316 



KNO^VLEDGE ♦ 



[Oct. G, 1882. 



..li.r tlie fartiiiT off tbty aro, 1 finr I can linriily apliiin mailers. 

 i. :.y liiHg a jiair of itni|>afs<K u| c« wiiUr ilin Inrihcr jou jmsli 

 tun-arxl* tho hin^ro any objict put between its lops ': Tliero ie no 

 " theory on this subject." (2) •' Were my reuiorkt! alout Colewayo 

 meant to lo Fsrrn.stical (Issue of Sept. 1) ?" I tliink I remarked 

 t' - -' '1 and semi-savape linps. all ripbt in tbcir own 



laipUnennt and iiilclUctra'Iy (in any matter 

 . .ntiniptible. 1 tliink 1 furi'lior expressed yet 

 ;.r ■ .r ratber disgust) for j>eople wlio, because tliesc 



men tin i:di.>i kinps, crowd round tbem, stare at them, bow down 

 to them, and so forth. That was not intended to be either sar- 

 cttstical or ironical, sarcasm and irony beinjf mtirelv different from 

 dispust and contempt.— W. O. Wooi.uojiiiE. (1) "l do not know 

 what are tho qualities of telescopes by Neprotti i Zambra. But 

 you could entirely trust Mr. lirowuinp's own estiniuto of tho 

 second-hand telescope yuu mention. (2) There is no complelo 

 history of tho Advance' of Science within the last f..rty or lifly 

 ycai^. (3) Lives of great men arc so numerous that it is impos- 

 sible to aeloct. (4) I do not know of any work in which all 

 reoemly-inventcd improvements arc described. — J. Uahgrbave. 

 Surely you must make a mistake. The child cannot have falUn 

 '• from the lips of a near relative ;" relatives, near or far off, do not 

 carry childrcD in their mouths. I have referred to the /fi/trcc, and 

 my mind is relieved. "Uaponet" does not say that tho child fell 

 that way, thouph what he says might be so intlrpreted. What he 

 says is that he had heard the account of the child " which fell out 

 of a movinp train, from the lips of a near rc'ulivo." — A Uriton. 

 Why, certainly. The .\merieans are always shootinp each other. 

 You ofi.n find it difficult to make yourself heard on Broadway, by 

 re:iM)n of the constant firing of revolvers. It is equally true that 

 It i« quite unsafe to walk beyond the suburbs of New York, lest an 

 Iiuliun sli'.uld tomahawk you. Seriously, you should read a littlo 

 more, and nad better books; yon would not then believe such 

 foolish stories. I have been, altogether, two years in America, and 

 durin;; all that time 1 only once heard a pistol fired in anger, and 

 that was firwi bv a lunatic.^jAS. Nicholson. Book received. 

 Th.ink». It shoU bo reviewed shortly .--JI. G. Wells would like to 

 have some information about emigration. But the information ho 

 pi^es about his friend who wishes to emigrate is not very full. This 

 friend baa £500, is strong, and intelligent. If that is enough to 

 vhiiw where he had better go, perhaps some corresiioiident will 

 kindly say.— Cu.«KLF.s James. The account you pivc of themeteir 

 is no! e.tact enough to be of use. For instance, what is " the velo- 

 city with which a body tc-enis to move from a state of rest?" — 

 A. r. S. Thanks. Will shortly start tho monthly astronomical 

 n<'t--< von wish for. — K. A. Law. Y'es ; there were many remark- 

 ab meteors in 17s3. Ciilbert White writes : " The summer of 17^3 

 w.is an omaiing and |K)rtentous one, and full of horrible pheuoniei;a ; 

 for Usides the alarming meteors and tremen<ious thunder-storms 

 that affrighted and distressed the different counties of this king- 

 dom, the [icculior haze or fog that jirevailed for many weeks in this 

 island, and in every fart of Europe, and even beyond its limits, 

 was a, most extraordinary appearance, unlike anything known 

 within the memory of roan." This agrees well with Cowper's 



" Fires from beneath, and meteors from above, 



Portentous, unexampled, unexj)lained, 



Havo kindled beacons in the skies." 

 .\. Stbai.li.vc. Thank.s ; you will see that we have already used 

 :: 1. :ter on the subject of the' lizard's name.— E. U. Cowlky. Neither 

 • f the works you name is now a trustworthy guide. In my bejok on 

 t !i'- son, the motions of the earth and sun are dealt with rather fully. 

 — W. H. Collins. Many thanks ; but articles of the came kind havo 

 alr> ady api>earcd. U.A. Billky. You mistake ; SirW. Thomson is 

 full;, aware of tho nature of lunar influence, ns understood by seleno- 

 ni' •■ ■.r<.:..-i.i- His way of expressing hiniKelf was simply a sort of 

 ^ for a more complete rejection. — J. A. Ollahii. 1. 

 .• yon con actually detect any incorrectness in the 

 iy that method :■' 2. Uave you done so large a slmro 

 ' ;,ou canform an opiniononthe paitIcularc|Ucstion 

 I . ! -.;(;ii by my " learned friend ?" 1 have myself tried the non- 

 ft' /lie drinks, and 1 consider their value for the purpose indicated 

 to i,.- uil. But then my exi>erienco maybe qnjte excepticnr'. 3. 

