Mat 26, 1882.] 



KNOWLEDGE 



631 



9n£tofr£S to Coiirsponlinitd. 



* ^* All commvmtfationa for the Editor requiring early attention thould reaek t\t 

 Office on or before He Suturjaf preceding lie cvrreut iitue oj' K.VOWLUDOS, the 

 \Acreanng circulation oJ rhicK compeU uM to go to prete earlg in the veetc. 



Hints to Cobbbspoudbsts. — 1. A'o gneitioni aal-ing for tcientijte inyormalion 

 can be antvered through the poet. 2. Lettrre rent to the Editor for correepondente 

 cannot be forvurded ; nor can the names or addreeiee t^ correipondenta be gtvcH in 

 anever to private inquiriee. 3. CorrenpondentM ehould terite oh one tide outit of 

 the paper, and put drateinga on a eepurnte leaj. 4. Each Utter thoutd have a titU, 

 and in replying to a letter, r^erence ahould he nzd* to ite number, the page on 

 vhick it appeartf and ile litU, 



J. Peekixs. Volume I. ends with mimbor 30. Tou ask wliy tlie 

 ]>rice is so high. Tlie answer, I am told, is simply that, to oblige 

 custoiners, the publishers have disposed of copies of the curly 

 numbers and parts (to complete sets) which would otherwise have 

 been available for volumes. Consceiueiitly, not one half, or oi:e 

 quarter so many volumes can bo bound as they would have wished. 

 To give you an idea liow small the stock of volumes, I may mention 

 that from a single colonial bookseller an order for more than one 

 ij\iarti'r the entire number of volumes then available was received 

 \sithin a few days after tho volume had been announced. — 

 K. G. You can readily express a number of many digits, all but 

 the first few being cyi)hers, in a contracted form by indicating the 

 1 lower of 10 corresponding to tho addition of so many digits. Thus 

 Mistead of 172slS3 followed by seventeen digits you may write 

 1728183x10". — A Keaokk kko.m tue First. Thanks for your sug- 

 -ostion respecting scientific instruments on the three years' system. 

 Will find room for it if possible ; but if not, pray consider that our 

 poverty (of space) and not our will declines. — H. H. You are not 

 ■ juite right, but the author of " The Stars and the Sun" is mistaken 

 in that matter. Wien wo see tho first rays of the rising sun, wo 

 MB him in the direction, with reference to the celestial sphere, which 

 ! i> had really occupied eight minutes earlier. The actual effect on 

 : lie time of apparent rising is not, as you suppose, measured by the 

 I iine light takes in travelling from the horizon to tho eye, or as the 

 :\uthor of " Stars and Karth " supposed, the light journey from the 

 -unj but is the time occnpied by the horizon ))lanc in shitting 

 ilirough 22" (the aberration angle) in the direction of the sun's 

 motion on the celestial sphere, and therefore varies with tho latitude 

 and time of year. — Corcagif.xsis. What has appeared about Earth's 

 Population is not Mr. Connall's statement, but mine. It can hardly 

 \>c said to need confirmation, for the calculation is sulficiently simjilo 

 and obvious. Nor doe.'i it contrast with other statements similarly 

 based on calculation. The present population of the globe could 

 'tand, 1 suppose, on five or six square miles. I would like to see 

 actual pro"/ that 36, 027,813,275,075, S45 persons have lived on tho 

 earth. Considering that the population of the earth has never pro- 

 bably e.tceeded 1,800,000,(00, that total would imply a very l«ng 

 period during which the earth Las been inhabited by man. — F. C. S. 

 Thanks ; but, alas, no space. I cannot see myself that the evidence 

 proves tho superiority of the boys, but that all work and no play 

 makes Jack a dull boy and Jill a dull girl. I can find space for 

 the extract alone. Your theory of the formatiou of the earth is not 

 reconcilable with astronomical facts. — JiMno. Your ''banter" is 

 good; I acknowledge with contrition that the stars in my monthly 

 „ maps do hol shine with all the brilliancy of stars in the heavens. 1 

 must pet the printers to use the blackness of night for ink, and 

 sections of the sun's surface for paper. Seriously, you would find 

 it a good plan to ])unch holes through the map for all the brighter 

 stars, then to paste tissue paper over the back, and hold the map 

 before a good light. The W you speak of (Cassiopeia) can, how- 

 ever, be very well seen, I find, at twice tho distance you mention 

 (a yard and a half). — Jas. De.is. The phenomena observed in cases 

 of so-called " mind reading" are curious, even when cases where 

 there may be deception are eliminated ; but where is the evidence of 

 magnetism? Will trj', however, to find room for your e.tperience. If 

 not, remember that tie reason will simply be want of space. — P. F. D. 

 Thanks. — W. Y'ou have not quite coiTcctlj- written your question ; 

 but it means that we are to find the value of tho series whoso general 



I n-l-r — 1 

 term is ( — 1)'"' — between the limits r = s and r=infinity. 



