July 12, 1883] 



NA TURE 



259 



Land Tortoise (Testudo marginata), South European, presented 

 by Lord Arthur Russell, M.P. ; an Indian Badger {Arctonyx 

 collaris) from Assam, a Rough-billed Pelican (J'Jeeanus trachy 

 rhync/ats) from Mexico, purchased ; two Red-crested Whistling 

 Ducks {Fnligula rufina], a Variegated Sheldrake (Tadorna 

 variegata), five Summer Ducks (Aix sponsa), five Chilian Pintails 

 (Dafia spinkauda), bred in the Gardens. 



OUR ASTRONOMICAL COLUMN 



The Constant of Aberration. — M. Otto Struve presented 

 to the Imperial Academy of Sciences of St. Petersburg, in 

 February last, a memoir by M. Nyren, of the Observatory at 

 Pulkowa, on the aberration of the fixed stars. He states it is 

 the result of researches made by M. Nyren during many years, 

 with the view to deter .nine the value of the con>taut of aberra- 

 tion, with the highe-t decree of accuracy which the most perfect 

 means of observation allow. The value 2o"*445, deduced by 

 W. Struve, has been so far generally accepted by astronomers 

 as the most exact, and has been employed in all astronomical 

 calculations. This is the value given in his memoir upon the 

 subject, but in 1852, by a new combination of his measures, the 

 constant was altered to 2o"'463, and with respect to this value 

 he remarked : " Kile me parait le vrai resultat pour l'aberration, 

 qui doit ctrc tire de raes observations du premier vertical." 

 (Preface to " Recaeil de Memoires presented a TAcademie des 

 Sciences par les Astronomes de Poulkova," t. i.) Notwithstand- 

 ing this statement, Struve's first value was retained in our 

 ephemeri les, &c. ; we have a suspicion that his correction, as he 

 appears to have considered it, was very generally overlooked. 

 M. Nyren was charged with the execution of a new series of 

 observations at Pulkowa, with the same instrument employed 

 by the elder Struve, and every endeavour was made to free the 

 new series from all objection that it was possible to bring against 

 the earlier one. Further, M. Nyren discussed a long series of 

 excellent observations made by M. Wagner with the great 

 meridian telescope in the years 1861-72, on the three stars, 

 Polaris, 5 Urs.r Minoris, and 51 (Hev. ) Cephei. M. O. Struve 

 remarks that with these two new determinations we now possess 

 seven separate series of observations executed with the three great 

 instru rients of the Observatory of Pulkowa, and he gives the values 

 of the constant of aberration resulting therefrom as follow : — 



W. Struve, prime-vertical instrument ... 20*463 ± 0*017 



Sckwdatr, meridian telescope 20*498 ± 0012 



Peters, venical circle 20*507 -fc 0*021 



Gvldin, „ 20469 ± 0*026 



Wagner, meridian telescope 20*483 ± o*OI2 



Nyren, vertical circle 20*495 ± 0*021 



Nyren, prime-vertical instrument 20517 **t 0*014 



M. O. Struve considers that these values sufficiently prove 

 that the constant of aberration is now known with a degree of 

 accuracy which it will be difficult to surpass ; it appears certain 

 that the mean of the seven determinations deduced by M. Nyren, 

 20" '492, will not be liable to an error of a hundredth of a second. 



If this mean value for the constant of aberration is combined 

 with the velocity of light determined by M. Cornu and Mr. 

 Michelson, the solar parallax is found to be 8""784, which, M. 

 Struve adds, only differs by a very few hundredths of a second 

 from the most reliable determinations lately obtained by the 

 geometrical process. 



With regard to W. Struve's alteration of the constant of aber- 

 ration assigned in his memoir, it may be remarked that his result 

 depended upon observations made with the prime-vertical instru- 

 ment upon seven stars, and the separate values accorded well. 

 liut, as he subsequently pointed out, this agreement of different 

 determinations, obtained with the same instrument, only guaran- 

 teed the accuracy of the final result under the condition that there 

 existed no common source of error. He examined all possible 

 sources of constant error, and convinced himself that none 

 existed which could exercise an appreciable influence. Never- 

 theless he said it must be admitted that there existed an agent 

 which possibly might prejudice the exactne.-s of his determina- 

 tion. Considering that the observations of the maximum of 

 aberration fall at a time of year when the star passes the 

 meridian near 6 p.m., while the observations of the minimum of 

 the aberration take place at 6 a.m., it is seen that the first are made 

 during a decreasing temperature and the last during an increasing 



one. " The zenith-distance of the star being determined from 

 the time between the two corresponding transits indicated by the 

 clock, it follows, if the clock has a defect of compensation and 

 if its effective rate during the interval differs from the mean daily 

 rate obtained by observations of consecutive days, that the error 

 produced acts in the same sense up-.n the resuhs obtained by 

 different stars." It is the same if between the two corresponding 

 pa-sages the azimuth of the axis of rotation changes. Fortunately 

 these two perturbing causes only exercise a minute influence upon 

 the zenith distances to be delermined. Yet, as Struve asks : 