 J'a.r;. riigs arc now nearly eihaunted.— T.' I>. W. M. Mr. Proctor does 

 n<'t p. t under the influence of his ixjison until after his work is done. 

 Bui I have n<>ked him your qucKlii.n ; and he begs me to say that the 

 moons attraction and the moon's influence in raining tides aro very 

 different things. The latter in part depends on the relative dis- 

 tal. . of the nearest and farthest paits of the enrlh, and the earth's 

 cei,lr<- from the mtxm. This introduces an element which increases 

 the < ffict of proximity. I eijiect to persuade Mr. Proctor, in 

 one ii'. his che<rful momenlx, to write a short article on the tides. — 

 M. B. A. I find it very difficult to answer your questions. You 

 •eetn so much more certain about everything than I feel about any- 

 thing. I cannot but think you are wrong about "the nasty oxygen." 



Surely oxygen cannot bo nltopether nasty and destruelive. — W. 

 lluK^. The idea that nothing can be destroyed — that "when the 

 brains are outthemnnis" yet not "dead," but his thoughts go about 

 in tho ether, affecting men in some way wo cannot understand — 

 is pleasant, but it can hardly be fa!d in re|iresiiit us yel any esta- 

 blished truth. I do not know of i<u\ 1 1 h u\ . llll^ :ii,il;,-iln about 

 thoupht-ivading. St KKKiiKRdesin n i i i stam- 

 mering. — A Peui'lkxkh Stidknt. i i |i n mildly, 



mistaken. If 0-»-2 is not equal to (i I ivln i! . i Ml rlrmuO'i' 

 1 suggested, in answer to another eornspnnih ni tliut tlial leviower 

 should be rewarded by the sum of i'l,OU0,0OU niiiUi|ilie(i by 0-1-2. 

 I would now suppes-t a different reward for him. Let him receive 

 the sum of £10,000,000, multiiilicd by tho difference between and 

 0-t2. — Yai.c. Y'uNEn BuTiiKA. Yes, there is a way of making up 

 one hundred out of the numbers 1, 2, 3, 4, 5, C, 7, 8, '.). Try again. 



KLECTRICAL. 

 H. Williams. The platinum wire ])a.iaing through the glass of an 

 incandescent lamp, to which tho filament is attached, does not 'be- 

 C(mio incandescent, because it is com]iarativoly thick, and does not 

 therefore offer the necessary amount of resistance. Tho best way 

 to repulato tho intensity of a current is to regulate tho speed at 

 which the dynamo is driven. Where there is a largo installation, it 

 is usual to have a lamp or two in tho cngino-room, which, by tho 

 light emitted, form a kind of gauge.- A. H. Keelino. A Diiniell 

 cell is easily made, (iet an eartlieowaro jar of about quart capacity, 

 costing Gd. tu'.M., a jiorous |iot sufliciontly high to stand half tothroo- 

 ipiarters of an inch above the level of the jiir. i'oiilie plates got a piece 

 of thin sheet-coijjier (say ali.'ni r, in, 1,. \.v \ .,y :, mhIk's), tlio thinner 

 t lie better, for cheupnoBs. I'iimIii ■.■..'• i- i ' t ,il lorm to go inside 



the porous pot. Clet also :.:!... i ., . :,i li ..f an inch thick, 



and about G inches by 9 or In un I., h, an. I 1.. n.l a into a cylinder to 

 Bland inside the earthenware jar, and outside tho porous pot. The 

 zinc should bo amalgamated (see answer to J. N. Frazcr, Knowledge, 

 No. (13). To charge the cell put crystals of sulphate of copper (in 

 buying it nsk for eU'Ctrii-al) into the porous pol, so as to half fill it, 

 then (ill up with water. Fill tho outer portion up with water 

 acidulated with about one-twentieth its volume of Bul])huric acid. 

 Then get a coujile of binding screws or terminals, ono for the zinc, 

 the other for the copper. If you can use a soldering iron you moy 

 save the shilling or so for terminals by attaching jii'jces of copper 

 wire direct on to the plates. Tho metal plates, although directed 

 to be six inches high, should stand half-an-inoh or BO above the 

 porous pot and earthenware jar resiiectivcly. 



(EHir iflatt)fmatical Column. 



EASY LESSONS IN THE DIFFERENTIAL CALCULUS. 



No. X. 



By RicnABD A. Psoctob. 



WE will consider a few more applications of tho differential 

 calculus to problems of maxima and minima. 

 A li, Fig. 1, is the section of a plane horizontal surface; L, B, 

 light vertically above A. If A B = I ft., at what height should L be, 

 that the illumination at the point 11 may be as great as jioHsiblo ? 



fit is clear there is a ma 



level ns B, the illumination 



zero if L is at an infinite liei 



LetAL-,/'. (The light i 



to act as if collected at u po 



Then BL-y/A H'-t 



- y.n + .c' 



And, according to the known laws of ill 



