Junk 23, 1882.] 



• KNOWLEDGE 



Gl 



anetotrs to CorrfSponlirnts, 



* ^* All eoumunications for tki Editor requiring early attention i\ould reaf\ tk« 

 Office on or before the S'ltttrday preceding tie current i»»ue of KnowlbdbJ, tlu 

 increasing circulation o/ ickich compels UJ to go to preit earlg in t\e iceek. 



HlIfTS TO CoBBBSPO^tDKlfTS. — 1. iV'o question) aiking for ecientife in/ormation 

 can be anevered tkrougk the post, 2. Lettere sent to tke Editor for correspondents 

 cannot be foncarded ; nor can ike names or addresses o/ correspondents be given in 

 anstrer to private inquiries. 3. Correspondents should write on one side onlg of 

 tke paper, and put dravings on a separate leaj. 4. Each Utter should have a title, 

 and in replying to a letter, r^erence should be made to its number, the page o» 

 which it apptarst and its titU, 



Peccavi. I had not been able to read the question relating to the 

 British Musenm, or I should not have failed to thank you warmly 

 for your very kind offer, but at present I have no occasion to avail 

 myself of your kindness. With regard to the beam question, you 

 must remember what the light has to do to make the optic nerves 

 sensible of its source. A wave may exist when it reaches the shore, 

 yet be too small to disturb the sand there. — J. Cram. Thanks for 

 magic cube on base of 11. Fear it may be .some time before we 

 find a vacant corner large enough for its insertion. — Takaxaki. 

 You need no forgiveness '-for being too plain spoken." Our waste- 

 paper basket is n t very ready to take offence. — Paesox. Thanks 

 for kind and encouraging letter. We agree with yon that the 

 mere dilution of science into amusing twaddle would not be 

 an improvement to Kxowleiue. We certainly do not intend 

 to become popular in that way. — TnouGiiTFuL. Yes ; pamphlet 

 safe. — SiLCiiESTEB. Fear we could not find space for any more 

 articles on health resorts than we have already provided for. — 

 F. W. Foster. Is there anything to show that the vapour is other 

 than a light mist, the condensed vapour of water ? — T. J. Hickin. 

 I do not think that a copy of Sir Isaac Xewton's " Opticfcs" (third 

 edition) has any special pecuniary value. — H. R. Weller. We 

 cannot understand your difficulty. It is proved that under the 

 attraction of the sun, a body, in whatever course it may be 

 moving, will travel in one of the conic sections, according to its 

 velocity at any given distance. The particular conic section 

 described will te an ellipse cr a parabola, or a hyperbola. 

 The planets have such particular velocities as correspond with 

 motion in an ellipse.— J. W. JaQies Attempting to indicate 

 the distance between two heavenly bodies or the apparent size of 

 one, in inches, feet, yards, furlongs, &c., is, as you say, utterly 

 absurd. One might as well indicate the distance between two 

 places in hogsheads and kilderkins. — H. D. The idea that there 

 is life on the farther side of the moon is an old one, but is quite 

 untenable. We have not yet seen it accompanied by the idea 

 that on the side turned from us the moon may have an 

 appendage like a comet's tail. You are mistaken about the 

 centrifugal tendencies existing on the moor. They are such 

 solely as result from her rotation on her axis once in 27' 

 days. The only cause to be considered as possibly tending 

 to produce a difference in the condition of the moon's invisible hemi- 

 sphere is the earth's attraction. This, however, can produce no 

 such effects as you imagine. The whole subject, however, has been 

 so often discussed that it is now ovei-wom. — J. M. Fisher. We 

 have not your letter. We did not insert it. because it promised to 

 initiate a controversy, and we have no room for controversy. Dr. 

 Wilson's answer was intended for you alone ; and the method we 

 adopted was the most space-saving one we could employ. We do 

 not know of any rule of "fairness" requiring us to insert everj- 

 letter of the kind which may be sent to us. We know that fairness 

 to the proprietors utterly forbids our doing so. If, however, you 

 still think Dr. Wilson mistaken, and will succinctly say in what 

 points, we will insert yonr remarks. — SixpEW. (1.) Are you sure 

 it was the zodiacal which you saw on the 7th soon after ten ? Juno 

 is not usually a good time to sec the zodiacal ; note the position of 

 ecliptic in oiu- June maps. (2.) The rainbow is not necessarily part 

 always of the same-sized circle. It can have two angular diameters, 

 and the apparent size, even for the same angular diameter, varies 

 according to the distance of the cloud on which the rainbow is seen. 

 — Ax Oxford UxDEBCRAPfATE. Those papers are rather letters 

 than articles; however, there will be no more of them. — M. B. 