 "Comment prouver que cette influence n'ait point alteie la 

 valeur trouvee de l'aberration de quelques centiemes de seconde ? " 

 He con-idered he had direct proof that there was no azimuthal 

 change, but with regard to change of clock rate, as already 

 stated, he was induced to rediscuss his series of observations 

 with the result above given. 



ON THE FUNCTION OF THE SOUND-POS7', 

 AND ON THE PROPORTIONAL THICKNESS 

 OF THE STRINGS OF THE FIOLIN 1 



CIR JOHN HERSCHEL says: " It (the bridge) sets the 

 *~ wood of the upper face in a state of regular vibration, and 

 this is communicated to the back through a peg stt up in the 

 middle of the fiddle and through its sides, called the ' soul ' of 

 the fiddle, or its sounding- post." z 



_ Savart says : " L'ame a pour usage de transmetire au fond les 

 vibrations de la table . . . son diametre est determine par la 

 qualite du son qu'on veut avoir ; il est maigre quand elle e=t trop 

 mince, et sourd quand elle est trip grosse." 3 



Daguin, in his " Trare de Physique," devotes a whole page 

 to the discussion of the fundi ins of the sound-post. The most 

 important sentences are the following : — " . . . l'ame n'agit pas 

 comine conducteur du son. . . 11 nous semble que Ton doit 

 expliquer l'effet de Tame de la maniere qui suit. L'ame, ou les 

 pressions exterieures par lesquelles on la remplace, a pour effet 

 de donner au pied du chevalet un point d'appui autour duquel il 

 vibre en battant sur la table de son autre pied. Si Tun des pied* 

 n'etait appuye sur un point fixe, il se releverait pendant que 

 1'autre s'abai serait, parceque les c irdes n'agissent pas normale- 

 ment a la table, puisque 1 archet les ebranle ties obliqu ment, ce 

 qui entraine le chevalet dans un mouvement transversal quand il 

 n'a pas de point d'appui fixe. Lorsque Tarchet et dirige nor- 

 malement aux tables, cet inconvenient n'exUte plus, et Tame n'est 

 plus neces-aire. " 4 



llel nholtz says : " The vibrating strings of the violin, in the 

 first place, agitate the bridge over which they are stretched. 

 This stmds 011 two feet over the most mobile part of the belly 

 between the two '/' hole-. One foot of the bridge re-ts upon 

 a comparatively firm support, namely, the sound-post, which is 

 a solid rod inserted between the two plates, back and belly, of 

 the instrument. It is only the other leg which agitates the 

 elastic wooden plates, and through them the included mass of 

 air. 6 



The experiments ,; which follow have been made for the pur- 

 pose of ascertaining whether it be any part of the function of 

 the sound-post to convey vibrations to the back, or whether this 

 post acts solely as a prop supporting the belly, so that its elas- 

 ticity is not injured by the pressure from the strings, and aLo, as 

 Daguin states, affords the firm basis which he con-iders necessary 

 for one toot of the bridge. 



Mr. Hill and other practical men maintain that the quality of 

 the w*ood of which the sound-po>t is made affects the tone of the 

 violin, a. undoubtedly do very minute differences of po-ition. 

 If the quality of the wood is important, we must admit that 

 vibrations are conveyed by the post. 



Whether or not tile sound-post exercise- the function of trans- 

 mitting vibrations, it is obvious (i) that it performs the impor- 

 tant duty of contributing to the support of the belly ; (2) that 

 the nodal arrangement of the belly and als 1 that of the back are 



1 Paper read at the Royal Society, May 24, by Willi. in Huggins, D.C.L., 

 LL.D., F.R.S. 



- " Encyclopaedia Metropohtana," Article '' Sound," p. 804. 



3 " Me"moire sur la Construction des Instruments a Cordes et a Archet/' 

 8vo, Paris, 1819. Also Bint's *' Report." Ann. de Chitnie, tome 12, pp. 

 225-255 



4 '* Traite* de Physique, Acoustique." t -me 1. p. 575. 



5 "Sensations of 'Tone " translated by Ellis, p. 137. In the 4th German 

 edition this passage remains unaltered. 



6 I wish to express my indebtedness to Mr. A. J. Ellis f jr some sugges- 

 tions in conneciijn wall these experiments, 



