena it is often necessary to 

 convert wave-lengths in frequencies. Can any of your corre- 

 spondents inform me if there exist in England tables of reci- 

 procals, by which this may be done easily and with sufficient 

 accuracy? V. A. Julius. 



Delft, Holland, May 19. 



On the Veined Structure of the Mueller Glacier, 



New Zealand. 

 The Mueller Glacier, in the Mount Cook district, has a total 

 length of between six and seven miles, with a breadth of one mile 

 in its lower portion. Like most, if not all, of the New Zealand 

 glaciers of the first order, the lower mile or two is so thickly 

 covered with rock debris that the ice can only be seen in the 

 crevasses. All through the lower portion of the glacier the 

 veined or ribboned structure is well marked, running nearly in 

 the direction of the glacier. But at the terminal face there are 

 two systems of veined structure, with the same strike but crossing 

 one another at angles between 15 and 20 . In one system the 

 blue bands are small, from a half to one inch thick, and separ- 

 ated from each other by bands of white icr, with large air. 



