398 



♦ KNOWLEDGE ♦ 



[Mat 8, 1885. 



THE FACE OF THE SKY. 



From May 8 to May 22. 



By F.R.A.S. 



SIGNS of solar activity are now diminishing, but still the Sun 

 should be watched daily, as spots and facula3 may at any time 

 appean The aspect of the night sky may be seen iu Map V. of 

 "The Stars in their Seasons." Mercury is a morning star, but 

 rises too short a time before the sun to be at all well placed for the 

 observer. Venus is practically invisible during the next fortnight, 

 a remark equally applicable to Mars. Jupiter must be looked at 

 as soon as ever it is dark enough, as he is travelling towards the 

 West. The planet is in "quadrature" with the Sun at 3 p.m. on 

 the 17th, so that the student may now look out for that shading on 

 the limb opposite to the Sun which was described on p. 102 of our 

 fifth volume. Jupiter is still travelling in the direction of Regulus 

 (" The Stars in their Seasons," Map IV.I. The phenomena of his 

 satellites during the succeeding fourteen days are .as follow : — 

 To-morrow night, the 0th, the observer should look out particularly 

 for the transit of Satellite III., which begins at llh. 5Gm. Unfor- 

 tun.ately, the planet will be low down. On the 10th Satellite II. 

 will be occulted at 8h. 46m. p.m. On the 12th the shadow of this 

 same satellite will leave Jupiter's disc at 8h. 23m. p.m. in the 

 twilight. On the 13th Satellite IV. will be occulted at Oh. 5Sni. 

 p.m., as will Satellite I. at lOh. 25m. ; while Satellite III. will 

 reappear from eclipse at lOh. 35m. 423. On the 14th the 

 transit of Satellite I. will begin in bright twilight at 7h. 40ra. 

 Its shadow will follow it at Oh. 3m. The satellite will leave 

 Jupiter's opposite limb at lOh. 6m., and the shadow at llh. 22m. 

 On the 15th perhaps the reappearance of Satellite I. from eclipse 

 may be caught at 8h. 27m. 49s. p.m. On the 17th, Satellite II. 

 ■mil be occulted at llh. 21m. On the 19th, the shadow of Satellite 

 II. will enter on to Jupiter's face in twilight at 8h. 4m. ; and 24 

 minutes later the satellite casting it will leave his opposite limb. The 

 egress of the shadow will happen at lOh. 59m. p.m. On the 20th, 

 Satellite III. will reappear from occultation at 9h. 33m. ; only, 

 though, to disappear in eclipse at llh. 10m. 3Gs. Satellite I. will 

 be occulted 21 minutes after midnight, but the planet will be 

 very low down. On the 21st, Satellite I. will begin its transit at 

 Vh. 41m. ; and be followed by its shadow at lOh. 57m. The 

 satellite will pass off at 12h. Im. Lastly, on the 22nd, the et'ress 

 of the shadow of Satellite IV. occurs at lOh. 4m. p.m. T. and 

 Satellite I. reappears from eclipse at lOh. 22ni. 55s. Saturn 

 has left us for the season. Uranus may be found from the direc- 

 tions so repeatedly given recently, to the west of jj Virginis. 

 The Moon is new at 3h. 17-5m. p.m. on the 14th,' and 

 enters her first quarter at 5h. 45-lm. a.m. on the 21st. Four 

 occultations of fixed stars will occur at convenient hours 

 during our prescribed period. The first is on the 16th, 

 when 130 Tauri, a 6th magnitude star, will disappear at the 

 Moon's^ dark limb at Gh. 31m. p.m. at an angle from the vertex 

 of 137 , reappearing at her bright limb at 7h. 27m. at a vertical 

 angle of 298'. On the 10th a Cancri, of the 4th magnitude, 

 will disappear at the dark limb of the Moon at lOh. 52m. at an 

 angle of 48° from her vertex. It will reappear at her bright limb 

 at llh. 15m. at an angle from her vertex of 350". On the 20th 

 B.A.C. a star of the Gth magnitude, will disappear at the dark limb 

 21 minutes after midnight at a vertical angle of 125°. The Moon 

 will have set ero it reappears. On the 21st another Gth magnitude 

 star, 35 Sextantis, will disappear at the Moon's dark limb at 

 8h. 48m. p.m. at an .angle of 2G' from her vertex, to reappear at 

 her bright limb at 9h. 18m. at a vertical angle of 340°. The Moon 

 IS in Aquarius when these notes begin, but leaves it for Pisces at 

 8^o' clock to-morrow morning. Just passing in and out of the extreme 

 N.W. corner of Cetus, she moves into Aries at 4 a.m. on the 13th. 

 Across this she travels until 10 a.m. on the 14th, when she enters 

 Taurus. Travelling through Taurus, she arrives at 8 p.m. on the 

 IGth on the confines of the northern strip of Orion. It takes her 

 until 7 o'clock the next morning to cross this, and at that hour she 

 emerges in Gemini. On the 18th, at 81i. 30m. p m., she quits Gemini 

 for Cancer; which she leaves, in turn, for Leo at 8 a.m. on the 20th. 

