Dec. -2, 1881.] 



KNOAVLEDGE 



93 





Xfttn-s; to tl)f etJi'tor. 



ITht Editor Joe$ noHolJhiiiuel/mponsible/or iheoptmoM ofhit corretpondenlt. 

 He caitnol uadfrlal-e lo return manuscrifit or to correspond irilh their rrtlert. Jie 

 TtquetU thai all cctamuniealiom ihould be as short at possible, cousisteiitli/ leith full 

 and clear statements of tlte icriter's meaning.'^ 



Alt cotnmi4«i<-atio,is should be addressed to the Editor cif KxowLIDGB, 71, Qreai 

 Queen-street, W.C. r- i. 



All Cheques and Fost-Office Orders to he made payable to Messrs. Wi/man e; 

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 and the page on which it appears. 



Ml Letters or Queries to the Editor irhich require attention in the current usue of 

 Kyowt.svax, should reach the Publishing Office not later than the Saturday preceding 

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" In knowledge, that man onlv is to be contemned and despised who is not in a 



Plate of transition 'Nor is there anything more adverse to accuracy 



than fiiity of opinion." — Faraday. 



" There is no harm in makine a mistake, but preat harm in making none. Show 

 me a man who makea no mistakes, and I will show you a man who has done 

 nothing.*' — Litbig. ^^^_^ 



0\\x CoiTfgponlifnrt Columns. 



THE IN\aSIBILITY OF LIGHT— JNCLIXATIOX OF THE 

 EARTH'S AXIS— THE ZOETROPE. 



[49] — Without entering — at all events for the present — into the 

 discussion of the question " Is the Sun hot ? " I should like to 

 suggest to " Tyro " (letter 6, p. 35) a simple experiment which 

 will give him ocular demonstration that light is invisible. Let your 

 correspondent, then, ntake a pasteboard tube, ten or twelve inclies 

 long, and three inches in diameter. Close the bottom with a disc 

 of cai'd, and line the whole with black velvet. This done, let him 

 cut two holes, say half an inch in diameter, ojiposite to each other, 

 two-thirds of the way towards the closed end of the tube ; and 

 between those a third hole. If he will hold tliis simple piece of 

 apparatus so that the sun shall shine across the tube, i.e.. so that 

 sunlight shall enter in at one of the side holes and pass out at the 

 other, and will look into the tube through its open end, he will see 

 — nothing; the interior being totally dark. Now, let him introduce 

 a strip of wTiting paper through the hole between the two others 

 (which should be at the lower side of the tube), and push this 

 l)aper up until it enters the beam of sunlight. Instantly the 

 interior of the tube will become illuminated, because the previously 

 invisible liglit will be reflected from the paper, and so become 

 perceptible. " Tyro " has presumably noticed that abeam of sun- 

 light entering a darkened room through a hole or chink in tlie 

 shutter is traceable as it cro.«ses the apartment. This, however, 

 arises from the reflection of the light by the particles of dust which 

 till the atmosphere of the apartment. Were it practicable to 

 eliminate these by burning, or other^^^se, the path of the ray would 

 be quite invisible, and the round sjiot of light on the floor or 

 opposite wall, would be the only inilication we should receive of 

 the entry of sunlight into the darkened chamber at all. 



I do not know whether the " Tyro " of letter 9 (p. 3C) is the same 

 " Tyro " as he to whom I have just been essaying a reply. At any 

 rate, I will suggest another experiment as a means of clearing up 

 this second ditiicultj-. It is this. Let your querist obtain a lamp, 

 an apple, and a knitting-needle. Fuinishcd with these, he must 

 thrust the knitting-needle diametrically through the apple, and place 

 the lamp in the middle of the table. The lamp will stand for the 

 sun, the apple for the earth, and the knitting-needle for its axis. 

 Now, he must incline the needle 23°, and we will suppose that, in 

 doing so, he causes the top of it to point to the north wall of his 

 room. Then he must carefully preserve the direction of the needle 

 constant in this position, and, doing so, carry it round the lamp. 

 An instant's reflection will show him that, should he start from the 

 north side of the lamp, the upper or northern end of his needle (and 

 obviously the northern half of his apple-earth) will be inclined from 

 the lamp ; and that w-hen he brings it round to the south side of 

 the lamp, the top of the axis, still pointing to the north wall of the 

 room, must be inclined towards it. The end of " Tyro's " query is 

 not quite so intelligible as might be desired ; but he seems to con- 

 ceive in some occult way that the earth's rotation must affect the 

 position of her axis. If, while studiously keeping his knitting- 

 needle axis parallel to itself during its revolutions round the lamp, 



he twists it so as to make the apple rotate on it, he will at once see 

 how the two movements may bo independent of each other. 



