500 



NATURE 



[January 15, 1920 



between plumage of all kinds that when the Anti- 

 Plumage Bill was introduced in 1913 the trade in- 

 security engendered was so far-reaching as to con- 

 stitute one of the principal causes of the serious slump 

 in the South African ostrich industry, involving the 

 loss of millions of pounds — a slump from which, the 

 war supervening, the Union is only slowly recovering. 

 The introduction of another Bill would be viewed with 

 alarm in South Africa, and would have a serious 

 international bearing, particularly upon our Ally, 

 France, involving thousands of workers and millions 

 of capital. It surely were wise not to attempt it when 

 other measures are possible which would afford a 

 wide stimulus to industry and to the studv of bird- 

 life. J. E. DUERDEN. 



Royal Colonial Institute, 



Northumberland Avenue. 



Musical Drums with Harmonic Overtones. 



It is well known that percussion instruments as a 

 class give inharmonic overtones, and are thus musicallv 

 defective. We ^nd on investigation that a special 

 type of musical drum which has long been known and 

 used in India forms a very remarkable exception to 

 the foregoing rule, as it gives harmonic overtones 

 having the. same relation of pitch to the fundamental 

 tone as in stringed instruments. Five such harmonics 

 (inclusive of the fundamental tone) can be elicited- 

 from the drumhead in this type of instrument, the 

 first, second, and third harmonics being spcciallv 

 well sustained in intensity and giving a fine musical 

 effect. The special method of construction of the 

 drumhead which secures this result will be understood 

 from the accompanying illustration (Fig. i). It will 



KiG. I. — iJrmnhead giving harmonic overtones. 



be noticed (i) that the drumhead carries a svmmetrical 

 distributed load, decreasing in superficial densitv from 

 the centre outwards (this appears as a dark circle in 

 the middle of the membrane, the load consisting of a 

 firmly adherent but flexible composition, in which the 

 principal constituent is finely divided metallic iron) ; 

 and (2) that a second membrane in the form of a 

 ring is superimposed on the circular membrane round 

 its margin. 



The character of the vibrations of this heterogeneous 

 membrane which give rise to its remarkable acoustic 



NO.i 2620, VOL. 104] 



properties have been investigated by us. It is found, 

 as might have been expected, that the fundamental 

 pitch and the octave are derived respectively frojn tlie 

 modes of vibration of the membrane without anv 

 nodal lines and with one nodal diameter. The third 

 harmonic, we find, owes its origin to the fact thai the 

 next two higher modes of vibration of the drumhead 

 (those with two nodal diameters and with one noda! 

 circle respectively) have identical pitch, this being a 

 twelfth above the fundamental. There is reason to 

 believe that the fourth and fifth harmonics similarlv 

 arise from some of the numerous more complex modes 

 of vibration of the drumhead becoming unified in 

 pitch in consequence of the distributed load at the 

 centre and round the periphery of the membrane. The 

 central load also improves the musical effect bv in- 

 creasing the energy of vibration, and thus prolonging 

 the duration of the tones. C. V. Raman. 



SivAKALi Klmar. 

 210 Bowbazaar Street. Calcutta, India, 

 rX'cember 10. 



Power from the Sun. 



.Mk. a. .\. Ca.mi'bell .S\M.\roN, in his letter on the 

 above subject in Nature of December 18, states that 

 "it is hopeless to expect to be able to effect anything 

 of this nature with the heat-engine, for with this we 

 should scarcely reach the 2 per cent, efficiency nearly 

 attained by vegetation." 



For nearly four years immediately preceding the war 

 I was engaged by the Sun Power Co. (Eastern Hemi- 

 sphere), Ltd., and the Shuman Engine Syndicate, 

 Ltd., on the problem of the utilisation of solar energy, 

 upon which these companies spent a considerable sum 

 of money, the experiments being conducted on a large 

 scale in America and Egypt, while the trials of the 

 necessary low-pressure engine (which made an easy 

 record for such engines) were made in this country. The 

 results of the whole of this work are recorded in m; 

 two papers both bearing the title "The Utilisation 

 of Solar Energy," one bein^ read before the Society of 

 Engineers in April, 1914, and the other before th<! 

 I Royal Society of Arts in April, 1915. 

 i At p. 540 of the Journal of the Royal Society 01 

 Arts of April 30, 1915, it was shown that the overall 

 thermal efficiency of the sun power plant erected in 

 Egypt was 432 per cent., which is to be compared 

 with the performance of the best steam-engine and 

 boiler of 11-5 per cent. At p. 560, ihid.. it was shown 

 that the theoretical efficiency of an engine working 

 between the same limits of temperature would bt 

 59 per cent., and that, consequently, the relative 

 efficiencv of the sun power plant to this ideal engine 

 was no less than 732 per cent. From this it will be 

 seen that Mr. Campbell Swinton's estimate of the 

 thermal efficiency of 2 per cent, for a sun-power steam 

 plant is more than 100 per cent, too low. 



It is well that any wrong impression which the lower 

 figure might give should be corrected, for in these 

 davs of extremelv expensive coal it is desirable that 

 inventors, experimenters, and financiers should not 

 be discouraged from attempting to utilise solar energy, 

 which some of us think is bound to be realised in the 

 future. In the Royal Society of Arts paper, it was 

 shown that the cost of solar energy was equivalent to 

 coal at 3(. 105. a ton, and from this it is obvious that 

 had the comoanies had more time in which to (develop 

 and construct plants before the war, they would have 

 been paving handsomely, with coal at its present 

 prices, in Egvpt, Chile, and other sun-bathed countries. 

 .\lfred S. E. .^ckermann. 

 25 Victoria Street, Westminster, S.W. i, 

 Januarv 6. 



