42 



NATURE 



[November 12, 1891 



owing to the more rapid movement of the belt in which they 

 occur. Prof. E. E. Barnard observed the spots so early as May last 

 {Astronomische Nachrichten, No. 3063). He found in September 

 that they were decreasing their longitudes about 10° daily. At 

 this rate they would describe a complete rotation round Jupiter, 

 relative to the great red spot, in about 36 days. The daily lo?s 

 derived from Mr. Denning's observations in August and Sep- 

 tember would bring the two spots in conjunction in about 39 

 days. 



Wolf's Periodic Comet. — The following ephemeris is from 

 one given by Dr. Thraen in Astronomische Nachrichten, No. 

 3064 :— 



Ephemeris for Berlin Midnight, 

 1891. Right Ascension. Declination. Brightness. 



h. m. s. „ , „ 



Nov. 14 ... 4 35 16 ... -8 29 28 ... IO-2 

 „ 17 - 33 27 ... 9 36 16 



„ 20 ... 31 31 ... 10 36 44 ... 9-3 



„ 23 ... 29 31 ... II 30 38 



„ 26 ... 27 31 ... 12 17 51 ... 8-3 



,, 29 ... 25 31 ... 12 58 22 



Dec. 2 ... 23 37 ... 13 32 15 ... 7-4 



„ 5 ... 21 48 ... 13 59 40 



,, 8 ... 20 8 ... 14 20 51 ... 6*5 



,, II ... 18 39 ... 14 36 10 



„ 14 ... 17 22 ... 14 45 59 ... 57 



„ 17 ... 16 17 ... 14 50 41 



„ 20 ... 15 26 ... 14 50 38 ... 4-9 



,, 23 ... 14 49 ... 14 46 14 



„ 26 ... 4 14 27 ... 14 37 52 ... 4-3 

 Although the comet is getting fainter and moving south, it 

 may probably be followed to the last date in the above ephe- 

 meris with instruments of moderate aperture. The greatest 

 southern declination of 14° 51' 8" is reached on November 18. 



The Total Lunar Eclipse of November 15.— If atmo- 

 spheric circumstances permit, a total eclipse of the moon may 

 be observed over all Europe on Sunday next, November 15. 

 The following are the times of contact with the earth's shadow 

 given in the Nautical Almanac : — 



G.M.T. 



First contact with the penumbra 

 ,, ,, ,, shadow 



Beginning of total phase 



Middle of eclipse 



End of total phase 



Last contact with the shadow 

 „ ,, ,, penumbra 



9 367 



10 35-1 



11 37-4 



12 18-9 



13 0-4 



14 27 



15 i-i 



The first contact with the shadow occurs at 55° from the most 

 northern point of the moon's limb counting towards the east, 

 the last contact at 95^ from the same point counting towards the 

 west. 



The Elements of the Minor Planets.— The Viertel- 

 jahrschrift der Astronomischen Gcscllschaft (first volume) con- 

 tains two interesting compilations, on the planets discovered 

 in the year 1890, and on the appearances of comets in the 

 same year. The first paper is contributed by Dr. Paul 

 Lehmann, and informs us that no less than fifteen new members 

 of our minor planet system were discovered last year between 

 February 20 and November 14. In the table that follows a 

 summary of all the days on which each individual planet was 

 observed is given, and this is succeeded by another which shows 

 their chief elements. By combining the elements of some of 

 the old planets with those of the new ones, some striking com- 

 binations are thus brought to light, of which we give the two 

 following cases, in which the new planets are 292 and 288 : — 

 Planets. 

 152 9, = 4i'3 i = 12-2 0=4-4 a = 3-15 

 13 43-2 16-5 5-0 2-58 



99 42-0 13-9 13-8 280 



15s 43"i 14-1 14-8 2-91 



292 43-1 147 2-4 2-53 



268 Q, = 121 7 i = 2-0 (^ = 7° -9 a = 3-09 

 288 121 -6 4-4 1 1 -6 275 



113 123-1 5-0 5-0 2-38 



213 122-4 68 8-3 2-75 



The next table shows the values that have been obtained after 

 computing the mean brightest and darkest magnitudes that the 



NO. I 1 50, VOL. 45] 



planets can attain. In the last form the tabulation is so arrangefl 

 that the following numbers can be directly seen : — (i) The num- 

 ber of oppositions in which, up to the present time, places have 

 been found, with the number of appearances since observed. 

