402 



NATURE 



[July 13, 19I6 



on definitely stated subjects, and who is, as a general 

 rule, expected to complete the duties within a more or 

 less definite time-limit. 



For investigations falling under the first category 

 the problem of remuneration presents serious diffi- 

 culties, and we may at least console ourselves with the 

 knowledge that a step in the right direction has been 

 taken by the Board of Education in requiring returns 

 to be made of researches conducted by the staffs and 

 graduates of our university colleges. In this connec- 

 tion it is, further, becoming recognised that teachers 

 in these institutions should have sufficient opportunity 

 in term time, as well as in vacation, for research. 



It is with regard to the second class of investi- 

 gation that the claim for remuneration is most 

 urgent. From personal knowledge, I consider that 

 it is impossible for an average skilled labourer in the 

 scientific industry to earn a living wage consistent 

 with his necessary expenses unless his whole time is 

 available for remunerative duties. It is true that 

 intervals occur, sometimes quite unexpectedly, during 

 which he may be temporarily unemployed, and these 

 can be utilised for purposes of research ; on the other 

 hand, there are certain periods of the year when the 

 work is extremely heavy, and latitude of time is 

 necessary even for the performance of paid work. 



There are probably very few scientific labourers who 

 would be justified in refusing an invitation to mark 

 500 examination papers at a fee of is. per paper in 

 order to complete an investigation for the Government 

 for which they received no fee. As soon, how- 

 ever, as the labourer accepts remuneration for a 

 definite undertaking, his employer has some guarantee 

 that he will not let future engagements interfere 

 with the fulfilment of his contract. This at least 

 applies to scientific specialists who are not members 

 of trade unions. 



I am very much afraid, however, that a great many 

 people are undertaking unpaid work under conditions 

 quite incompatible with the present depressed con- 

 ditions of the scientific labour market. In some cases 

 this is being done from a sense of patriotism. Un- 

 doubtedly their labours may have the effect of reducing 

 the duration and the severity of the lesson which the 

 enemy countries are teaching us in regard to our 

 national neglect of science — a lesson which is the one 

 good turn the Huns are doing us. But they are 

 certainly tending to diminish the efficacy of that 

 lesson. G. H. Bryan. 



Negative Liquid Pressure at High Temperatures. 



In my paper with Lieut. Entwistle on the effect of 

 temperature on the hissing of water when flowing 

 through a constricted tube (Proc. Royal Soc, A. 91, 

 1915) I h'-vc determined the temperature coefficient 

 o^ £.11 effect which indicates that the tensile strength 

 of water would be zero at a temperature between 

 279° C. and 363° C, with a mean from all the experi- 

 ments published of 328° C. Sir Joseph Larmor's 

 calculated result, 265^0., quoted by him in his letter 

 in Nature of June 29, agrees satisfactorily with the 

 experimental value if we take into account the difficulty 

 of getting the precise point at which hissing ceases, 

 and that the result was obtained by extrapolation 

 from observations taken at temperatures between 

 12° C. and 99° C. Lieut. Entwistle and I have ex- 

 perimented with other liquids — alcohol, benzene, 

 acetone, and ether — and obtained results of a similar 

 character. Experiments are now in abeyance, for my 

 colleague is otherwise engaged. 



My own view, formed from physical conceptions, 

 was that the tensile strength of a liquid would become 

 zero at its critical temperature. It is of very great 



NO. 2437, VOL. 97] 



interest that Sir Joseph has been able to show mathe- 

 matically that the negative pressure can only subsist 

 at absolute temperatures below 27/32 of the critical 

 point of a substance. 



The conclusions appended to our paper are : — 



1. That the phenomenon of hissing of water passing 

 a constriction is due to a true rupture of the stream- 

 at the point where the pressure is lowest. 



2. That the temperatures at which the hissing just 

 occurs, between 0° and 100° C, follow a law which 

 may be expressed V = C(0— f), where V is the velocity 

 of the stream at a temperature t, 6 the critical tem- 

 perature of water, and C a constant. 



If we adopt Sir Joseph Larmor's view the latter 

 law will require to be expressed 



V = C;27/32(e + 273)-(t + 273);, 



or by a slightly more complex formula. 



Sidney Skinner. 

 South-Western Polytechnic Institute, Chelsea. 

 July 3- 



THE PROPAGATION OF SOUND BY THE 

 ATMOSPHERE. 



SINCE the beginning of the war the sound of 

 gun-firing in Flanders and France has often, 

 been heard in the south-eastern counties of 

 England. There can be little doubt as to the 

 origin of the sounds, for the reports of distant 

 heavy guns have a character which is readily 

 recognised. A correspondent of the Daily Mail 

 (July 6) states that at Framfield (near Uckfield), 

 in Sussex, it is easy to identify the particular kind 

 of gun which is being used. The great distance 

 to which the sound-waves are carried under 

 favourable conditions is evident from the letters, 

 recently published in the Daily Mail. As firing, 

 has occurred lately over a great part of the 

 Western front, the exact position of the source 

 of the sound is uncertain. But if it were in the 

 neighbourhood of Albert the waves must have 

 travelled about 118 miles to Framfield, 150 miles, 

 to Sidcup, and 158 miles to Dorking. 



Of far greater interest are the form and dis- 

 continuity of the sound-area. A remarkable- 

 example of the inaudibility of neighbouring, 

 reports in the face of a gentle wind was given 

 in the last number of Nature (p. 385). This is 

 a subject on which many observations have been- 

 made since the beginning of the present century, 

 especially in connection with the sounds of vol- 

 canic and other explosions. The source of sound 

 is always surrounded by an area of regular or 

 irregular shape within which the sound is every- 

 where heard, though the source is not always- 

 situated symmetrically with reference to the 

 boundary of the area. On several occasions a 

 second sound-area has been mapped, separated' 

 from the former by a " silent region " in which 

 no sound is heard. Sometimes this second area 

 partly surrounds the other, sometimes it consists 

 only of isolated patches. As a rule, according 

 to Dr. E. van Everdingen, who has made a de- 

 tailed study of the subject, ^ the least distance of 

 the second area from the source is much more- 



1 "The Propagation of Sound in the Atmosphere." Koninklijke Akad.. 

 van Wetenschappen te Amsterdam, Proc., vol. xviii., 1915, pp. 933-960. 



