Makcu 3, 1882.] 



♦ KNOWLEDGE '• 



387 



irttcisf to t!)c eiJitor, 



TTk* Editor dots not \old himself re*po}i*iUe for the opinions of his eorrenpondentt. 



H' ■ 'innol undfrtake to return uinnuscriptit »r to correxpotid vith their tcritem. Atl 



, -mytinitioHg ahoulJ be as whort a* po$s\blet conrisfeniljf Kith full and clear ttate- 



>» of the writer's meaning.'] 

 {■I Editorial communication* should be addretsed to the Editor o/* EnowledCiB; 

 JiMuxera communicalione to the Publithert, at the OJice, 74, Great Queen- 

 '. W.C. 



: Bemittances, Chequer^ and Tott-Office Order* should he made payahU to 

 ri. Wwmun 4* Son*. 

 •/All lettera to the Editor iciU be XHinbered. For convenience of reference, 

 corrftpondenttf irhen r^erring to any letter, tcill oblige by mentioning itt number 

 and the p-igt on which it appears. 



All Lftier* or Queriej> to the Editor ichich require attention in the current intue of 

 K50WLBDGB,«AoH/il reach the Publishing Office not later than the Saturday preceding 

 the day qf publication, 



(I) Letters to have ftchance of appearing must be concise; they must be drawn 

 up in thi* form adopted for letters here, so that thev may go untouched to the 

 printers ; private communications, therefore, as well as queries, or replies to 

 queries (intended to appear as such) should be written on separate leaxes. 



(11.) Queries and replies should be e^en more concise than letters ; and drawn 

 op in the form in which they are here presented, with brackets for number in case 

 of queries, and the proper query number (bracketed) incase of replies. 



(III.) Letters, queries, and replies which (either because too long, or unsuitable, 

 or dealing mth matters which otners have discussed, or for any other reason) can- 

 not 6nd place here, will either be briefly referred to in answers to correspondents, or 

 ackBonledged in a column reser\'ed for the purpose. 



"In knowledge, that man only is to be contemned and despked who is not in a 



gUte of transition Sot is there anything more adverse to accuracy 



vhiD fiiity of opinion.'* — Faraday. 



**There is no harm in making a mistake, but great harm in making none. Show 

 B6 a man who makes no mistakes, and I will show you a man who has done 

 nothing." — Lieb i<j. 



"(jod's Orthodoxy is Trnlh."— Charles Slngsley. 



(Bttv Corrrsponirnrf Columns. 



PEESKRVIXG FOSSILS FROM THE LOXDOX CLAY.— 

 EXCKINITES. — VEXTRILOQUISII. — STRATA ON THE 

 GREAT WESTERN RAILWAY. — DIAMETER OF THE 

 MOON'S IMAGE IN THE FOCUS OF A 42-INCH OBJECT- 

 GLASS. 



[293] — I can sympathise ^vith " Lepidodendron " (query 2S4, 

 p. 316) inasmuch as I once had the pain of seeing a quantity of 

 beautiful fossils of my own from the London clay effloresce and 

 cnunble to a greenish powder. The only thing to do with those 

 who behave in such a way is to preserve them in closely stoppered 

 Tials of water as soon as they are cleared from the matri.'c in 

 irhich they are embedded. Sheppey flints are terrible things to 

 decompose in this fashion. The blackish metallic nodnles about 

 which your correspondent asks is iron pyrites, or snlpherised iron, 

 called '■ Copperus " by the people who collected them. They do fre- 

 quently exhibit traces of vegetable remains. I am ignorant of the 

 neighbourhood of West Drayton. 



"Wilfred" (([nery 231, p. 346) of course knows that various 

 forms of encrinitc abound in our British mountain limestone. So 

 far as I know though, the £nfriH«.5 liUiformis is not found in our 

 English rocks at all, but is confined to the German rn'a.--'.Mc .1/uj,-- 

 chelkalk. Its sole surviving representative is the Pentacritius Caput- 

 lled\isx in the Carriboan Sea. This has a long-jointed stem lixed 

 to the rock and supporting a cup-like receptacle containing the 

 soft body and viscera. In the plates covering the upper part of 

 this is an opening for the mouth. From the edge of the cup pro- 

 ceed line jointed arms which ramify and ultimately terminate in 

 articulated cirri or feelers. The joints composing the columns are 

 pentangular, and groups of fine articulated tentacula issue from 

 the column at intervals between the root and the top of it. It is 

 really an echinoderm mounted on the top of a stem. 



" Erin " (query 259, p. 364) will find all that is communicable in 

 print on the subject on which he inquires in a little book called 

 " VentriIoqui.sm made Easy," published- by Wamc it Co. 



In reply to <|uery 264 (p. 364), the Great Western Railway runs 

 over the London claj- between Paddington and Reading. From 

 Reading to Wallingford the subsoil is all chalk and chalk marl. 

