June 



1882.] 



KNOWLEDGE ♦ 



9 



volves a fractional loss of current, but, at the same time, 

 it removes an element of danger. The ordinary commu- 

 tator consists of a cylinder of metal cut crossways, and so 

 dividing it into two parts. This, however, involves the 

 short-circuiting of the armature for a lirief fraction of time 

 twice during each revolution ; heat is in consequence pro- 

 duced. We have preferred, therefore, to recommend a 

 total disconnection of the circuit twice in each revolution. 

 This may seem a great loss, but really it is very trifling. 

 It also tends to keep the armature cool, and, above all, it 

 has been found to answer well in the Brush dynamo- 

 machine. We must, however, take care that our discon- 

 nections are made at the right time. Our readers will 

 recollect that in the introductory article we stated 

 that a certain current was induced as one pole of 

 a magnet was inserted in the coil, while the opposite 

 current was induced by withdrawing the magnet. 

 Similarly and conversely the approach of one pole, 

 say the north, produces the same electrical effect as 

 withdrawing the other or south pole. Referring once 

 more to Fig. 6, we must regard the coil of wire as 

 surrounding an ordinary soft iron core, whose ends (A. B., 

 Fig. 1, May .ith) become north or south, according to the 

 relative position of the armature. Let us for a moment 



Fig. 9. 



associate ourselves with one end or side of the core, and let 

 the armature perform a quarter of a revolution, so that 

 our end is uppermost As it starts revolving (say to the 

 right) it gradually leaves the north pole of the magnetic 

 system, and at the same time approaches the south pole. 

 Going from the north and to the south, it is necessarily 

 subjected to the same magnetic influence, accompanied by a 

 similar etiVct on the coil of wire. At the moment that our 

 end is crossing the vertical line, no current is produced. Of 

 course, as the end ascends on the opposite side, passing 

 from south to north, the opposite magnetic, and therefore 

 electrical, effect is produced. We must, therefore, to get 

 the best effect, so arrange our commutator that the 

 " brushes " or " collectors " rest on the sniall insulated 

 pieces of metal (C. D., Fig. .5), when the core is in the 

 vertical i>lane. How shall we do this? Space compels us 

 to defer the answer to this question till ne.xt week. 



Ekbatim.— In last week's article, page G19, second colnmn, 

 line 19, read Tib. for 71b. 



THP] ECLIPSE. 



"IT'E have received the following interesting coinniunica- 

 \> tion from Mr. A. C. Ranyard, who kindly under- 

 took to forvvard communications to us respecting the 

 eclipse. 



" Sohag, Upper Egypt, 



".lAri/ 17, 1882. 



" 1 ain sorry that you were not able to come out to the 



eclipse. The weather has been all that could be desired, 



and the Corona magnificent, in a perfectly clear sky, with 



a bright comet within about half a degree of the moon's 



limb. The diflBculties have been very slight, and the 

 trouble caused by heat and flies much less than we were 

 led to e-xpect. We have lived in great comfort on 

 Vioard a steamer anchored alongside our tents. The 

 Khedive has entertained us at his personal expense, 

 and has had pillars built for our instruments, besides 

 giving us a guard of soldiers and water-carriers, who 

 have perseveringly endeavoured to keep down the dust 

 round our station by means of water carried up from the 

 hill and distributed from goat skins. I had made every 

 preparation to defend my instruments from the wind which 

 frequently springs up during totality, but instead of a wind 

 there was a dead calm. 



"The Ccrona'was of the sun-spot maximum type, with no 

 very marked rifts. As seen in the telescope it was full of 

 structure, delicate lines of light curving hither and thither, 

 and arranged in great groups. There was one very marked, 

 long, straight ray, and several structures like great promi- 

 nences stretching to a height of 15 or 20 minutes. Tliese 

 are shown in my photographs, as well as the comet, which 

 has registered itself, together with about half a degree of 

 its tail, on some of the plates. I did not succeed in obtaining 

 a photograph of the spectrum of Young's reversing stratum, 

 though the bright lines were well seen for about three- 

 seconds. — Yours sincerely, A. 0. RANYARD.' 



