April 2,0, 1874] 



NA TURE 



513 



The Russian Scientific Expedition to the Amu Daria was to 

 set out on Monday last. The expedition will be commanded by 

 the Grand Duke Nicholas Constantinovitch, assisted by Colonel 

 Stolctoff and Dr. Moreff, secretary. It will include 25 persons, 

 whose work will be divided into four sections : — (i) The Trigo- 

 nometrical and Topographical. (2) The Meteorological Section, 

 which will construct, two stations on the Amu Daria, at one of 

 which hourly observations will be made of all the meteorological 

 phenomena. (3) The Ethnographical Statistical Section. (4) The 

 Natural History Section. 



The meeting of French Astronomers took place last week 

 at the Ministry of Public Instruction, under the presidency of 

 M. Leverrier. It was composed of M. Dumetil, director of the 

 Enseignemcnt, the astronomers from Paris, Toulouse, and 

 Marseilles Observatories, and Officers from the General Staff 

 of the Trigonometrical Survey. Four sittings were held, and 

 an account of them will be issued shortly. Steps have been 

 taken for the determination of the latitude of Algiers, by tele- 

 graph. M. du Barail, Minister of War, and M. Saget, his Staff- 

 Officer, visited the Observatory last Saturday, in order to see 

 for themselves how the work may be begun without further 

 delay. 



The additions to the Zoological Society's Gardens during the 

 last week include four Bladder-nosed Seals (Cystophora cristaia] 

 from Greenland, presented by Capt. Alex. Gray ; a White- 

 winged Whydah Bird (Urohrachya albonolus) from West Africa, 

 presented by Mr. J. Fairchild ; a Rose-crested Cockatoo (Caca- 

 tua tiwlticceiuis) from the Moluccas, presented by Mr. H. Bald- 

 win ; an AzarajS Fox {Cants azara) from South America ; a 

 Snowy Owl (Nyctea nivca) from South America ; a Green- 

 cheeked Amazon {Chrysotis viridigenalis) from Columbia, pur- 

 chased. 



ON THE REFRACTION OF SOUND* 



'T'HE principal object of this paper is to show that sound, in- 

 ■*■ stead of proceeding along the ground, is lifted or refracted 

 upwards by the atmosphere in direct proportion to the upward 

 diminution of the temperature ; and hence to explain several 

 phenomena of sound, and particularly the results of Prof. Tyn- 

 dall's recent observations off the South Foreland. 



The paper commences with the explanalion of the efiect of 

 wind upon sound, viz., that this efTect is due to the lifting of the 

 sound from the ground, and not to its destruction, as is generally 

 supposed. The lifting of the sound is shown to be due to the 

 different velocities with which the air moves at the ground and 

 at an elevation above it. Owing to friction and obstructions the 

 air moves slower below than .above, therefore sound moving 

 against the wind moves faster below than above, and the bottom 

 of the sound waves will thus get in advance of the upper part, 

 and the effect of this will be to refract or turn the sound up- 

 wards ; so that the rays of sound which would otherwise move 

 horizontally along the ground actually move upwards in circular 

 or more hyperbolic paths, and may thus, if there is sufficient dis- 

 tance, pass over the observer's head. This explanation was 

 propounded by Prof. Stokes in 1857, but it was discovered 

 independently by the author. 



The paper then contains descriptions of experiments made 

 with a view to establish this explanation. 



These experiments were made with an electric ball, over a 

 nearly flat meadow, and again over the same when it was nearly 

 covered with snow, and it was found (as indeed it was expected) 

 that the condition of the surface very materially modified the 

 results in two ways. In the first pl.ice, a smooth surface like 

 snow obstructs the wind less than grass, hence over snow the 

 wind has less effect in lifting the sound moving against it than 

 over grass ; and it is inferred that a still greater difference would 

 be found to exist in the case of smooth water. In the second 



• On the Refraction of Sound by the A mosphere, By Prof. Osborne 

 Reynolds, Owens College, Manchester. Abstract of paper read before the 

 Royal Society April 33.— Communicated by the Author. 



place, the ends of the waves of sound travelling along in contact 

 with the rough ground are continually destroyed by the rough- 

 ness, and the sound from above slowly diverges down to replace 

 that which is destroyed, and this divergence gradually weakens 

 the intensity of the lower parts of the waves, so that, under 

 ordinary circumstances, the sounds which pass above us are more 

 intense than those we hear. The general conclusions drawn 

 from these experiments are : — 



1. The velocity of wind over grass differs by i at elevations of 

 I and 8 feet, and by somewhat less over snow. 



