166 



KNO'nA/'LEDGE 



[March 16, 1883. 



THE FACE OF THE SKY. 



From March 10 to March 30. 



By F.R.A.S. 



THE Bnn may bo watched for spots and facula^, but they aro 

 neither so larpo nor so frequent as they were a few months 

 since. The Zodiacal light should bo looked for after sunset on every 

 clear evening. The aspect of tho night sky may bo determined, 

 and the constellations now visible identified from Map III. of " Tho 

 Stars in Their Seasons." Neither Mercury, Venus, Mars, nor 

 Neptune are availably placed for the observer. Jupiter is still the 

 most conspicuous object in tho night sky, but he sets soon after 

 2 a.m. now. He will be found above ? Tauri. On the night of tho 

 20th, the student should look out for tho transit of Satellite 111., as 

 it has on previous occasions been seen to change from a bright to a 

 dai'k spot on Jupiter's face. It will enter on his fullowina limb at 



10 h. 3 m. p.m. Subsequently Satellite II. will be occulted at 



11 h. 4 m., and the transit of I. begin at 12 h. 20 m. On the 21st, 

 Satellite II. will be occulted at 9 h. 35 m. The 22nd is a night pro- 

 lific in Jovian phenomena. The ingress of the shadow of Satellite I. 

 will occur at 8 h. G m., and that of the shadow of II. at 8 h. 41 m. 

 At 8 h. 54m., Satellite II. itself will pass off the planet's face, as 

 will Satellite I. at 9 h. 3 m. Its shadow will follow it at 10 h. 23 m., 

 and lastly the shadow of Satellite H. will leave Jupiter's disc 

 at 11 h. 28 m. On tho 23rd Satellite I. will reappear from 

 eclipse at 7h. 36 m. 34 s. p.m. Satellite III. will reappear from 

 eclipse at 8h. 2 m. 35s. in the evening of the 24th. On the 28th 

 Sat. I. will be occulted by the planet at 11 h. 32 m. The night of 

 the 29th will be another rich one for the young observer of .lovian 

 phenomena. Sat. I. will begin its transit at 8 h. 45 m. ; as will 

 Satellite II. four minutes later. The shadow of Sat. I. will enter 

 on to the planet at 10 h. 2 m. ; the Satellite casting it passing off at 

 Uh. 2 m. At 11 h. 19 ra. the shadow of Satellite II. will leave 

 Jupiter's disc ; as will the Satellite itself at 11 h. 35 m. The egx'ess 

 of the shadow of Satellite I. will happen at 12 h. 19 ra., but Jupiter 

 will be very close to the horizon. Finally, on the night of the 30th 

 Satellite I. will reappear from eclipse at 9 h. 32 m. 20 3. Saturn is 

 very rapidly approaching the west now, but may still be observed 

 an hour or two after sunset. He is situated to the S.E. of S Arietis. 

 If the reader will refer to Map III. of " The Stars in Their Seasons " 

 (spoken of above), he will find a small star marked r in the con- 

 stellation Leo ; a little to the left of this in the sky is a 6th Mag- 

 nitude one, called 89 Leonis, and a very little above this last-named 

 .star lies Uranus, whose pale blue planetary disc in the telescope 

 will reveal him at once. 



The Moon is 7'3 days old at noon to-day, 8'3 days old at the same 

 hour to-morrow, and so on until the 30th, when she is obriously 21'3 

 days old. During the next twenty-four hours she passes from Leo 

 into Tirgo, in which constellation she remains during the whole of 

 the 17th and 18th, travelling into Libra on the 19th. She is in 

 Libra on the 2Uth, and a good deal of the 21st, but moves into the 

 confines of Scorpio towards the end of that day. Then she crosses 

 the south-west part of Ophiuchus on the 22nd, moving out of Ophiu- 

 chus into Libra on the 23rd. She occupies the 21th in crossing 

 Sagittarius, where she remains on the 25th, leaving it, to enter 

 Capricornus, on the 26th. She passes into the confines of 

 Aquarius on the 27th, occupies the 28th in crossing that con- 

 stellation, enters Pisces on the 29th, and is there during the 

 whole of the 30th. Four occultations of stars by the Moon 

 occur at tolerably convenient hours during one specified 

 period. On March 20th the 6th Mag. Star, 14 Sextantis, will 

 disappear at the Moon's dark limb at 12.3 p.m., at an angle of 

 88° from her vertex, and reappear at her bright limb at 1 h. 17 m. 

