January i6, 190^] 



NA TURE 



247 



rule — no inslrumi-'nt has yet been devised whiih will in- 

 variably do this — but should not be too sensitive, or the 

 record of important disturbances may be lost. 



(4) The records should be capable of easy and rapid 

 reproduction. 



Of the instruments which have been designed or sug- 

 gested for this purpose, four types are in use to a greater 

 or less extent. 



The Wiechert so-called astatic pendulum is an inverted 

 pendulum with a bob weighing more than a ton, kept in 

 position by two springs, and provided with an ingenious 

 system of air-damping of its vibrations. This instrument 

 has been recommended for general use, because its sup- 

 posed astatic nature is believed to make it record the 

 movement of the soil in an accurate manner ; as already 

 pointed out, this condition is immaterial, and, moreover, 

 cannot be completely fulfilled. The instrument is un- 

 doubtedly a fine one, and gives valuable records, but its 

 proper place is in an observatory specially devoted to 

 seismology ; for general use it is too heavy, requires too 

 much attention, and gives records which are not adapted 

 for ready and rapid reproduction. 



The Rcbeur-Ehlert instrument is a horizontal pendulum, 

 of the form devised by v. Rebeur-Paschwitz, combined 

 with a recording arrangement devised by Prof. Ehlert. 

 This instrument is an e.xtremely sensitive one, and there 

 seems to be none better for recording small disturbances ; 

 in the case of large earthquakes the record is apt to be 

 lost. The record is photographic, and the seisniograms 

 are readily reproduced by photography. Its cost of main- 

 tenance and too great sensitiveness are the points in which 

 it fails to meet the requirements of an instrument for 

 general adoption. 



The so-called Omori pendulum is a horizontal pendulum 

 presenting no special peculiarities, and is a modification, 

 in details only, of a type of instrument in very general 

 use. It fulfils all the first three requirements, being 

 moderate in size, needing little attention, and gives good 

 records, easy of interpretation and measurement. It fails 

 in the fourth requirement only ; the record, being taken on 

 smoked paper, is not readily reproduced by photography, 

 and is on too small a scale to obviate the risk of intro- 

 ducing error when copied by tracing. 



The fourth type of instrument is the Milne pendulum, a 

 horizontal pendulum with photographic record on a prin- 

 ciple quite different from that adopted in any other instru- 

 ment. This fulfils all the first three requirements, and 

 the fourth too ; the seismograms are easily reproduced by 

 contact printing on to the same photographic paper that 

 is used for recording, and the copies are practically as 

 serviceable as the originals. This alone, if the instrument 

 had few other merits, would mean much ; but in addition 

 to this I have found its seismograms the most convenient 

 of any for determining the exact time of any point on the 

 record, and had it not been for the general adoption of 

 this type of instrument, and the ease with which its 

 records can be reproduced, a considerable part of what 

 seismological work I have been able to do could not have 

 been attempted. The only improvement I have ever 

 desired to see is an increase in the rate of movement of 

 the recording surface, and this has now been introduced. 

 I have examined and studied hundreds of records of this 

 instrument from different stations; from Victoria, Toronto, 

 Cape Town, Bidston, Paisley, and many other places, its 

 records are consistently good ; at a few stations, whether 

 from a defect in the particular instrument, a want of 

 proper adjustment, or, more probably, something in the 

 foundations or the subsoil, its records are less satisfactory, 

 but from none do they seem to be so bad as at Strassburg ; 

 having never seen a seismogram of that instrument — it is 

 not easy to get copies from Strassburg — it is impossible 

 to hazard a suggestion of the reason for the failure of the 

 instrument at this station. 



\o one would wish to see one pattern of instrument 

 adopted to the exclusion of all others, nor has it ever been 

 pretended that the pattern adopted by the organisation 

 which has grown up under the auspices of the British 

 Association is faultless : but for the purpose of securing 

 a large number of records for comparison with each other, 

 and thereby determining the rate of transmission of earlh- 

 <)uakis across, through, and around the earth, it is no 



NO. 1994, VOL. 'J']'\ 



more faulty than any other pattern, and has one crowni:;.^ 

 merit which they do not possess. Can it be to this, to 

 the ease of reproduction of its records, which renders 

 unnecessary the centralisation of seismological research, 

 that we must attribute the continuous vilification of a 

 valuable type of instrument? R. D. Oldh-im. 



