248 



NATURE 



\y21ly 14, 1 88 1 



OUR ASTRONOMICAL COLUMN 

 The Comet. — By favour of Dr. W. L. Elkin of the Royal 

 Observatory, Cape of Good Hope, we are able to subjoin obser- 

 vations of the great comet made at that establishin;nt before 

 the perihelion passage, which it is stated will admit of improve- 

 ment when the places of the comparison-stars have been more 

 accurately determined : 



From the position-; on May 31, June 4 and 9, Dr. Elkin has 

 calculated the following elements of the comet's orbit, by the 

 side of which we place for comparison others deduced by Mr. 

 Hind, from post-perihelion observations up to July i ;— 

 Elkin I Hind 



Perihelion passage, June 16-29839 G.M.T. June 16-415190. M.T. 



Longitude, perihelion. 

 ,, node 



Inclination 



Log. perihelion dist. .. 



264 55 13 



270 54 27 



63 27 4 



9-866656 



Direct. 



265 9 4 



270 58 o 



63 29 9 



9-S65516 



Direct. 



The longitudes are reckoned from the mean equinox of I SS i 'o. 

 Mr. Hind's orbit gives the following expressions for the comet's 

 heliocentric co-ordinates x, y, z to be used with the X, y, z of 

 the Nautical Almanac in the calculation of geocentric right 

 ascensions and declinations ; they apply to apparent equinox 

 1881-5. 



X = J- [9-65000] sin. {v -f 356 22-5) 

 y = >-[9-99i87] sin. (v -)- 243 20-7) 

 2 = ^[9-96142] sin. (v 4- 328 24-3). 



Here, v is the true anomaly, r the radius vector, and the quanti" 

 ties within square brackets are logarithms. 



We may take this opportunity of correcting a singular mis- 

 statement with which Admiral Mouchez, the director of the 

 Observatory at Paris, is credited in the Comptes rendus of the 

 Academy of Sciences. He is there made to say that the period 

 of revolution of the comet of 1807, which had elements resembling 

 those of the present comet, was found by Bessel to have been 

 reduced to 174 years alter he had taken account of "new per- 

 turbations." Any one who refers to Bessel's treatise will see 

 that this is an eiTor. Bessel fixed the period of revolution at 

 1713I years, on September 22, 1807, and in continuing the com- 

 putation of the perturbations by the planet Jupiter to March, 

 1815, when they had become very small, he found that the revo- 

 lution had been diminished thereby, about 1 70 years; he con- 

 sidered that the period he had assigned for September 22, 1S07, 

 was not liable to a greater error than 100 years. In the Cciiiptis 

 rendus the effect of perturbation on the period appears to have 

 been quoted, instead of the revolution itself, as perturbed to 

 March, 181 5. In the communication to which vie refer, the 

 identity of the comet of 1881 with that of 1807, is pretty distinctly 

 assumed, but the weight of evidence is certainly in the other 

 direction. 



The Variable Star U Cephei. — Prof. Julius Schmidt has 

 published an ephemeris of the last variable star discovered by 

 Ceraski in Cepheus, extending to the end of the present year. 

 From hii later ob ervalions he has found that the gradual increase 

 of period which he had formerly suspected is not confirmed, and 

 he now fixes the period at 2d. Ilh. 4911. 33-35S. On August iS 

 commences a series of minima, which may be observed at inter- 

 vals of something less than five days to the end of December : 

 the following are the Greenwich mean times to the end of 

 October : — 



h. m. h m. h. m.- 



Aug. 18, 14 44-9 ... Sept. 12, 13 o-S ... Oct. 7, II 166 



23, 14 24-0 ... 17, 12 399 ... 12, 10 558 



.28, 14 3-2 ... 22, 12 191 ... 17, 10 35-0 



Sept. 2, 13 42-4 ... 27, II 58-3 ... 22, 10 142 



7, 13 21-6 ... Oct. 2, II 37-5 ... 27, 9 534 



The letter T, at first used to designate this star, will be 

 properly applied to another variable also discovered by Ceraski, 

 the place of which for 1855-0 is in R.A. 2[h. 7m. 33s., 

 Decl. 4- 67° 54' '4, which seems to have a period of about 400 

 days, and was at a maximum on December 30, 1880. 



