6io 



NATURE 



[January 6. 1921 



includes convenient data relating to specific gravities, 

 solubilities, strengths of solutions for analysis, etc. 

 A section is also devoted to scientific societies and 

 departments, in which a brief account is given show- 

 ing the address and the officials of each. The diary 

 is well bound, and makes a convenient pocket-book 

 for ready reference and notes. 



Messrs. H. F. and G. Witherby are about to pub- 

 lish vol. i. of " .■\ Manual of the Birds of Australia," 



edited by G. M. Mathews and T. Iredale, illustrated 

 by coloured and monochrome plates. 



.\ NEW edition of Sir Edward Thorpe's "■ .\ Dic- 

 tionary of Applied Chemistry " is announced by 

 Messrs. Longmans and Co. Vol. i., A to Calcium, 

 is promised for January, and vol. ii. for early in 

 the coming summer. The work, which has been 

 carefully revised, will be in six, and possibly seven, 

 volumes. 



Our Astronomical Column. 



Comets. — Mr. Woods has communicated by cable 

 the following elements of Skjellerup's comet. Ele- 

 ments deduced by M. Ebell from observations on 

 December 13, 17, and 18 last are also given. Both 

 are for the equinox of 19200 : 



Woods Kbcll 



T in G.M.T. Dec. irii Dec. 13-46512 



<o 341° 10' 344° 9' 47" 



ft 107 47 107 38 7 



/ 22 12 24 4 7 



log j? 006047 006476 



Mr. Woods's elements make the position at mid- 

 night on January 20: R.-'V. iih. 7m. 17s., N. decl. 

 36° 22'. The position given last week was : R.A. 

 iih. 17m. 42s., N. decl. 38° 24'. 



Herr Hoffmeister observed the comet at Sonneberg 

 on December 18. He described it as a circular nebu- 

 losity 80' in diameter, total light 9th magnitude, 

 nucleus nth magnitude. 



.\n observation by Mr. R. L. Waterficld on Decem- 

 ber 31 indicates that Mr. Woods's ephemeris is very 

 near the truth. 



The two longest-known comets of short period 

 (those of Encke and Pons-Winnecke) are both due at 

 perihelion in 1921. There is a simple method of pre- 

 dicting the date of perihelion of the former comet 

 within a day or so. Eighteen revolutions of the comet 

 occupv 50^ years, equal approximately to five revolu- 

 tions of Jupiter and two of Saturn, so that the per- 

 turbations nearlv repeat themselves after this period. 

 The following have been the duration of eighteen 

 revolutions at recent returns : 



Perihelion should occur about July 13-4. 



Winnecke's comet is subject to large perturbations 

 by Jupiter, and no simple cycle is available in this 

 case. The best estimate that can be made of the 

 date of perihelion is the end of June, but this may be 

 a month in error. The period of this comet has in- 

 creased bv four months in the last century, and the 

 oerihelion distance has increased from 077 to unity. 

 This has resulted in introducing it into a new shower 

 of meteors, first seen on June 28, 19 16, the connection 

 of which with Winnecke's comet was quickly detected 

 bv Mr. Denning. These meteors should be in evi- 

 dence next June. The radiant is about 240°-!- 50°. 



Another periodic comet may return this year, 

 1846 IV (de Vico). The most probable period is 

 ;5-7 years, which would make the next perihelion 

 November, 192 1, but the uncertainty is fully three 

 years. The comet will first become visible to southern 



NO. 2671, VOL. 106I 



observers, but it will move northward very rapidly, 

 its inclination to the ecliptic being 85°. 



DisAPPEARAN'CE OK Saturn's Rings. — The earth 

 passed through the plane of the rings on November 7, 

 and a joint pajser on the phenomena presented about 

 that date, by Messrs. Hepburn, .'\inslie, Steavenson, 

 and Waterfield, was read at the R.A.S. meeting on 

 December 10. They observed with the 28-in. equa- 

 torial at Greenwich, by kind permission of the 

 Astronomer-Royal. On November 6 the ring was 

 easily visible, but on the following night no trace 

 whatever was visible outside the ball. This was con- 

 firmed by Prof. Barnard, observing with the 40-in. at 

 Yerkes Observatory, who estimated that the thickness 

 of the rings could scarcely exceed 40 miles. A few- 

 days later, in spite of the dark side of the rings being 

 turned towards us, the observers at Greenwich could 

 clearly see a number of luminous patches outside the 

 ball, which thev were able to identify with the brighter 

 regions of the ring ; they noticed that these regions 

 continued visible when twilight was far advanced and 

 long after the inner satellites had disappeared. This 

 would appear to indicate that an appreciable amount 

 of sunlight is able to penetrate the ring, illuminating 

 the particles on its remote side. It will be remembered 

 that a star was recently seen through the ring, so it 

 is not surprising that sunshine should penetrate it. 



The ring will again be edgewise to the earth on 

 February 21 and .August 2, and to the sun on .April 10. 



KoDAiKAXAi. Observatory. — Bulletin Ixiii. of this 

 observatory deals with the direction and aspect of the 

 dark filaments which are such a conspicuous feature 

 of spectroheliograms in Ha light, and are known 

 to be prominences projected on the solar disc. 

 Their azimuth is first studied, and shown to be a 

 function of latitude. Near the equator they lie along 

 a meridian ; as we recede from the equator the polar 

 end of the filament swings eastward. The mean in- 

 clination to the equator is 40° in latitude 30°, and 

 0° in latitude 60°. 



The explanation suggested is the easterly drift pro- 

 duced by the excess of easterly motion at the equator. 

 It was inferred from this that the high-latitude fila- 

 ments are older than the equatorial ones. However, a 

 study of the history of particular filaments from the 

 photographic records did not give clear evidence of 

 this difference of a£je with latitude. 



Two interesting features of the prominences are dis- 

 cussed : d) Those on the disc are flanked by a bright 

 strip, which is easiest to see near the limb. It is 

 interpreted as showing that the prominences rest en 

 a bright base which is hotter than the ordinary 

 chromosphere. (2) The lower portion of prominences 

 .seen in profile outside the disc is freauentlv obscured 

 by a dark strip. It is suggested that the central 

 region of a prominence is the hottest, and that the 

 lower portions of the cooler outer envelope may be 

 dense enough to absorb light. 