In this there is no difficulty : making V successively eqnal to «, s -f 1, 

 8 + 2, &c., we get the series. 



\n + s — l t r> + n In-fs-fl 



+ , &c., ad inf. 



]„ + .,_ if (1 + 1)- c + -'% liL±£:tl 

 L J 2"+' 



J. V. Eliasson, W. M. Williams, E. Cuaytob, and others. It ia 

 likely enough that a change from 2d. to 3d. weekly, with certain 

 iniprovemouts which you suggest, would find us with a scarcely 

 diminished circulation, and ho considered a not unfair charge. 

 But I should myself have strong objections to the change, and I 

 shall do my best to prevent its being Iriffd. So far, indeed, as mere 

 fairness is concerned, such a change in price without any increase 

 in the average expense of Knowledge, would btill, I think, leave 

 readers nothing to complain of ; for, of course, the cheapness of 

 K xowlkdce must bo regarded asan experiment, and (it may be) a risky 

 one, even with the presently constantly growing tasto for knowledge. 

 But we have faith and patience, which will, we believe, bo justi- 

 fied. Our readcra can do a great deal to help us, and to put any 

 change such as you suggest entirely out of the question. If each 

 reader in three obtained but one new reader, or if each in twelve 

 obtained four, the publishers would turn an entirely deaf ear to all 

 suggestions of changa of price, and nearly all the suggested im- 

 provements would be mado at once. As matters go, we look for- 

 ward to that state of things a little later ; and, to resume the 

 first person, readers may rest assured I shall do my best to 

 see that no change of the former kind takes place. 



PHOTOGEAPHY. 



WiscAr asks f jr information as to painting photographs on glass 

 in oil or water-colours by a process called " Crystoloum." We 

 never heard of the process. Any one who can paint in oil-colours 

 could apply the process to glass. We doubt the practicability of 

 painting on glass in water-colours. — II. 11. The wet collodion 

 process is not the thing of the past you suppose it to be, and, tor 

 certain pm-poses, is not likely to bo supplanted by gelatine. Our 

 scheme includes a series of articles on tho gelatine dry-plate 

 process. That no dark rcom is needed while working with gelatine 

 plates is new to us. 



ELECTRICAL. 



H'sETT. There is a wide difference of opinion as to the origin of 

 earth currents, and accordingly, om* ideas concerning them are of 

 the vagnest kind. During the earlier portion of the disturbance 

 you refer to, tho currents affected only the wires running N.E. to 

 S.W., but subsequently wires in all directions were affected by cur- 

 rents which doubtless owed their existence to a number of con- 

 spiring and conflicting causes. How far the geological structure of 

 the superficial strata affect, by their varying roaistance, tho passage 

 of earth currents, it is impossible to say, but I should imagine very 

 little indeed. Discarding the earth plate, and substituting a return 

 wire (technically termed a loop), prevents almost entirely currents 

 passing from the earth to the line. This demonstrates that, so 

 far as the wire is concerned, the currents a-o not induced, but 

 imparted. I heard of no warning being given by our meteorological 

 oQice to the collieries, of coming magnetic and electric storms ; and, 

 in fact, such warnings never have been given by the society. The 

 American Bureau is, however, a little less antiquated, and ventures 

 to predict these storms, and that they may bo attended by colliery 

 disasters. One or two men in England are also working on this 

 line. — J. F. U. I really should advise you not to attempt making a 

 machine to support a single Swan lamp. Y'ou would find it a com- 

 paratively expensive affair. A Swan lamp requires a current of 

 1'2 amp^res (see articles on Generators). — T. Sington. Better use 

 small rectangular rods of good metal, and solder tho junctions. 



ECLECTICUS AND READERS OF KnOWXEDGE GENERALLY. — As BOOU 



as we have finished our description of simple forms of electric 

 generators, the subject of " Electrical Units and Simple Methods 

 of Measurement" will occupy our column for the "Amateur 

 Electrician." 



BIOLOGICAL. 



" G. S." calls attention to what he is pleased to term " a couple 

 of inaacuricies " (sic) in a biological note in K.nowledge (page 013) 

 of last week. The first inaccuracy refers to the statement that 

 " with the exception of the Camclida>, all quadrupeds have round 

 nucleated blood-globules." The word "nucleated" should have 

 been rendered nori-jntrJea'ccJ, the omission of the "non" being a 

 printer's error. We differ from " G. S." both in his spelling of the 

 word " inaccuracies " and in his contention that the word " round " 

 does not describe the blood-globules of mammals. He adds the 

 expression should havo been " circular and bi-concave." If " G. S." 

 sees any difference between "round " and " circular" (applied to a 

 blood-globule) ho is very welcome to his contention. We did not 

 describe the globule as to its form (bi-concavo), since that question 

 was not asked by tho inquirer. 



By a misprint, " three molars " were said to exist in each jaw of 

 the old world apes as in man. The statement should have read six 

 molars in each jaw. 



[About two pages of " Answers " have been unavoidably held over.] 



