70 



KNOWLEDGE 



[MABfH, 1902. 



A. E. F. — Vou will find the information you require in an 

 article on "Tlio Specific (Iravities of Saturated Solutions of 

 Stains," given in detail in the "Knowledge Diary and Scientific 

 Handbook for l',)0:J." This is puldislied at " Kn(iwi.ki)<;i; " 

 Otiice and contains a variety of useful information in addition 

 to the diary itself. 



C. S. Piirrrll. — Anj' of the methods in use for the demonstra- 

 tion of llagella will show those of the /i. Suhtilln. Perhaps the 

 best and easiest is the Pitfield modified by Muir (rifle page lllj 

 of the last edition, Jluir A Ritchie's Manual of Bacteriology). 



Communicadonit and enquirirx on Microscopical matters are 

 cordially ittrltrd, and Dhoultl be addressed to M. I. CROSS, 

 Knowi.epgk Office, 32t'., Ili;ih llolhorn, II'. C. 



NOTES ON COMETS AND METEORS. 



By W. F. Denning, f.e.a.s. 



Next Rkthbn of IIah.kt's Comet. — As wc draw nearer to the 

 time of perilielion passage in 1910, the interest in t.lii< objeet will 

 continue to inerense. There is no other eoniet which has so many 

 importjint historical associations and whose apparitions can be traced 

 from so remote a period. It was visible in the year of tlie Norman 

 Conquest, and its fir.st well-recorded appearance through more 

 than nineteen centuries to nearly twelve yeai-s before ths Lhristian 

 era. This comet is only perceptible once witliin the allotted term of 

 human existence, since it revisits the sun every 77 years, and there 

 are very few persons who have lived in the particular period and to 

 the great age necessary to afford them the privilege of viewiiig the 

 comet at two returns. We may discern that small " Mercury of comets," 

 calle<l after the illustrious Eneke, again and again, for its solar visits 

 arc repeated at short intervals of 3J years, but tlie comet of Halley 

 ordinarily furnishes but one spectacle at most during a lifetime. 

 Since the year 12 B.C. the latter comet has nearly accomplished 25 

 revolutions, and Mr. J. R. Hind gave some interesting details of its 

 previous apparitions in Yol. X. of t lu^ Monthly Xotices (January, It^oO). 

 Its period varies, owing to planetary perturbation, between T-iV and 

 794 years, and the revolution now being performed will be one of the 

 shortest if not the shortest known in the history of the comet, for the 

 last periheliou passage occurred in 1835 (November loth), while the 

 next is predicted for 1901 (May 23), the interval being equivalent to 

 74 years and 189 days. Dr. t). Smart has furnished some useful 

 particulars in reference to the next return of the comet (Journal of 

 the British Astronomical As!tociation,\o\. XII., p. 134-';), and on the 

 assumption that the perihelion will he readied on 1910, May 23rd, 

 has computed an approximate e]ihemeris, from which are extracted 

 the following positions : — 



Hallet's Comet, 1910. 



Distauee Apparent 



In December, 1909, the comet will be placed in the region of the 

 Hyades and thereafter traverses Aries aud Pisces. 1 here is a meteoi-ie 

 shower connected with it !>nd this should be looked for before sunrise 

 during the first week in May. The meteors are directed from a 

 radiant near i) Aquari, whicli docs not rise in the latitude of Green- 

 wich initil nearly 2 a.m. on May 4th. 



TitH Leonids of 1901. — From a number of accoimts received from 

 American observers, it seems that a moderately rich display of these 

 meteors was seen. For several hours preceding sunrise on November 

 15th the rate of apparition was three or four per minute for one 

 ohsen'er, while during the period of greatest frequency there were 

 about twice that number. At several of the stations the maximum 

 appears to have occuiTed at about 1 1.30 a.m. G-.M.T., or four hours after 

 sunrise in England, and this sufllciently explains how it was that the 

 disjday proved a meagre one as observed in this country. There were 

 about ten times uiorc Leonids seen in America than in England, aud 

 it is certain that tlie earth encountered the denser part of tlie stream 

 at a time when it could only be observed at places far west from 



Greenwich. At some of the American stations meteors appeared 

 t>he increasingly plentiful in the morning twilight of November lotli, 

 and it seems )io«8ible that the rial maximum may have occurred even 

 later than the time given above, in which case it could have been 

 witnessed from tlie Pacific Ocean, but wc have no description of it 

 from voyagers in this region. The fact that the sliowcr showed a 

 marked increase in strenglh as compared with the feeble exhibitions 

 of the few previous years, encourages the hope that a further revival 

 may occur in November next. The parent comet passed through 

 pcriliclion in the spring of 1899, but the meteors appc.ir to be plenti- 

 fiillv clustered along a considerable section of tlie orbit in rear of 

 thc'eomet. There were showers in 902, 1002, 1202 and 1602, and 

 observers will therefore do well to make observations in the coming 

 November, though the full moon will offer a serious hindrance. 



Rei ATIVE Ubiohts OF PEnsBius AND Leosids. — The real paths 

 of a large number of meteors have been computer! in past years, and 

 especially at those particular periods marked by the occurrence of 

 special displays like the Peraeids and Leonids. Our accumulated 

 records are becoming very extensive of the former shower, occurring 

 as it does at a genialseason when the skies are often clear, and every 

 year presenting a moderately rich display of long duration. The 

 November Leonids are confined to a more limited interval and are 

 often obliterated by clouds ; in some years the display is so feeble 

 that it passes unrecorded. From a comparison of the real paths of 

 Perseids and Leonids it appears certain that the former are rather 

 lower in the atmosphere than the latter, though wc have scarcely 

 secured a sufficiently large number of Leonids to institute a fair com- 

 parison. Tlie following arc the resulting heights, &e., of a number 

 of Perseids and Leonids doubly observed in England during the last 

 fifteen veal's, and computed by the writer : — 



The observed velocities of 42^2 and 40-8 miles per second exceed the 

 theoretical velocities (38 and 44 miles), but the discordance may well 

 be inducetl by errors in the estimate! durations of flight. 



THE FACE OF THE SKY FOR MARCH. 



By W. Shackleton, f.k.a.s. 



The Sun. — On the 1st the sun rises at 6.50 a.m., and 

 sets at .5.37 p.m. Ou the 31st he rises at 5.42 a.m., and 

 sets at 6.28 p.m. 



The Vernal Equinox occurs on the 21st, when the sun 

 enters Aries, and Spring commences at 1 p.m. The disc 

 of the sun promises little of interest for spot observations. 



The Zodiacal Light may be looked for in the ivest for 

 some two or three hours after sunset. 



The Moon : — 



The following are the more interesting occultations 

 visible at Greenwich : — 