An Eaily Acoustical Analogue of Michelson's Echelon 

 Grating. 



In the " OEuvres completes " of Christiaan Huygens 

 (tome X., p. 571) occurs the note given below. It was 

 destined for Ph. de la Hire, and of date November, 1693. 

 Huygens 's remarkable observation and his ingenious ex- 

 planation of the musical note produced by reflection from 

 a large flight of steps of the noise of a fountain in the 

 park of Chantilly will be read with interest also by those 

 who, though having no ready access to the " Qiuvres 

 completes," are still concerned with the (reflecting) echelon 

 grating : — 



" Je veux adjouter icy au sujet de la reflexion du son 

 une observation assez singulifere, que j'ay fait autrefois 

 estant ii la belle maison de Chantilly de la Cour oil est 

 la statue Equestre on descend avec un degr^ large de . . . 

 marches dans le parterre ou il y a une fontaine de celles 

 qu'on appelle gerbe d'eau, qui fait un bruit continuel. 

 Quand on est descendu en has et qu'on se tient entre le 

 degr^ et la fontaine on entend du cost^ du degri une 

 resonance qui a un certain ton de musique qui dure con- 

 tinuellement, tant que la gerbe jette de I'eau. On ne 

 s^avoit pas d'ou venoit ce son ou en disoit des causes peu 

 vraisemblables ce qui me donna envie d'en chercher une 

 meilleure. Je trouvay bientost qu'il proc^doit de !a reflexion 

 du bruit de la fontaine contre les pierres du degriS. Car 

 comme tout son, ou plustost bruit, r^it^r^ par des inter- 

 valles ^gaux et trte petits fait un son de musique, et 

 que la longueur d'un tuyau d'orgue determine le ton qu'il 

 a par sa longueur par ce que les battements de Pair arrivent 

 ^galement dans les. petits intervalles de temps que ses 

 ondoiements emploient ^ faire deux fois la longueur du 

 tuyau scavoir quand il est ferm^ par le bout, ainsi je 

 concevois que chaque bruit tant soit peu distingu(? qui 

 venoit de la fontaine, estant r^flechi contre les marches 

 du degr^, devoit arriver k I'oreiUe de chacune d'autant 

 plus tard qu'elle estoit plus ^loignte, et cela par des 

 differences de temps justement ^gales h. celuy que les 

 ondoiements de I'air employent h. aller et venir autant 

 qu 'estoit la largeur d'une marche. .Ayant mesur^ cette 

 largueur qui estoit de 17 pouces, je fis un rouleau de 

 papier qui avoit cette longueur, et je trouvai qu'il avoit le 

 mesme ton qu'on entendoit au has du degr(5. 



" Je trouvay comme j'ay dit que la gerbe n'allant point 

 Ton cessoit d'entendre ce ton. Et aiant eu occasion d'aller 

 a Chantilly pendant I'hyver, qu'il estoit tombd beaucoup 

 de ni^ge qui ostoit la forme aux marches, je remarquay 

 que on n 'entendoit rien quoyque la gerbe allast et fit du 

 bruit i I'ordinaire." 



A slight confusion is caused by Huygens's first referring 

 in his note (apparently only drafted) to a closed organ- 

 pipe and later to an open one. Taking a pouce = 2-7 cm., 

 the depth of the steps becomes i7X2-7 = 45.9 cm. At 

 10° C., the corresponding sound of about 368 vibrations 

 per second would be given by an open pipe of 46 cm. 



The effect of gratings on impulsive motion of light is 

 now well understood, thinks to the labours of Lord 

 Rayleigh, Gouy, Schuster, and others. It remains 

 interesting, however, to contrast the opinion concerning the 

 supposed regularitv of white light, held by some high 

 authorities before these discussions, with Huygens's state- 

 ment that the regularity in the nature of the sound which 

 he observed has been impressed upon it bv outside 

 influence. P. Zeeman. 



.Amsterd.Tm, January 6. 



The Inheritance of " Acquired " Characters. 



May I ask for information upon the interpretation of 

 two sets of facts? 



(i) Prof. Henslow states that the garden parsnip 

 " known in the trade as ' The Student ' was raised from 

 seed of the wild plant by Prof. J. Buckman in 1847 at 

 the Agricultural College, Cirencester," and also that M. 



