Aug. 8, 188L] 



o KNOWLEDGE 



111 



layer, therefore, supposing a current to traverse such series 

 of coils, tends to produce a north pole at one end and a 

 south pole at the other. This is the principle involved in 

 the construction of electro magnets. 



Before dilating upon the size and length of wire for any 

 particular coil, let us turn our attention to the practical 

 details of construction. It may, in the first place, be stated 

 that, generally speaking, the best effects are produced when 

 the length of the core is six times its diameter. Its .=uiface 

 should he as even as possible, not absolutely smooth, but 

 free from marked irregularities. If such irregularities are 

 present, they are likely to cut through or wear away the 

 cotton or silk, whichever it may be, that is used for insu- 

 lating the wire. If, again, the current is one of excep- 

 tionally high electromotive force, there will Le a consider- 

 able risk of its passing from the wire to the core, rejoining 

 the coil at some other point. It is almost superfluous to 

 say that when it behaves in this way, the coil might as 

 well be absent for all the good it is capable of doing. Where 

 any of these dangers are present the iron should either 

 he coated with a layer of shellac varnish (made by dissolving 

 good shellac in methylated spirit), or closely covered with 

 paper which has been saturated with melted paraffin wax. 

 Either of these cour-ses will, in the great majority of cases, 

 be sutlicient to prevent the current taking a short path 

 through the iron. The wire should be well insulated with 

 cotton or silk, and wound as closely and regularly a.s possible. 

 When the first layer is finished it should, more particularly if 

 the wire is cotton covered, and currents of high EM.F. are 

 to be used, be immersed in a bath of melted paraffin wax, 

 and insulation is further assured by covering the layer 

 o£ wire with a piece of paraffined paper. This will not 

 want tying on or securing by extraneous means, but 

 may be quite as effectually fixed by winding it round 

 tightly, and then with the aid of a hot poker, or other 

 piece of iron, heating one edge of the paper over the other. 

 The hot iron melts the wax, which, on cooling, holds the 

 paper firmly. The second layer may then be applied and 

 treated in identically the same manner as the first. Layer 

 should then succeed layer until their entire thickness 

 equals the diameter of the core ; or, in other words, 

 until the diameter of the coil measures three times that 

 of the core. The length of the coil will then be twice 

 its diameter. It will be apparent that, to get the greatest 

 possible number of layers on, so as to come within these 

 dimensions, the space occupied by the insulating material 

 must be as small as is practicable. An electro magnet is 

 then produced, care being taken, obviously, to leave free 

 sufficient wire at the ends of the coils for purposes of 

 •connection. 



Electro-magnets are very powerful, much more so than 

 ordinary permanent steel-magnets, and are capable of 

 producing marvellous effects. The shape of the magnet 

 may be just whatever the experimentalist pleases, remem- 

 bering, of course, that the simpler the shape the more 

 easily will it be wound. The effects produced by magnets 

 ■oi equal length wnth equal quantities of wire are not by 

 any means identical. Even in simple straight bar-magnets, 

 a number of different effects may be produced, and Pro- 

 Jessors Ayrton and Perry have published particulars of 

 very interesting experiments on this point, with which, 

 however, as the allotted space is filled, we must deal a 

 fortnight hence. 



(To he continued.) 



It is a. sad commentary on the danger of railroading to limbs, to 

 say nothing of the disaster to life, that the Toledo Railroader, the 

 xjrgan of the railway employes, contains the advertisements of nine 

 different manufacturers of artificial limbs. - • 



THE CAPTURE THEORY OF COMETS.* 



Bv Ricn.vBD A. Proctor, 



"XTTE start from the conception that all comets originally 

 * V entered our solar system from without. They came, 

 says Heis, Schiaparelli, and others, who have advanced the 

 Capture Theory, fiom out of interstellar space. Now, it is 

 no valid objection to this view that it gives us no idea how 

 cometary matter came to exist in interstellar space, for in 

 all inquiries into the past condition of the celestial bodies 

 we must always come short of their actual origin. Thus, 

 in considering the past of our solar system we may start 

 from a chaotic vaporous state, or from a past condition in 

 the form of cosmical dust, or from a condition in which the 

 vaporous and the dust-like forms are combined ; but if we 

 are asked whence came the vapour or the cosmic dust we 

 are obliged to admit that we cannot tell. If, hereafter, we 

 should be able to say that it, carae from such and such 

 changes io a quantity of various forms of matter, which we 

 may represent by X, Y, and Z, we should still be unable 

 to say how X, Y, and Z came into existence. So that I 

 make no serious exceptiou against the supposed origin of 

 comets on the ground that it really leaves very much to be 

 explained. Interstellar space is a convenient place to which 

 to assign the origin of bodies so mysterious as comets. Cela 

 exprime bMucoup de c/wses. Almost anything might happen 

 in regions of which we know so little, or, rather, of which 

 we know absolutely nothing. 



Yet it may be worth while to remark that, on the whole, 

 the interstellar regions are less likely to be the regions 

 whence comets originally came to visit suns and sun 

 systems, than to be regions whither comets strayed after 

 leaving originally the neighbourhood of solar systems. 

 The most probable idea about the interstellar spaces is 

 that they are the most vacuous regions within the range of 

 the sidereal .system. The mere circumstance thit comets 

 came from out of them affords no belter reason for regard- 

 ing them as the original home of comets, than tlie circum- 

 stance that comets pass from the solar system into these 

 interstellar spaces affords for rejecting that assumption. 

 There is, in fact, simply no reason whatever for imagining 

 that the place where comets came into existence is the vast 

 unknown region around the solar system which we call 

 interstellar space. Most comets come to us from thence ; 

 as many comets are travelling into that unknown region as 

 are coming out of it. To form an o|)iniou about the origin 

 of comets from no better evidence than their last journey 

 (out of millions, very likely) can afford, woidi be as absurd 

 as for a day-fly to reason that the river flowing past the 

 home of his race came out of the sky because a few drops 

 of rain came thence. 



Suppose, however, we admit that in interplanetary space 

 there have been in the past, and still exist, such flights of 

 meteoric matter as the theory we are considering assumes. 

 Let us grant them, also, such motion as may save them 

 from what otherwise would inevit>ibly be their fate, viz., 

 a process of direct indrawiiig towards the nearest sun, and 

 consequently destruction (with mischief probably to his 

 orb), after a period of time which must be regarded as 

 utterly insignificant compared with the time intervals 

 measuring the ditration of a solar system. 



It follows, then, that each flight of meteors would in the 

 long run draw near some sun, without, however, rushing 

 directly upon him ; and sweeping round his <;l<ibe upon 

 such path as chanced to result from the combination of its 

 original movement and its attractive influence, would pass 



* From an article on the " Origin of Comets," in the North 

 American Review, 