 Alder. It must be satisfactory to know so certainly about 

 the moon. I wish astronomers did. Your remarks about warm 

 weather some time ago read strangely now, after so much unsea- 

 sonable cold. — A.B. Solution correct, but autlior's (shorter) has 

 appeared. — A Stitdent. We should in that case see the 

 moon turn slowly on her axis (apparently backwanis) in such sort 

 that every part of her surface would in turn become visible to us. — 

 AsMODEis. It is hardly in our lino to give rules for preventing the 

 beard from growing. — E. M. The number of stations at present 

 existing is far too small to give even the roughest approximation to 

 annual rainfall for whole earth. The sun-spot theory of rainfall is 



not believed in. It was only started, I fancy, to get a pull on 

 national |>urse-strings. — G. T. M. E. Thanks. — H. E. Kilbv. Yes; 

 that method, like the others, is theoretically sound, bat practically 

 unavailable, because of the relative smallnees of the moon's dis- 

 tance. Very small errors in determination of longitude and latitude 

 of stations, and of times of contact, Ac, would introduce very largo 

 errors into the estimates of the sun's distance. The conception 

 is. however, very ingenious. — Erix-go-Bk.igii. It is the case some- 

 times ; so also old hens are adorned occasionally with imperfect 

 sort of comb, and some old ladies with monstachios and beards. — 

 A. BiTTi-E. The Leprosy communication did not seem quite so 

 generally interesting and important as that on Tubercle, and our 

 space ran short.— W. Geaxdy. Thanks for encouraging letter. On 

 the other point, I can assure you I would gladly see my books sold 

 at very low prices ; but publishers, with whom the matter rests, 

 cannot venture — yet, at any rate. — A. E. W. Our arrangements for 

 indexing already made when your letter received. — F. Morris. 

 Berry's Polishing Paste and Domestic Black-Lcad are rather outside 

 our lines. — T. (Jreex. Do not know of any book or books on Gum, 

 its properties and uses. 



BOTANICAL. 

 E. C. H. (Worthing). Host cultivated " geraniums " belong 

 really to the genus I'elargotiium, which has slightly irregular 

 flowers (two upper petals differing from three lower), a spurred 

 sepal (scarcely noticeable), no glands, and few stamens (generally 

 five). The true Geraniums have regular flowers, no spurs, five 

 glands on the disk, and ten stamens. A few perennial true Oera- 

 niums from southern Europe are still cultivated in old-fashioned 

 gardens, but most of the plants so-called are I'elartioniums from the 

 Cape of Good Uope. I'dargnnivm U, in fact, a larger and more 

 specialised insect-attracting type. When in doubt, look for the 

 irregular sepal and count the stamens. Gba.nt Allen. 



-ELECTRICAL. 

 Igxuuamis. See article in Kxowlepge, Xo. 30 ; ebonite can be 

 procured from the India-rnbber and Gutta-]x>rcha Company, ICO, 

 Cannon-street, London, E.C. The same article (see also errata, No. 

 31) answers your (juery anent length of wire. The generator de- 

 scribed will "make a current quite strong enough for plating — ii» 

 fact, rather too strong in proportion to its other features. It will 

 not be strong enough for a .Swan lamp, which rec|uires an electro- 

 motive force of forty volts, or the force produced by twenty good 

 Bunsen cells joined up in series. — U. Bardslev. Get a glass or 

 earthenware jar, about three pints capacity, into this put a xine 

 rod for the positive element. The negative element is a small 

 carbon ))late placed in a porous pot, filled up with a mixture in equal 

 ]>arts of crushed carbon and black oxide of manganese, about the 

 size of parched-peas. Great care is requisite in making this part 

 of the cell, and yon would find it cheaper to buy — well, the )>orous 

 pot with carbon plate in situ. A saturated solution of sal ammo- 

 niac is put in the outer jar, half filling it. At Silvcrtown, where 

 these batteries are made in very large quantities, the upper ex- 

 tremities of the carbons are dipped, at a high temperature, into 

 melted paraflin, and subsequently placed into a mould containing 

 molten lead, which forms a cap for the carbon, a brass terminal 

 being simultaneously fixed into the lead. Marine gluo is run over 

 the carbon-manganese mixture, the carbon rod being in position. 

 Vent-holes are made in the glue to allow any gases to escape. M\ 

 bare metal connections must bo well coated with pitch or varnish, 

 otherwise they will soon be corrodml by the gases given off by the 

 battery. Should you buy a coll, I would n-commend the new- 

 agglomerate LecIanchiS w'ith u largo zinc cylinder, instead of the 

 zincrwl. There are at the present moment some thou.<iands of this 

 latter form in use, comparing verj- favourably with the Fuller or 

 double liijuid bichromate battery.— Chablks Blsnv. You have 

 been anticipated long since. Inventors have not yet been able to 

 overcome the apparent partial volatilisation of the carbon, as is 

 evidenced by the thin carbonaceous film deposited on the surface 

 of the gloss globe.— liiXOBAMis. 1. Yes, the same. 2. S« articles 

 on " Generators." 3. No, not on nn extensive scale. 4. The reversi- 

 bility of the dynamo can be easily explained. It is due to a very 

 simple truism, viz., the convertibility of forces. This point, we hope 

 soon to be nblo to deal with under the head of " Electric Motors." 



AccoRniNQ to tho RaiU-ay Aje (Chicago, April 20) the American 

 Bell Telephone Compsny has in ojwration 18!t,37i instruments and 

 49.168 miles of wire. 'These are conuiHled with 5".i2 exchanges. 

 The tigiires show a great increase on the preccling year. If the 

 company is able to organise a satisfactorj- sjstom of underground 

 wires, a'groat further development nmy be exj>ected. Tli- rlmrv-.-a 

 will in this case be considerably reduce*!. 