 the next morning. On the 21st 36 Sextantis will disappear at the 

 Moon's dark limb at 6 h. 37 m. p.m., at .an angle of 31° from her 

 vertex, reappearing at her bright limb at 7 h. 48 m. p.m., at an angle 

 of 215°. 50 Virginis will be occulted by the bright limb 38 minutes 

 after midnight on the 24th, at an angle of 91° from the Moon's 

 vertex. Both these stars are of the 6th Magnitude, tf/ Ophiuchi, 

 a 5th Magnitude star, will di.sappear at the bright limb at mid- 

 night on the 2Sth, at an angle of 87° from the vertex of the Moon, 

 to reappear 54 minutes later from behind her dark limb at an angle 

 of 190°. We do not give the times of reappearance, &c., when the 

 phenomena occur after 1 a.m. 



The latest development of Now Zealand cultivation is known as 

 liquorice-farming, and wc hear that many colonists are giving much 

 attention to tho subject. This plant, the ghjcyrrhiza (derived from 

 two Greek words, signifying sxceet root), was well known to tho 

 ancients as a medicine ; and ever since the days of Queen Elizabeth 

 it has been extensively grown in England, especially about I'on- 

 tefract.— P. E. 



" Let Knowledge grow from more to more." — Alfred Ten.vtsos. 



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ON THE FORMATION OF COMET'S TAILS. 



[750] — I remember, on receiving Mr. A. S. Davis's paper, 

 feeling very pleased to find that any one was paying attention 

 to the subject of repulsion due to evaporation, and in writing 

 to thank Mr. Davis for his courtesy in sending me a separate 

 copy of his communication to the Philosophical Magazine, I 

 have no doubt that I complimented him on his numerical 

 results very much in the language he has described ; but I can- 

 not admit that from the date of the publication of his paper 

 Mr. Davis obtained anything like a prescriptive right to discuss 

 " the adequacy of the force of evaporation to produce the repulsion 

 observed" in comet's tails. The formula which I made use of in 

 my paper in Knowledge of Feb. 16th, for expressing the relation 

 between the velocity of the molecules of hydrogen and the tempera- 

 ture of the gas, is a direct consequence of the kinetic theory of 

 gases, and has been used by very many persons besides Mr. Davis 

 and myself, long before the date of Mr. Davis's paper, or of my 

 communication to the Astronomical Society. With the exception of 

 the use of this formula, our methods of treating the subject and 

 our theories differ very materially. 



Mr. Davis supposes the force of repulsion to be due to the evapo- 

 ration of solid matter, while I, having a diSiculty in conceiving of 

 the precipitation and subsequent evaporation of the same substance 

 in the neighbourhood of a comet's nucleus, have been driven to 

 assume that a mist of solid or liquid particles is precipitated by the 

 cold of space, and the particles are then acted upon by freely 

 moving molecules of other substances which still remain in the 

 gaseous state. Though one can conceive of the heat of the sun 

 producing evaporation from the sunward side of a particle, it is 

 difficult to conceive that the heat radiated from the nucleus would 

 be sufficient (especially in the case of comets which exhibit tails 

 when far outside the orbit of the earth) to give rise to energetic 

 evaporation from the face of the particle turned towards the 

 nucleus, and without repulsion from the nucleus the small particles 

 would not be driven towards the hyperbolic envelopes which fre- 

 quently form round the nucleus. According to my theory the 

 repulsion from the nucleus is due to a bombardment of gaseous 

 particles, while the repulsion from the sun is due to the recoils 

 accompanying evaporation, not of the substance of the liquid or 

 solid particle itself, but of matter deposited on its shaded side. 



Mr. Davis is right as to tho velocity produced by slow evapora- 

 tion of the halt of a particle, but he has omitted to notice that I 

 assumed that the half of the mass was " rapid?;/ thrown off." I 

 should have said suddenly thrown off, and then my numerical 

 cxanqile would have been perfectly accurate ; but the theory in no 

 way rests upon this numerical result. I am only concerned to 

 prove that recoils from evaporation will give rise to velocities which 

 are commensurable with the velocities of gaseous molecules at 

 similar temperatures. 



Mr. Davis's example, in which he supposes tho repelled body to 

 be a gramme of sand contained in a block of ice equal to a cubic 

 mfctre of water can hardlj- correspond to tho probable condition of 

 matter about the nucleus of a comet which has made several returns 

 to perihelion, yet such comets occasionally throw out largo tails ; 

 and by making this extraordinary and somewhat artificial assump- 