 She is travelling through Leo until 1 p.m. on the 21st, when she 

 descends into Sextans, re-entering Leo at 11 o'clock th6 same 

 night. She finally quits Leo and enters Virgo at noon on the 

 22nd, and we there leave her. 



E.xPERiMEXT.s are being made by the Government with a new 

 locomotive torpedo steered by electricity from the shore. Very 

 good results are said to have been obtained. 



The anthracite coal trade in the United States has been in a far 

 more prosperous condition just lately. Orders are plentiful, and 

 the number is increasing. 



" Let Knowledge grow from more to more." — Alfred Tbnntbon. 



Only a amall proportion of Letters received cari possibly he »n- 

 ierted. Correspondents must not he offended, therefore, should their 

 betters not appear. 



All Editorial communications should he addressed to the Editor Of 

 Knowledge; all Business comniunicotiens to the Publishees, at the 

 Ofice, 74, Great Queen-street, Tl'.C. If this is not attended to 

 delays arise for which the Editor is not responsible. 



All Remittances, Cheques, and Post Office Orders should be made 

 oayable to Messrs. Wyman & Sons. 



The Editor is not responsible for the opinions of correspondents. 



No COMMDNICATIONS ARE ANSWERED BY POST, EVEN THOUGH STAMPED 

 AND DIRECTED ENVELOPE BE ENCLOSED. 



■WAVE MEASUREMENT. 



[At the special request of "Excelsior" — but also because the 

 subject of his letter is one of great intrinsic interest — his letter ia 

 given in full, though in reality his reasoning is based on a miscon- 

 ception, and therefore falls to the ground. — R.P.] 



[1G96] — I have just been giving myself the pleasure of reading 

 your valuable book, " The Sun," and should have regarded it with 

 unmitigated satisfaction but that I find one argument in it which 

 I cannot reconcile with facts, so shall be glad if you will kindly bear 

 with me while I point out its inaccuracies. I refer to the foot-note 

 on page 1^^, which treats of the breadth of water-waves on a canal 

 as an example of the action of light-waves. As you may, perhaps, 

 not have a copy of the book by you to refer to, I will, with 

 your indulgence, quote in full the paragraph to which I take 

 objection, and it is this : — " Let the reader imagine himself on 

 the bank of a canal observing a series of waves uniformly 



propagated along the stream Let the observer, fixing 



his eye on a certain wave, walk any measured distance 

 (say 100 yards) at the same rate as the wave is moving. 

 Suppose he accomplishes this distance in 65 sees. He 

 knows then that the velocity of transmission of the waves 

 is 100 yards in G5 sees. Let him now, standing still 

 for 65 sees., count the number of crests that pass him in that 

 time. Suppose 360 pass him, then, from his first observation, he 

 knows that the first which passed him has travelled 100 yards 

 from him. Within that distance all the 360 waves are uniformly 

 distributed. Thus the breadth of each is l-360th part of 100 

 yards, or 10 in. This result is perfectly reliable if, during his 

 second observation, his position on the bank has been unchauged. 

 But let us imagine that he has made his observations from a truck 

 — on rails by the canal's edge — and that, unnoticed by him, the 

 truck has glided uniformly along the rails. First suppose that 

 this motion has taken j^lace in .a direction contrary to that of the 

 wa%'es, and that, while he is counting the passing crests, the truck 

 glides a distance of 20 y,ards. It is evident that when the last 

 wave passes him the first is 120 yards, instead of 100 yards, from 

 him. Thus the 360 waves are distributed over 120 yards, and the 

 true breadth of each is l-360th part of 120 yards, or 12 in. If, on 

 the other hand, the truck had moved over 20 yards in the same direc- 

 tion as the waves, it is equ.ally obvious that the 360 waves would be 

 distributed over only 80 yards, and the breadth of each would be only 

 8 in. Similarly, at whatever rate the truck moves it is evident that 

 the observer can no longer depend on the result of his observations. 

 If it moves in a direction opposite to that in which the waves travel, 

 they appear narrower [? broader by foregoing example] ; if it 

 travel icifh them they appear broader [? narrower by foregoing 

 example] than they really are. Indeed, it is not difficult to con- 

 ceive the truck to move in the same direction and at the same rate 

 as the waves travel, in which case (if we could suppose the observer 

 to remain unconscious of that motion) all undulation would appear 

 to him to have ceased, and the water to have a waved but unmov- 

 ing surface." That is the paragraph ; and now I will show, in as 

 few words as are consistent with clearness, that it is inacurate, 

 to say the least of it. Well, in the first place, I will just briefly 

 repeat your first illustration, which supposes an observer standing 

 at a given point (hereafter referred to by me as the "starting- 

 point") on a canal bank, and counting the waves as they pass him 



