344 



KNOWLEDGE 



lOcT. -JO, 1882. 



as E." This is only true when the external resistance is 

 greatly in excess of the internal resistance. The error is, 

 however, corrected further on in the excellent lecture on 

 " Resistances." 



AltogetluT the book comniands a front rank amongst 

 elementary textbooks, ami is well wortliy of our recom- 

 mendation. It is written in language which is simple Imt 

 to the )>oiut, and our praise is due for the manner in 

 which so much reliable information has been intelligibly 

 compressed into so small a compass. 



TuK Electric Liuht at Watekloo House. — The 

 ■ \tcnsive premises, situated at the corner of Cockspur- 

 street, in the occupation of Messrs. Hailing, Pearce, A 

 Stone, have just V)een titled with sixteen arc lamps by the 

 Metropolitan (Brush) Electric Light t'ompany, the motive 

 power being derived from a 1 2 horse-power (nominal) Otto 

 gas-engine, 



Wb shall have HanifKlen and the three-headed Parallax 

 (Ooulden-Birley-Kowbotliam) upon us. The daily papers 

 tell the following story : — " An interesting experiment 

 in signalling by sunshine has been made by our army 

 in Egypt. Colonel Keyser ascended one of the Pyramids 

 near Cairo, and setting up a heliographic mirror, reflected 

 a ray of sunshine all the way to Alexandria, a distance of 

 some ISO miles. The signals, a]>pearing like pin-points of 

 brightness, were distinctly readable on tlio coast, and took 

 the form of a mc-^age from Sir Garnet Wolselcy to the 

 Khedive" The distance is scarcely 120 miles; nearerllO: 

 but taking six inches for the depression in one mile from 

 earth-cur\-ature, corrected for refi-action, we find for 5.") 

 miles a depression of more than l,40O feet; so that a 

 tangent to the earth's surface at a point midway between 

 Alexandria and the Pyramids, would pass 1,100 feet above 

 each of those places. Thus, if the mirror were 1,400 feet 

 al)Ove the sea-level at the Pyramids, it might have reflected 

 sun-rays to a point 1, HilJ feet above the sea level at Alex- 

 andria. But a le.ss height at either end would not serve, 

 or if there were a less height at one. then there must be a 

 great<r height at the other. 



Elements of the Comet. — As we expected. Dr. Hind's 

 elements of the new comet, as given in our last number, 

 have had to receive considerable modification. Instead of 

 making the logarithm of the perihelion distance ^ 00201, 

 he makes it now T'.iUC-'i.'i. The eflect of this on the esti- 

 mated p<'rihelion distance in miles may be seen from the 

 two following little sums : — 



10 + log. j>erihelion distance=8 00201 or 7-90G.')3 

 log. earth's distance =7-90848 790848 



C-OG049 .5-87501 

 These logarithms correspond to a perihelion distance of 

 1,11 ."i/JOy miles and 7">0,000 miles respectively. We believe 

 the distance we adopt, d, viz., .500,000 miles, is nearer the 

 mark. At any rate, this approach liy 3C.'5,000 miles to our 

 e»timat4- is satisfactoiy. Dr. Hind remarks that his lat<;r 

 elements atrord indication of disturbance of the comet's 

 motion n<ar the time of passag'- through perihelion. It 

 can l>c shown from his elements that, at the time when 

 Mr. Gill sivw the conx.t touching the sun's limb (the edge 

 of the sun's disc), it should ha^e been projected more than 

 -■>' within the limb. It follows that the comet was much 

 ntardwl fjuring perih<lion passage, as wi; shall show more 

 f^articularly next week. All yet known aljout the comet 

 tfcnds to show that the comet of 100."^, of 1843, of 1880, 

 and of I "82 are <//<-, and tliat at the place of nearest 

 approach to the sun this comet is very seriously distorbed. 



lettfrs to tf)t ©liitor. 