 Prom Wallington to Didcot we pass over the upper greensnnd. 

 A few miles to the west of Stoventon Station we get on to the 

 lower green-^and. After passing Uflington, we pass through the 



npper oolite, and at Shrivenham get into the middle oolite. From 

 Swindon to Wootton Bassett we are again on the up|>er oolite ; 

 from Wootton Bassett to Chippenham in the middle oolite ; and 

 from Chippenham to Corsliam on the lower oolite. Thence to Bath 

 is tlu'oiigli the lias stratification, which continues to Bristol. The 

 triassic strata are next met with between Bristol and Yatton, 

 whence a run over alluvium brings us to the mountain limestone. 

 After this, the country is covered with alluvium, until we get to 

 Bridgewater, between which to«ni and Taunton our journey is 

 mainly over the triassic dei)osits again. Finally, from Taunton to 

 Exeter we practically traverse the new red sandstone for the entire 

 distance. " Cams " should look up the account of these various 

 formations in any elementary work on (ieology. 



X Fellow of the Rov.vi, .\stroxomical Society. 



GREAT COMET OF 1861. 

 [294] — I ought to have asked admission before, but perhaps it 

 may not be too late now, for an observation of mine on the tail of 

 the Great Comet of 1861. I was fortunate enough to see it on the 

 night of June 31, when its magnificence was at its height, and I 

 obtained a very interesting sketch, mth a 5i inch object-glass, of 

 the unsymmetrical stnicture of the head, which seems so frequently 

 to characterise the larger comets. But what I wish particularly 

 to mention was the appearance of the tail, which, as represented 

 in the graphic sketch in KxowLEnGE, No. 5, p. 87, was spread 

 out like a fan. The eastern edge of this, owing perhaps 

 to trees or a rising moon, escaped my notice ; but the 

 central ray I traced for at least 90°., first curved to the left, 

 and then straightened near Polaris. Abont midnight my wife 

 pointed out to me a great separate beam, some 3' or 3^° broad, 

 lying far W. under the square of Ursa Major, having)// Urs» in 

 the lower edge, and Cor Caroli about 1° above its upper, traceable 

 about half-way from the latter star to Arcturus, and pointing with 

 its other extremity to the head of the comet, though, owing to the 

 summer twilight, no connection could be made out. In about 20m. 

 I found that it had risen higher, so as to stand midway between ^ 

 and 7 Ursae, and its termination, now much more distinct, was 

 plainly visible near f Bocitis; but some times afterwards the beam 

 was no longer visible. This unexpected change of position, contrary 

 to the general motion of the comet, led me at the time to think 

 that, notwithstanding its similarity to the rest of the train, it might 

 be only a cirrus cloud lighted by the risen moon, and coming up 

 with the breeze ; but subsequent comparison with a drawing 

 kindly sent me by George Williams, Est|., of Liverpool — much 

 resembling the sketch in Kxowledge — led mc to the conviction 

 that it was part of the outspread tail, and that the observed move- 

 ment was the effect of perspective, this long streamer having passed 

 so swiftly and closely over the earth, that the .apparent closing up 

 of the great fan from increasing distance had been distinctly per- 

 ceptible. ~ T. W. Webb. 



IS SPACE BOUNDLESS? 



[295]— In Clifford's Essays and Helmholtz's Lectures (Second 

 series) are considerations on flat, spherical, and pseudo-spherical 

 surfaces, which seem to destroy the certainty of Euclid's postulates 

 and Kant's intuitions. Clifford is fragmentary, but his conclusion 

 is evident ; he prefers to believe that space is limited and spherical. 



Helmholtz's lecture is obscure, perhaps owing to the translation. 

 I know his first series suffered grievously in translation, but I can 

 gather that the idea of limited space involves the idea of bodies 

 and movements diminishing as they near the circumference of such 

 space. Now, could Clifford entertain a belief involving such a con- 

 dition ? Helraholtz seems to hold that pseudo-spherical surfaces 

 may be infinite, and may be imagined so. 1 cannot imagine them 

 so, and it seems to me that a surface consisting of two opposite 

 curves must come to an end by the curves re-entering themselves, 

 just as a spherical surface does. 



Could you do anything to make an ordinary intellect grasp these 

 new and revolutionary views ? Considering the transcendent im- 

 portance of the subject, and that Gauss and|Lobachewski are not 

 to be attempted lightly, might you not give us a paper ? 



A word of congratulation on your journal. Its weekly appearance 

 is quite an event here ; it more than fulfils the high expectations 

 which those who were familiar with your writings entertained on 

 hearing of its being projected. J. S. T. 



PURPLE OF THE ANCIENTS. 



[296] — Mny I ask, in reference to your article on "The Purjile 

 of the Ancients," what cnlour this ancient purple was r I once 

 attended a lecture in which the lecturer proved, to his own satisfac- 

 tion at least, that the ancient purple was red — a sort of vermilion 



