232 



NA TURE 



{Jan. 21, 1875 



wjlli shoals, until in the reacli wheie Eend is situated, jinJ where 

 the maximum volume h^s been abstracted for purposes of irriga- 

 tion, the entire breadth of the Amu Darya is obstructed by a 

 mass of sandbanlcs intersected by narrow and tortuous channels. 

 It appears, then, that such in'ormation as we have, regarding 

 the change and the existent conditions of the old and new 

 courses ol the Amu Darya, presents a picture precisely the con- 

 verse of that delineated in and quoted from Sir Charles Lyell's 

 work. In lieu of a constant increase to the transporting capacity 

 of the waters of the river, we see that in the Amu Darya such 

 is replaced by a con.stantly diminishing transporting power, and 

 that the old bed has been filled up and destroyed by the deposi- 

 tion of silt. This deposition of silt and deterioration of the bed 

 can only have been caused by the abstraction of its waters for 

 irrigation. Whether other circumstances assisted the consequent 

 change of the flow of the Amu Darya is a question it is not my 

 purpose to examine in this place. Enough has, I would submit, 

 been adduced to show that the practice of irrigation, as con- 

 ducted on the banks of the Amu Darya, produces phenomena 

 whose action furnishes a probable explanation of a very curious 

 and interesting geographical problem. Herbert Wood 



THE PARIS INTERNA TIONAL CONGRESS OF 

 GEOGRAPHICAL SCIENCE 



THE meeting of the International Congress, of which vjre 

 published the programme a few months ago (vol. x. 

 p. 267), has been postponed, owing to the large number 

 of demands from foreign parts for room in the Exhi- 

 bition. It will not take place in the beginning of spring 

 as intended orginally, but will be opened on the ist of 

 August, perhaps by the President of the Republic, who 

 seems to be deeply interested in the success of the enter- 

 prise. It will be held in the Pavilion de Florc. This 

 magnificent building was left unfinished when the Empire 

 was upset, and could not be burned by the Communists, 

 as the woodwork had not been begun. It is now being 

 decorated most tastefully, and will be inaugurated by 

 the Congressionists. 



An exhibition will also take place in the Pavilion 

 de Flore and Orangerie situated close to the Place 

 de la Concorde. All the Terrace du Cord de I'Eau, 

 from the Pavilion de Flore to the Orangerie, will form 

 part of the Exhibition. Temporary sheds of every 

 description will be constructed in that splendid situation 

 along the banks of the Seine and under the four rows of 

 lofty trees. The coup d'a-il will be splendid, and is sure 

 to attract an immense number of spectators. The Ex- 

 hibition will be opened on the 19th of July, and will last 

 until the 4th of August. A very large number of gentle- 

 men ot all countries have been appointed members of the 

 honorary committee. The president of the Congress is 

 M. Delesse, a French engineer in the mining service, 

 and a great geologist. M. Delesse is now the president 

 of the Central Committee of the Geographical Society. 

 Up to the present moment the vice-president has not 

 been elected. 



The Exhibition and Congress, as we fonnerly notified, 

 have been divided into seven different groups : (i) Ma- 

 theipatical ; (2) Hydrographical ; (3) Physical ; (4) His- 

 torical ; (5) Economical ; (6) Didactic ; (7) Travels. 



A programme of 123 questions has been published, and 

 all these, as far as possible, will be discussed by the 

 members of the Congress. The principal questions will 

 te found in the article referred to. 



ON THE ALTERATION OF THE NOTE OF 

 RAILll'A V WHISTLES IN TRAINS MEETING 

 EACH OTHER 



I AM not aware whether the following explanation of 

 this curious acoustical phenomenon has ever appeared 

 in print ; if it has, it will, I think, bear repetition, as 

 offering an interesting illustration of some of the laws of 

 propagation of undulations through aerial media. 



If two railway trains meet and pass each other at 

 tolerable speed, and the driver of one of them is sounding 

 his whistle, any person in the other train accustomed to 

 music will notice that the moment the whistle passes him 

 its note will be lowered i)i pitch in a marked degree. 



It was at first supposed that, at the time of passing, the 

 driver lowered his whistle intentionally, as a salute to the 

 other train (like " dipping the ensign ■' at sea), but this 

 was found not to be the fact, the driver himself being un- 

 conscious of any change. I believe the true explanation 

 was first given by Mr. Scott Russell, but I do not know 

 when or where. 



It is an exactly parallel case to one which has recently 

 attracted attention in astronomy, namely, the evi- 

 dence afforded by the change in position of certain 

 spectral lines, owing to the vapours which produce them 

 approaching or receding from the observer. The expla- 

 nation of this will be familiar to most of the readers of 

 Nature, and I have only to apply it to the case in 

 question. 



Every musical note propagates aerial waves succeeding 

 each other with a known rapidity, corresponding to the 

 pilch of the note ; the higher the pitch, the greater the 

 rapidity of succession of the waves, and vice versa. Now, 

 when a person advances to meet these waves, more of 

 them will pass him in a given time than if he stood still, 

 on the same principle that if a man meets a file of 

 soldiers on march, more men will pass him per minute 

 than if he were stationary. Thus the apparently increased 

 rapidity of the waves will give him the impression of a 

 sliarpcr note. 



On the other hand, when the trains have passed each 

 other, the listener will be moving in the same direction 

 as the sound-waves, and consequently a less number will 

 pass him in a given time, causing the note to appear 

 flatter. 



The sum of these effects will be the sudden drop of the 

 pitch of the note at the moment the listener passes the 

 whistle. 



We may reduce the effect to numerical calculation, 

 premising that, in order to simplify the reasoning, we 

 will suppose the source of the sound to be stationary, and 

 the observer to move towards it with a given velocity. 



Let.;/ = number of sound-waves propagated by the 

 given note per second ; and let ii-^ = the number which the 

 listener will gain by his advance in the same time, which 

 is the number he would pass by his own proper ino(ioii 

 if the waves were standing still. 



Then the effective number of waves per second which 

 will meet his ear will be = « + «j, this number determining 

 the pitch of the note he hears. This may be called (by an 

 astronomical analogy) \^& apparent pitch, as distinguished 

 from the true pitch. 



To find the value of «;, let L = the length of the sound- 

 wave (= — where V = velocity of sound in feet per 

 second). Then, \i v = velocity of motion of the listener 



V 



he would pass, by his own proper motion, -■ waves per 



second ; whence //i = -J = t;- 



Hence the apparent pitch of the note is what will cor- 

 respond to the number of vibrations 



But we may simplify this by applying the harmonic 

 principle, that a musical interval is measured by the ratio 

 of the vibration numbers of its higher and lower limiting 

 sounds. Let therefore 5 = the interval between the real 

 and the apparent sound ; then 



K' + f) 



