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NATURE 



[June 13, 1918 



In the chapter on water much attention is devoted 

 to the important question of sterilisation, and 

 although the authors give a brief historical sketch 

 of the subject of chlorine sterilisation, they fail 

 to^ mention the pioneer work of Houston, who, so 

 far back as 1905, was the first to apply the treat- 

 ment to the whole water supply of a town when 

 he undertook the sterilisation of the water supply 

 of Lincoln, and who now controls the chlorination 

 of a large part of the London water supply. The 

 question of dose in relation to period of contact of 

 the water with the sterilising agent .seems to 

 require some modification. 



The authors show great ingenuity in finding a 

 use for. all sorts of waste materials, such as empty 

 oil-drums, biscuit-boxes, and petrol-cans; in fact, 

 it appears that the complete sanitary officer must 

 not only be highly skilled in medical and sanitary 

 science, but also have some considerable know- 

 ledge of such trades as bricklaymg, carpentry, 

 metal work, and a host of others, besides know- 

 ing something of allotment gardening and 

 poultry farming. 



The hook Is well illustrated with clear and well- 

 drawn diagrams, and concludes with what ap- 

 pears to be a most complete and useful index. 



D. B. B. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 thts or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Propagation of Sound and Light in an Irregular 

 Atmosphere. 



I SUPPOSE that most of those who have listened 

 tO' (smgle-engined) aeroplanes in flight must have 

 noticed the highly uneven character of the sound, even 

 at moderate distances. It would seem that the 

 changes are to be attributed to- atmospheric irregu- 

 larities affecting the prop'agation rather than to vari- 

 able emission. This may require confirmation; but, 

 in any case, a comparison of what is" to be expected 

 in the analogous propagation of light and sound has 

 a certain interest. 



One point of difference should first be noticed. The 

 velocity of propagation of sound through air varies 

 indeed with temperature, but is independent of pres- 

 sure (or density), while that of light depends upon 

 pressure as well as upon temperature. In the atmo- 

 phere there is a variation of pressure with elevation, 

 but this is scarcely material for our present purpose! 

 And the kind of irregular local variadons which can 

 easily occur in temperature are excluded in respect of 

 pressure by the mechanical conditions, at least in the 

 absence of strong winds, not here regarded. The 

 question is thus reduced to refractions consequent 

 upon temperature variations. 



The velocity of sound is as the square root of the 

 absolute temperature. Accordingly for 1° C. differ- 

 ence of temperature the refractivity (fx-i) is 000183. 

 Jn the case of light the corresponding value of (fx- i) 

 is 0000294 X 0-00366, the pressure being atmospheric. 

 The effect of temperature upon sound is thus about 

 2000 times greater than upon light. If we suppose tht 

 NO. 2^37, VOL. lOl] 



system of temperature differences to be altered in this 

 proportion, the course of rays of light and of sound 

 will be the same. 



When we consider mirage, and the twinkling of 

 stars, and of terrestrial lights at no very great dis- 

 tances, we recognise how heterogeneous the atmo- 

 sphere must often be for the propagation of sound, and 

 we need no longer be surprised at the variations of 

 intensity with which uniformly emitted sounds are 

 received at moderate distances from their source. 



It is true, of course, that the question is not ex- 

 hausted by a consideration of rays, and that we must 

 remember the immense disproportion of wave-lengths,, 

 greatly affecting all phenomena of diffraction. A 

 twinkling star, as seen with the naked eye, may dis- 

 appear momentarily, which means that then little or 

 no light from it falls upon the eye. When a telescope 

 is employed the twinkling is very much reduced, show- 

 ing that the effects are entirelv different at points 

 so near together as the pai'ts of an object-glass. In 

 the case of sound, such sensitiveness to position is 

 not to be expected, and the reproduction of similar 

 phenomena would require the linear scale of the atmo- 

 pheric irregularities to be very much enlarged. 



June 7. Rayleigh. 



The Drift of Meteor Trails. 



In the Astronomical Column of Nature of May 23 

 there appears a note on the currents in the upper air 

 as revealed by the direction of drift of the streaks left 

 by meteors.. Before we can say with certainty, how- 

 ever, that such drift represents movement of the air, 

 we require to know the real nature of a meteor trail. 

 The ordinary view seems to be that the trail is com- 

 posed of air heated by the meteor in its flight through 

 the atmosphere, the heating being produced not so 

 much by friction as'by the compression of the air in 

 front of the meteor. But is it physically possible for 

 a mass of air so heated to retain its heat so as to 

 remain luminous for any length of time? Streaks 

 have been seen which remained luminous for more 

 than two hours, and though this is exceptional, yet 

 any explanation which would account for long-endur- 

 ing trails would apply also to the more transient kinds. 

 Is it not possible that the trail is an electrical pheno- 

 menon akin to an auroral streamer, or to the patches 

 of light seen during an aurora? The movement of 

 both trails and streamers is usually towards the east, 

 but both more rarely move in other directions. The 

 movement in the case of the aurora is presumably due 

 to the passage of electrified particles moving in the 

 earth's magnetic field, and deflected by it. Is It pos- 

 sible that a meteor trail is due to the passage of elec- 

 tricity through rarefied air that may have been ionised 

 bv the passage of the meteor? 



It Is ' difficult to Imagine that there are definite 

 air currents In the upper part of the atmosphere. It 

 Is true that balloons have not explored the atmosphere 

 much above twenty miles, and that meteor trails are- 

 far higher. But it is difficult to suppose that condi- 

 tions are other than isothermal. In a vertical direction, 

 above the base of the stratosphere, however high one 

 may go. If this is so, there would be no vertical cir- 

 culation ; and if there Is no vertical circulation, could 

 there be anv horizontal circulation? There Is usually 

 a marked falling-off of the wind as a balloon enters 

 the stratosphere. Perhaps some of your readers more 

 versed in dynamical meteorology, and in the question 

 of the passage of electrlcltv through rarefied air than 

 I am. can throw light on the problem. 



June 4. C. J. P. Cave- 