[Tk0 SJitor {lofiMotMoId himsflfrefponiiibU/ur the opiuioniiofhi$ corretpondent», 

 le coHNot utiJertakt to rrtum mtinutrrit'tt or 'to corr<ff>omi tcith their vrit«r$. AH 

 Bible, coiitiMtentltf with full and cleur ttatt- 



ttrett, W.C. 



Alt Remittane€$, Chequee, and Pott-OMce Orders ahouU be made pavahle io 

 Uesert. H>miiH 4" Son*. 



\* All letter* to the Editor %rill he Kumhered. For convenience qf reference ^ 



" In knowledf^e, that man onlv is to be contonmod nn< 



statoof traoBition Nor ia there Biiylhiiig 



than fliity of opinion."— Jiirarfwj/. 



" Show me a man who makes u'o mistakes, and I will 

 done Dothint;," — Liebig. 



SATURN'S RINGS, 



1 592] — As I am interested in the inquiry which has 

 been made respecting the discovery of the primary division 

 in Saturn's ring, will you kindly inform me where the copy 

 of J'/iildso/iJitriif Tnitisdctions, vol. i., which contains that 

 drawing of the planet of which you have given a repre- 

 sentation in Knowledge, No. 49, p. 307, can be seen, or 

 in whose possession it is ( A friend of mine has examined 

 no less than seven copies of the said volume, and has not 

 been able to find it. C. L. Piuncb, 



The Obser\atory, Crowborough Beacon, 

 Tunbridgc Wells. 



Ii-ncl to wliich Mr. 

 :i(r ilio imrticulars 

 ) ■ ii-om tlio first 

 I lir condiMiacd or 

 :.ir. ,N"oblc, hut t)ie 

 Mr. Ijynn romarka 



[Mil. T. Wahii, who kindly sent \.^ u 

 l^rince refers, will, we liavo no doulii • 

 rcinn'ri'd. The extract is (Mr. \\ i 

 vohinie of the i'hifosojj/iicaJ rivi» 

 cjiitomised one which was rcfcnod i > 

 original issue thnt came out in monthly 



in the Alhenaum on the difleronco between the picture, cf which 

 Mr. Ward has Biipplicd a facsimile, and the one jjrofessinK to bo a 

 copy of it, " 80 that some carelossncss was committed in roproduc- 

 ing it." It seems clear that the engraver carefully rouiovod the 

 very fcaturo which IJuU had been careful to indicate as anomaloQs. 

 Kvery one who has had much to do with scientilic illustrations will 

 understand how tho thing was done. Tho engraver removed what 

 he regarded as a fault in drawing — result, a valuolcss picture. The 

 picture sent by Mr. Ward shows what Ball really saw, — or rather 

 what, by a curious optical delusion, he scomod to boo. Wo must 

 thank Captain Noble for restoring to Cassini tho credit (whatever 

 it moy be wortli) which is due to him in this matter.] 



UIGU TIDES. 

 [593] — In your issue of Sept. 29 you kindly inserted my pri'- 

 dictions of the high tides of Sept. 2030. It may interest some 

 of your readers to know how far these predictions were fulUllod. 

 I take tho computations as they stand in your pajx)r, with Hft. (iin. 

 deducted, as there directed, and the observations arc thoHc made at 

 the self-registering gauge at (icorgo's I'ior, Jiivorjiool, from the 

 level of tho Old IJock sill. It will bo noticed how accurately the 

 "iliumal inequality" is accounted for: — 



Dale. Calculation. Olmiirvalioli. Error. Bar, Wind, diroclion and fo 

 1SH2 ft. in. ft. in. in. 



Hc'pt. 20.., 1« !> 18 G 29.G R.E. fresh 



20 7 20 t; 1 29.4 S.S.W. Blight 



„ 27... 20 20 27.4 S. calm 



22 3 

 20 7 



29.5 W.K.W. 



very ( 



29.0 W.N.W. fresh 



) 2 29.0 W.S.W. slight 

 i 2 29.0 W.S.W. fresh 

 ; 7 29.0 W.S.W. very strong 



Jamks Peabson, M.A., F.K.A.S. 



I 



