April 2^, 1878] 



NATURE 



507 



"Mimicry" in Birds 



If Mr. Young will look at the fourth edition of Yarrell's 

 " British Birds," he will find that the fact he mentions {««/<•, 

 p, 486) has already attracted notice, for he will there read (vol. i. 

 p. 616) :- 



"In places near Thetford, where the ringed plover is 

 common, skylarks often imitate the note of that bird, making it 

 part of their own song." Alfred Newton 



Magdalene College, Cambridge, April 19 



Our starlings here, which are a numerous and accomplished 

 colony, have acquired the following notes of other birds :— Cur- 

 lew, red-shank, blackbird, thnish, magpie, swallow, swift, 

 chaffinch, house sparrow, hedge spaiTow. The most successful 

 imitations are those of the curlew, red-shank (the note uttered 

 by the latter on taking wing), and the swallow. I have several 

 times this year been certain that I heard a swallow twittering on 

 the house-top, and found that the note proceeded from a 

 starling. 



The jays also in this neighbourhood, which are very plentiful, 

 are very able mimics ; the note of the carrion crow is about 

 their most successful effort. II. H. S. 



Riding Mill-on-Tyne, April 22 



The Westinghouse Brake 



Under the heading, "Notes," in Nature, vol. xvli. p. 140, 

 there is a paragraph describing the automatic brake of the 

 Westinghouse Brake Company, St. Stephen's Palace Chambers, 

 Westminster, the latter part of which refers to a ball which 

 performs certain functions under different circumstances. In a 

 previous account in the Times, three balls are mentioned as 

 being used in the experiment ; it also states that several gentle- 

 men were investigating the mathematical principles under which 

 these functions fell. I have not seen any results of their work, 

 neither is there any comment upon it in Nature. I therefore 

 take occasion to mention it, in order that if any account of it 

 has passed me, I may be informed of it, or that, if no results 

 have appeared, this may lead to the subject being investi- 

 gated by some of the mathematical correspondents of your 

 esteemed paper. G. O. K. 



Sound and Density 



Since velocity of sound does not vary with density (Balfour 

 Stewart, Chap. IV., "Elementary Physics"), would you kindly 

 state the answer that should be given to the question. Why does 

 sound travel quicker m water and -wood than in air, and what is 

 the relation between density and velocity of sound in water, wood, 

 air? J. Cameron 



The Academy, Montrose, April 18 



[The velocity of sound depends on the ratio between the mass 

 and the elasticity, and in air (to which Prof. Stewart refers) it 

 does not vary with the density of the air if its temperature only 

 remain constant. In this case the denser the air the greater the 

 mass, but the greater the elasticity in the same proportion. The 

 ratio between mass and elasticity is thus unaltered, and therefore 

 the velocity remains under these conditions the same. — Ed.] 



OUR ASTRONOMICAL COLUMN 

 The Transit of Venus in 1882.— In addition to inde- 

 pendent calculations of the circumstances of this pheno- 

 menon, founded upon Le Verrier' s tables of the sun and 

 planet, to which reference has already been made in 

 Nature, we have to record the publication of two 

 memoirs upon the same subject, the first by Herr Bruno 

 Peter, who is attached to the Observatory at Leipsic, the 

 second by Dr. Karl Friesach, of Graz, which has been 

 received within the last week. As was to be expected 

 where practised calculators are working upon the same 

 data, the direct results from the tables are in very close 

 accordance with those previously published ; indeed the 

 advantage of so many repetitions of such work is not very 

 evident. The differences which the calculated times of 

 the geocentric contacts exhibit are almost wholly due to 

 the employment of different semi-diameters of sun and 



planet. Le Verrier suggested {Annates, vol. vi. p. 40) 

 that for the present the values to be employed should be 

 respectively 958"'424 and 8"-30S for the mean distance. 

 Herr Peter has used 96i"-2i and 8" '472, and Dr. Friesach, 

 960" 'o and 8"-305. Their results for Paris mean times 

 of contacts and least distance of centres are subjoined ; — 



Transit of Venus, 1882, December 6. 



Least distance of centres ... 



13 29-9 \ ... 5 13 27-3 

 64x"7 S 64i"-5 



Encke's Comet in 1878.— Observers in the southern 

 hemisphere may be reminded that this comet is likely 

 to be a pretty conspicuous telescopic object in their 

 evening sky, in the first days of August. According to 

 Dr. von Asten's latest researches on the motion of this 

 comet, the period of revolution at the last perihelion 

 passage was I200"8 days, which, without taking any 

 account of perturbations (not likely to be very material 

 during the present revolution), would bring it again to 

 perihelion on July 27. Mr. Tebbutt, of Windsor, 

 N.S.W., has once found Encke's comet without assist- 

 ance beyond his own calculations, but it will probably be 

 Dr. von Asten's intention to furnish southern observers 

 with a reliable ephemeris commencing with August next. 

 Observations will not be practicable before the perihelion 

 passage, the comet being too near to the sun's place. 



The "Berliner Astronomisches Jahrbuch" and 

 THE Minor Planets. — The volume of this ephemeris 

 for 1880 has just appeared under the joint editorship of 

 Professors Forster and Tietjen. The general contents 

 are similar to those of preceding volumes. The ephe- 

 meris of the moon is again transferred, with full acknow- 

 ledgment from the Nautical Almanac, and a great 

 amount of labour of computation is thereby saved, which 

 is made to tell upon the specialty of the work, the pre- 

 paration of ephemerides of the small planets as far as 

 their orbits are sufificiently determined. The reader who 

 may be in search of the elements of these bodies will find 

 in this new volume of the Berliner fahrbuch the most 

 complete and reliable table yet in the hands of astro- 

 nomers. It includes orbits of all the minor planets to 

 No. 172, with the exception of No. 155, Scylla, for which 

 the necessary materials for calculation are wanting; and 

 while referring to Scylla, it may be remarked that the 

 four observations on November 8, 9, 22, and 23, 1875, 

 cannot be represented by an elliptical orbit, which raises 

 a suspicion that those of November 8 and 9 may belong 

 to one planet, and those of November 22 and 23 to 

 another, not, so far, recognised in the list. On examin- 

 ing the table of elements it is seen that No. 153, Hilda, 

 has by far the longest period, while No. 149, Medusa, is 

 credited with the shortest, according to the calculations 

 of Prof. Tietjen. The observations of Medusa, however, 

 extend over a period of eight days only, but they appear 

 very exact, and it has happened that from a similar short 

 interval of accurate observation, very close approxima- 

 tion to the true elements of an elliptical orbit has been 

 attained ; we may especially note the case of the short- 

 period comet of De Vico in 1844 : from eight days' very 

 precise observations, M. Faye deduced an orbit which, 

 as was pointed out by Prof. Briinnow, was almost iden- 

 tical with the result of his own elaborate investigation of 

 the elements from the whole extent of observation. It is 

 unfortunate that Hilda has escaped observation at the 

 last opposition, since of all the small planets it is most 

 desirable to keep this one in view, from the fact of its 

 orbit allowing of a very much closer approach to the 

 planet Jupiter than is possible in the case of any other. 