PHYSICAL NOTES 



In they««-«a/of the FrankUn Institute Prof. S. W. Robin- 

 son has recently described some experiments upon the effect 

 produced upon sound-waves by repeate i oblique reflections at 

 membranes forming the boundary of two gaseous media. These 

 experiments, as far as they go, would appear to show that after 

 repeated reflection at oblique surfaces set in vertical planes a 

 sound-wave acquires new properties by virtue of which it is 

 reflected at another such oblique surface with an intensity which 

 is a maximum if this surface be also in a vertical plane, and a 

 minimum if the surface be equally inclined to the direction of 

 propagation of the wave, but turned through a right angle about 

 that direction as an axis. If this be indeed established as an 

 experimental result it is equivalent to a proof that sound-waves can 

 be /»/a;vVt-j' by reflection. The apparatus consisted of a series 

 of L-shaped tubes of tin plate, one inch in diameter and three 

 inches long, the parts joined at an obtuse angle, but having the 

 outer angle cut away and covered by a thin membrane. This 

 membrane was fixed so that a sound-wave coming in either direc- 

 tion should be incident on it at the angle of suppo ed maximum 

 polarisation, the angle being calculated by Brewster's Law so 

 that its tangent should represent the ratio between the velocities 

 of wave-propagation in the two media, namely, coal-gas within, 

 air without (14 : II). The series of tubes was so set that at first 

 the membranes were all parallel, and then a "pulse" w-as sent 

 along the tube in the following manner : — The initial and final 

 openings were closed by membranes stretched across the tubes 

 orthogonally. Against each a small ball of ivory or glass was 

 hung by a thread. The ball at one end was raised to a given 

 height and dropped on to the membrane, and the impulse given 

 to the ball at the other end was noted. Then the second half of 

 the system was turned round so that the membranes of this por- 

 tion were at right angles to their former position, and the ball 

 was again dropped. The impulses received on the second b.ill 

 were in general feebler when the second system, or "analyser," 

 stood at right-angles to the first system, or " polari-icr," the 

 diminution varying in different experiments from 16 to 38 per 

 cent. No diminution was observable when the interior of 

 the tube was filled with air instead of coal-gas. It remains 

 to be seen whether the results are capable of being repro- 

 duced under other circumstances, or whether they are due 

 to some mechanical peculiarity of Prof. Robinson's apparatus. 

 Whether this be so or not we must absolutely reject the very 

 unwarranted conclusion at which Prof. Robinson arrives, namely, 

 that the vibrations of light as well as of sound are longitudinal 

 until they are polarised, and that they become transversal only 

 in the act of polarisation. 



M. Mercadier has examined the resistance of selenium a' 

 different temperature-, using for this purpose a photophonic 

 leceiver of selenium spread between the edges of platinum sheets, 

 the resistance being measured in the dark by the ordinary bridge - 

 method. The receiver, which hai been well annealed, had at 

 15^ C. a resistance of 54,000 ohms. This amount decreased as 

 the temperature rose with great regularity to 36° C, when its resis- 

 tance was less than 1500 ohms. From this point the diminution 

 of resistance was less rapid; but at about 125" C. it had fallen 

 to less than 500 ohm-, rising slightly to 163° C, from which it 

 again fell as the temperature was raised to 20S". These results 

 accord with the earlier experiments of Werner Siemens. 



M. Lll'PMANN finds (Jour, de Phys., May) that galvanic 

 polarisation, which modifies so much the capillary properties of a 

 metallic surface, causes no perceptible variation of its optical 

 properties. He directed a beam of polarised light towards a 

 platinum or silver mirror in acidulated water, or copper sulphate 

 solution, and it was received, after reflection, in a Jamin quartz 

 compensator, followed by an analyser-Nicol. The dark fringe 

 in the compensator was not displaced when the mirror was in- 

 cluded in the circuit of a Daniell or Bunsen element, and the 

 cuirent passed. Again, Neuton's rings, formed by a glass plate 

 on the platinum surface, and observed under various incidences, 

 showed no displacement when the polarising current passed. 

 (M. Lippmann adds some observations as to the mode of pro- 

 duction of gas-bubbles by electrolysis.) 



A FEW years ago M. Montigny called attention to the fact 

 that the scintillation of stars is considerably increased during 

 aurora borealis. Further data on this subject are afforded in a 

 recent issue of the Belgian Academy's Bulletin, No. 3. Inter 

 alia, he has observed that the phenomenon is more pronounced 

 in winter than in summer, and that stars in the northern region 