As " Moonstruck" (query l-, p. 38) has a question on a cognate 

 subject, I may say that perpetual spring, rather than perpetual 

 summer, would reign at the poles of the earth, were its axis perpen- 

 dicular to the plane of the ecliptic. Summer would, of course, be 

 continuous at the Equator, where the sun would be always verti- 

 cally overhead. 



If "Zulu" (query 10, p. 38) will reflect, imprimis, how much 

 what we call seeing is a matter of inference, and, in the next place, 

 remember that the image of any object is retained by the retina 

 for something like 01 second after such object has disappeared, 

 he will get some idea of the way iu which the images in the 

 Zoetrope are caused, as it were, to shade into each other, the mind 

 unconsciously supplying the intermediate steps. The real images, 

 in the case of an actually moving object, must, so to speak, overlap 

 in a way which must render it impossible to predicate definitely 

 that any one given instantaneous attitude of the body iu motion has 

 been actually observed. 



A Fellow of xnE Kov.^l Asteoxomical Societt. 



THE JIISSIXG LIXK. 



[50] — Dr. Antlrew Wilson does good service in making known to 

 youi" readers the erroneousuess of the widespread notion that man 

 is descended from monkey. Concerning the question to which he 

 makes excellent reply, I think the following remarks in Professor 

 Huxley's Preface to Hoeckel's " Freedom of Science " (p. xiii.) will 

 be serviceable. 



"All the real knowledge which we possess of the fossil remains 

 of man goes no further back than the quaternary epoch, and none 

 of these remains present us with more marked pithecoid* characters 

 than such as arc to be found among the existing races of mankind. 

 But then the equine quadrupeds of the quaternary period do not 

 differ from existing Equidw in any more important respect than 

 these last diiier. Yet it is a well-established fact that in the course 

 of the tertiary period, the eqnine'quadrupeds have undergone a series 

 of changes exactly such as the doctrine of evolution requires. 

 Hence sound analogical reasoning justifies the expectation that 

 when we obtain the remains of pliocene, miocene, and eocene 

 anthropidce, they will present us with the like series of gradations." 

 — Edward Clodd. 



THE 1-lXCH OKDXAXCE MAPS. 

 [51] — In the first numbers of Knowledge, attention has been 

 called to the fact that the 1-inch to the mile Ordnance maps are not 

 trustworthy. A very good case in point has come to my knowledge. 

 From Handcross, in Sussex, three roads run to Brighton, one 

 through Crabtree and Heufield, one through Cuckfield, and the 

 other through Hicksted and Bolney. The one through Crabtree 

 leaves the other roads about a mile south of Handcross Gate, and 

 is showH in the right place on the map. The one through Cucklield 

 leaves the one through Bolney about a furlong lower do^vn ; but on 

 the map it is a good mile and a quarter, and, consequently, for 

 about three miles run is shown entirely in the wrong idace. — I am, &c., 



G. W. BCCKWELL. 



PALIZSCII AXD IIALI.KVS COMET. 

 [52] — Allow me to point out that, in an article on Comets in the 

 first number of Knowledge (p. 10), you have inadvertently adopted 

 an oft-repeated error, that when Palizsch re-discovered Bailey's 

 comet on Chiistmas Day, 1758, he found it " without telescopic 

 aid." A complete account of his discovery, in his own words, will 

 be found in the " Berliner Jahrbuch " for 1828, by which it appears 

 that it was made with a telescope of 8 ft. focal length, and was the 

 result of a search for the comet in the part of the sky where he 

 expected it to appear. With this he noticed, about six o'clock on 

 the evening in question, a nebulous-looking object, between i and 

 S Piscium, which he had not seen there before ; and subsequent 

 observations, on Dec. 26 and 27, proved that it was indeed a comet. 

 Thus did Palizsch (who, though certainly a farmer, was not a 

 peasant, for he was a man of education, and an amateur in botany 

 and other sciences, besides astronomy) first observe the predicted 

 return of a comet nearly a month before anjone else ; Messier 

 being the next, and observing it at Paris, mth a 4i ft. X'ew- 

 tonian, on Jan. 21, 1759. I presume the mistake that Palizsch found 

 it casually, and without telescopic aid, was founded on a misunder- 

 standing of a note iu Uerschel's "Outlines," which certainly conveys 

 that impression, but docs not actually state it, so that it is difficult to 

 be sure whether Sir John really thought so. But at any rate, it has 

 been repeated in many books on astronomy, and Miidler thought it 



* Ape-like. 