BUTTERFLIES AND MOTHS. 

 By W. J. H. Clabk. 



WE have now arrived at one of the busiest months of the year 

 for the entomologist ; every bush and tree teems with insect 

 life, and both butterflies and moths in the imago or perfect state, 

 and larx-iv and pup<r, are to be found ererrwhere. The collector 

 has only to take out his net or sngaring-pot, as the case may be, 

 and a profusion of game is sore to meet his eyes. 



Among the butterfles that are out now wo may notice the hand- 

 some, but rather scarce, S«-allow Tail (I'apilio Machaon), seldom to 

 be found but in the fens of Cambridge, the Brimstone (Gnnepteryr 

 lihamni), the well-known Large Garden White (VierU Brafgic(r\, 

 the Small White (Pieris Iiap<r), the Green-Veined White (PiVri-- 

 Napi), the Orange Tip {Eucheliw Cardamines), the Pearl-Bordered 

 Fritillary (Argijnnis Euphrostjne), the Greasy Fritillary (Uelitaa 

 ^r(emis), the Glanvillo FritilJarv PMilaa Cinxia), the Speckle* 

 ^Yood_{Satllrus .E.jeria), the Wall Brown (Saturuf Jfcj.rrn), the- 

 Green and the Purple Hairstreaks(T/i<'<-;<i Riiliand Thecia Quercug), 

 the Small Copjwr (I'olyommatrts Phla-as), Brown Argus (Li/c<enti 

 A'jestis), Silver-Studded Blue, and Small Blue (Li/f<rna .E;)o» ami 

 Lycana Alsus). 



Great numbers of the moths make their appearance now, ami 

 many of the common hawk moths will be out some time during the 

 month. In the course of the ensuing weeks we may probably find 

 the following -.—The Kyed Hawk (gmeriulhua Ocellaius). the Poplar 

 n.-»wk (SiOfrinlhus Vopuli), the Lime Hawk (Smerinthus Tili.r). 

 the rare Spurge Uawk (/ViVt-pli/a Euphnrbur), the very pretty 

 small Elephant nawk(C'/iarofaiiipa Porcflius), the lluraming-birtl 

 Hawk {ilacroiiloasa Stellalarum), remarkable for its rapid flight 

 and habit of hovering steadily over a flower whilst thrusting in its 

 exceedingly long proboscis to obtain its food. The Broad and 

 N'arrow-bordered Bee Hawks (ilacmglofsa Fueiformis and ilacro- 

 glofsa BomluU/nnniii), two insects which take their names from a 

 striking resemblance they possess to the Wild Bee. 



The commonest of our Clcanving Moths, the little Currant Clears 

 wing {Sesia Tipiilifnimis), is now on the wing, and is generally to 

 be found on or near currant bushes. 



The Common .Swift {llepi'ilus ti.jiiiJiiiiKi) and Ghost Swift 

 (HrpiaUig //iirini/i) are both out now. The male Huniuli is n most 

 Imnd.ionio insert, the wings being |iurc white, with yellow c<Igea on 

 the np|H"r, and of a dusky black on the under side: it is a Tery 

 abundant species. The p'n-tty little GnTn Forester (fnn StatirtM, 

 the Least Black Arches (.Vo(n Cn.«rH/<i;i..). the Cinnabar {E«ehtli.r 

 Jao^h.T,r). Wooii Tiger (Chelonitt Planlnginis). White Ermine (Antia 

 Menthratti). Pale Tusisock (Or.jyio Piitiibunda), and Lappet 

 (Laaioeampn Qiirrdjolia) arc all on the wing. 



Among the Ukosiktr-K Family, the SjMitted Yellow (r«fli7i4i ifaru- 

 lata), Scorched Wing (EuryiMB* Dolohraria), Scalloped Uaa-I 

 {Odontopttra Bidentata), Brimstone (Riimia Crat.riJalo), the very 



