362 



NATURE 



TMay 19, 192 1 



facturers' Association, which has issued the dictionary, 

 "is one of the industrial associations working in con- 

 nection with the Department of Scientific and Indus- 

 trial Research." Will you permit me to correct a slight 

 misunderstanding here? The British Optical Instru- 

 ment Manufacturers' Association is a trade associa- 

 tion, and is independent of the Department of Scien- 

 tific and Industrial Research. The industrial research 

 association formed under the scheme of the Privy 

 Council for the promotion of scientific and industrial 

 research is the British Scientific Instrument Research 

 Association. Most of the leading British manufac- 

 turers of scientific instruments are members of both 

 associations, but the credit of publishing the dictionary 

 referred to is due wholly to the British Optical Instru- 

 ment Manufacturers' Association. 



J, W. Williamson, 

 Secretary, British Scientific Instrument 

 Research Association. 

 26 Russell Square, W.C.i, May 13. 



Picture-hanging Wire. 



I SHOULD be glad to know the best kind of wire and 

 the best form in which to use it for hanging pictures, 

 etc., on walls. 



Some ten years or so ago I was advised to use 

 twisted brass wire of five strands, which was then 

 immensely strong with a breaking strain of probably 

 more than 100 lb., but it has become so rotten as to 

 break under a weight of a pound or two. This wire 

 has been in use in a very dry room with electric light 

 only. My own experience has proved that plain copper 

 wire in one strand has lasted three times as long as 

 the twisted brass wire, though bearing far heavier 

 weights. Before the war a "wire" consisting of a 

 steel core with some other wire braided over it was 

 recommended, but it is soon affected by rust, and 

 appears to be much stronger than it really is. 



. . R. B. Marston. 



Surrey Lodge, 160 Denmark Hill, S.E.5. 

 Mav 12. 



The Occurrence of Bombus in the Indian Plains. 



As it is generally agreed among naturalists that 

 the genus Bombus — the "bumble-bees" of Europe — 

 . is in India entirely confined to the hills, and never 

 descends below 3000 ft., I write to record its occur- 

 rence in the plains. 



Nearly three years ago, when my entomological 

 knowledge was yet in a rudimentary state, I remem- 

 ber occasionally seeing a bee, which I considered a 

 species of Bombus, at Sukna, situated at the base of 

 the hills of the eastern Himalayas. The few friends 

 to whom I mentioned the incident generally politely 

 turned the conversation aside, but the actual capture 

 a few days ago in Calcutta of two specimens of 

 Bombus tunicatus seems to indicate that my first 

 observation was probably correct, and that "bumble- 

 bees " do (very rarely, of course) occur in the Indian 

 plains In the cold season. Cedric Dover. 



Indian Museum, Calcutta, December 28. 



Symbols in Vector Analysis. 



In books on mathematics and physics where vector 

 analysis is used it is customary to use clarendon or 

 thick-letter type to distinguish vector from scalar 

 quantities. This practice has, among others, the dis- 

 advantages that it reduces the number of symbols 



NO. 2690, VOL. 107] 



available for other purposes, and is impossible to re- 

 produce in manuscript. 



It is justified only by the fact that it prevents con- 

 fusion between the two types of quantities and the 

 consequent application of algebraic operations to 

 vector quantities and vice versa. 



Another means of reaching the same results without 

 the above disadvantages would be to replace the 

 symbols +, -, and = by new symbols in vector 

 analysis. This would be of itself sufficient to dif- 

 ferentiate vector from algebraic symbols, and would 

 be more logical, as the symbols stand for quite 

 different ideas in the two systems of analysis. 



R. H. NiSBET. 



Kut, March 26. 



Young's Interference Experiment. 



I HAVE read with considerable interest Dr. Houstoun's 

 letter on Young's experiment in Nature of April 28, 

 p. 268, and I beg to state that we have been using 

 the spectrometer for some time in the University 

 College of Science, Calcutta. For making the double 

 slit, a rectangular slit, about 2 cm.x2 mm., is cut 

 in a piece of cardboard. Two Gillette razor-blades 

 are placed on two sides of this slit by small pieces of 

 wax. At the centre a fine cocoon fibre, or preferably a 

 spider thread, forms a double slit. By mounting- the 

 cardboard on the prism-table the fringes are easily 

 seen, and as the rotation of the table alters the width 

 of the slit the change in the nature of the fringes can 

 be easily examined. P. N. Ghosh. 



97 Finborough Road, S.W. 10, 

 May 9. 



The Origin of " Churning at 62° " on Dairy 

 Thermometers. 



Mr. Hedger Wallace's question (Nature, April 28, 

 p. 2b8), "Why do makers of dairy thermometers mark 

 their thermometers 62° F. as churning tempera- 

 ture?" interests us as thermometer-makers who are 

 frequently asked to supply floating dairy thermo- 

 meters to a particular pattern. In many cases the 

 customer decides the pattern, and we are prepared to 

 satisfy our customers' requirements. We make and 

 sell a large number of dairy thermometers not marked 

 at -any particular temperature for churning, and we 

 advise this pattern, as we are told by dairv experts 

 that any temperature between 45° and 62° F. may 

 be required, according to conditions. It appears that 

 no definite temperature can be fixed ; therefore, to 

 mark 56° F. as a fixed point for churning would be 

 equally in error. A. C. Cossor and Son. 



Accoson Works, Vale Road, London, N.4, 

 May 9. 



Organism in Flint. 



In reference to Prof. Cole's suggestion (Nature, 

 May 12, p. 333), the possibility of the organism being 

 a radiolarian was considered long ago and rejected. 

 The consensus of opinion is now in favour of its 

 being a beetle. Under higher powers the clavate and 

 merismatic antennae are very conspicuous. There is 

 no niicro-slide of the fossil ; the photographs are 

 taken direct from the flint-surface. Special photo- 

 graphs of the organism's separate parts are now 

 being prepared under more favourable conditions, and 

 will be available shortly. C. Carus-Wilson. 



May 13. 