 (2) The number of every known opposition in which the planet 

 has been observed. (3) Every planet to which the foregoing 

 statement refers. (4) The number of these planets. 



SOME EXPERIMENTS MADE WITH THE 



VIEW OF ASCERTAINING THE RATE OF 



PROPAGATION OF INDUCED MAGNETISM 



IN IRON. 

 "T^HE question, considered in a simple form, may be put thus : 

 -^ Suppose a mj^net were suddenly brought up to one end of 

 a long iron rod, what length of time intervenes between the 

 occurrence of magnetization at the near end and at the far end ? 



Everyone, probably, would at first be inclined to say that the 

 speed along the bar would undoubtedly be about the same as 

 the velocity of light, and this supposition would naturally follow 

 if the energy to places along the bar be supposed transmitted 

 through the surrounding space ; but, on the other hand, the 

 speed may be much less if the energy of magnetization is trans- 

 mitted from particle to particle in the iron — the orientation of 

 the molecular magnets being, as it were, passed from each to the 

 next along the bar. In such case we would, of course, expect 

 the velocity of propagation to be comparable in speed with that 

 of molecular phenomena rather than that of disturbances in 

 the ether. 



The velocity of sound, with which we may, perhaps, compare 

 it, is in iron about i6,coo feet per second. The transmission 

 of sound resulting from vibratory movement can be said to de- 

 pend on the mass of the molecule, and on the mutual forces 

 keeping the molecules in position ; while the rate of propa- 

 gation of a magnetic disturbance of the kind supposed would 

 depend on the moment of inertia of the particles (assumed 

 to be molecular magnets) round their axes of rotation, and on 

 their mutual magnetic moments. 



The propagation of such a disturbance can be observed in 

 Prof. Ewing's magnetic model. The model, which consists 

 essentially of a great number of small compass needles placed 

 within each other's action, but not near enough to touch, can be 

 disturbed at one place by bringing a magnet near, or otherwise. 

 The disturbance then is seen to spread throughout the model, 

 much in the same manner as we have suggested a magnetizing 

 disturbance to be propagated in iron. 



The method proposed to test matters depended upon the 

 principle of the interference of waves travelling in opposite 

 directions observed through the production of stationary waves. 



Thus, if a bar of soft iron have two coils of wire placed one at 

 each end, and if the same alternating current be passed through 

 both coils, disturbance of opposite signs travelling in opposite 

 directions along the bar should interfere, provided the rate of 

 alternation and the length of the bar are chosen suitable to the 

 rate of propagation. 



It was proposed to detect the nodes or places of interference 

 by means of a telephone in circuit with a third coil which could 

 be slid along the bar. 



Instead of employing two alternating coils, the bar can be bent 

 round to form a ring, and one coil will be then sufficient. 



Some preliminary experiments with a straight bar having 

 given faint indications of the existence of places of minimum 

 intensity, closed magnetic circuits or rings, formed of a 

 great number of turns of soft iron wire,^ were then tried 

 with more decided results. When the alternating coil was 

 in certain positions on the ring the telephone coil could 

 be placed at points where no sound, or if any very slight, 

 could be heard — the sound reaching a maximum in places 

 somewhere between these points. These nodes and inter- 

 nodes occupied about half the ring — the opposite half of 

 the ring from that in which the alternating coil lay. On 

 ' approaching nearer the alternating coil, apparently the very 

 unequal length of the paths prevented any effect being observed. 



It was without difficulty ascertained that these were not the 



I ^ Two rings were made of No. 21 soft iron wire, one about 10 feet and the 

 other I4"5 feet in circumference. Both had 8 pounds of wire wound on. 

 The wire used in a third ring was No. 32 This ring was about 12 feet in 



I circumfcence. There was about 4 miles of wire put on. The wire of this 



I and the 14-feet ring was well coated with shellac before winding, so as to 



1 minimize Foucault currents. 