2. That when there is no wind, sound proceeding over a rough 

 surface is destroyed at that surface, and is thus less intense below 

 than above ; owing to this cause the same sound would be heard 

 at more than double the distance over snow at which it could be 

 heard over grass. 



3. That sounds proceeding with the wind are brought down to 

 the ground in such a manner as to counterbalance the effect of 

 the rough surface (2), and hence, contrary to the experiments of 

 Delaroche, the range of sound over rough ground is greater with 

 the wind than at right anj^les to its direction or than when there 

 is no wind. When the wind is very strong it would bring the 

 sound down too fast in its own direction, and then the sound 

 would be heard farthest in some direction inclined to that of the 

 wind though not at right angles, 



4. That sounds proceeding against the wind are lifted off the 

 ground, and hence the range is diminished at low elevations. But 

 that the sound is not destroyed and may be heard from positions 

 sufficiently high (or if the source of sound be raised) with even 

 greater distinctness than at the same diijtances with the wind. 



5. In all cases where the sound was lifted there was evidence 

 of diverging rays. Thus although on one occasion the full inten- 

 sity was lost when standing un at 40 yards the sound could be 

 faintly and discontinuously heard up to 70 yards. And on raising 

 the head the sound did not at once strike the ear with its full in- 

 tensity nor yet increase quite gradually ; but by a series of steps 

 and fluctuations in which the different notes of sound were vari- 

 o".sly represented, showing that the diverging sound proceeds in 

 rays separated by rays of interference. 



On one occasion it was found that with the wind sound could 

 be heard at 360 yards from the bell at all elevations, whereas at 

 right angles it could be only heard for 200 yards standing up, 

 and not so far at the ground ; and against the wind it was lost at 

 30 yards at the ground, at 70 yards standing up, and 160 yards 

 at an elevation of 30 feet, although it could be distinctly heard at 

 th:s latter point from a few feet higher. 



It hence appears that these results agree so well with what 

 might be expected from the theory as to place its truth and com- 

 pleteness beyond question. 



The author then goes on to argue from the action of wind upon 

 sound to another phenomenon which admits of a somewhat simi- 

 lar explanation. The effect of wind together with that of a rough 

 surface in lifting the sound may be shown to account for many of 

 the apparently capricious variations in the intensity with which 

 sounds can be heard at different times ; and it gives a reason for 

 the custom which prev.ails of elevating church bells, platforms, 

 &c., where the pounds are intended to be heard at a distance. 

 But it does not explain a fact, which has often been observed, 

 namely, that distant sounds can be heard much better during the 

 night than during the day, and on dull cloudy days better than 

 on bright hot days. This phenomena has engaged the attention 

 of Humboldt, Delaroche, and recently of Prof. Tyndall, who 

 have all assumed that the sound is obstructed or destroyed in the 

 bright hot air, and have suggested causes which they thought 

 might produce this effect. These suggestions are all more or less 

 open to objection, and none of them meet the difficulty that any 

 heterogeneous condition ol the air which could obstnict sound must 

 more or less refract or reflect light and so render vision indistinct. 

 In this paper the author gives another explanation, in which he 

 shows how, as in the case of wind, the sound may be lifted and 

 not destroyed. 



It is argued that since wind raised the sound simply by causing 

 it to move faster below than above, any other cause which pro- 

 duces such a difference in velocity will lift the sound in the same 

 way. And since the velocity of sound through air increases with 

 the temperature — every degree from 32 to 70 adding i foot per 

 second to the velocity — therefore an upward diminution in the 

 temperature of the air must produce a similar effect to that of 

 wind and lift the sound. Whereas Sir. Glaisher has shown by 

 his balloon observations that such a diminution of temperature 

 exists, and further he has shown that when the sun i<; shining 

 with a clear sky the variation from the surface is i^ for every 



