yan. 



1889J 



NA TURE 



^n 



ASTRONOMICAL 



WEEK li 



PHENOMENA FOR 



Zc) JANUARY 6-12. 



THE 



/pOR the reckoning of time the civil day, commencing at 

 ^ Greenwich mean midnight, counting the hours on to 24, 



is here employed.) 



At Greenwich on January 6 



Sun rises, 8h. 7m. ; souths, I2h. 6m. I7*2s. ; sets, i6ti. 6.n. : 



right asc. on meridian, igh. 11 •3m. ; decl. 22° 26' S. Sidereal 



Time at Sunset, 23h. 12m. 

 Moon (at First Quarter January 9, ih.) rises, lih. 2m. ; 



souths i6h. 22m.; sets, 2ih. 53m.: right asc. on meridian, 



23h. 27-9m.; decl. 8° 22' S. 



that of the following morning 



Saturn, January 6, — Outer major axis of outer ring = 45""i : 

 outer minor axis of outer ring = ll"'o : southern surface visible. 



NOTES ON METEORITES} 



VII. 



Possible Connection between the Jets and Envelopes 



SEEN IN CoMETARY SWARMS. 



TTHE jets observed in comets when near the sun are very various 

 in form. The concentric envelopes seen at times are much 

 more regular ; an idea of their appearance will be gathered from 

 the accompanying illustration of Donati's comet. 



It has not yet been clearly ascertained whether the jets and 



' Continued from p. 142. 



envelopes are connected phenomena— that is, whether the jets 

 are true whirls of the meteorites themselves— or whether they 

 represent volatilization of the vapours of the nucleus in a 

 particular direction, which vapours subsequently assume a con- 

 centric form. In Halley's comet, at all events, this was not 



Fig. 



-Concentric envelopes as illustrated by Donati's comet. 



observed. Sir John Herschel writes concerning this: "The 

 bright smoke of the jets, however, never seems to be able to 

 get far out towards the sun, but always to be driven back and 

 forced into the tail, as if by the action of a violent wind rolling 

 against them — always from the sun — so as to make it clear that 

 this tail is neither more nor less than the accumulation of this 



Fig. 22. — Combination of jets and envelopes (comet of 1861). 



sort of luminous vapour, darted off in the first instance towards 

 the sun, as if something raised it up, as if it were exploded by 

 the sun's heat, out of the kernel, and then immediately and 

 forcibly turned back and repelled from the sun." 



The Concentric and Excentric Envelopes. 



While in Donati's comet we get perhaps the finest exhibition 

 of concentric envelopes successively thrown off from the nucleus 

 towards the sun, in Coggia's comet, on the other hand, we 

 had the most striking instance which has been yet observed in 

 which the envelopes put on an appearance as if they belonged 

 to two different systems of concentric envelopes cutting each 

 other. 



It is important here to enter into some details. In 

 Coggia's comet (as observed with Mr. Newall's 25-inch re- 

 fractor, with a low power), next to the nucleus the most brilliant 

 feature was an object resembling a fan opened out some 160'. 

 The nucleus, marvellously small and definite, was situated a 

 little to the left of the pin of the fan— not exactly, that is, at 

 the point held in the hand. If this comet, outside the circular 

 outline of the fan, offered indications of other similar concentric 

 circular outlines, astronomers would have recognized in it a 



