ilAEcn 17, 1882.] 



♦ KNOWLEDGE - 



425 



other portion may more than make up for the tletieiency, 

 if the opening is wide enough. The mine method of deter- 

 mining the earth's mean density is, in any case, suKject to 

 great uncertainty ; and few astronomers now attacli much 

 weight to the result <jf the Ilarton Colliery experiment. 



There is a singular mistake in the following statement 

 respecting Venus in transit : — " It would not be possible 

 to see any indications of an atmosphere in such circum- 

 stances ; for the reason that the light passing through its 

 denser portions would be refracted entirely out of its 

 course, so as not to reach an observer on the earth at all." 

 By similar rea.soning, it could lie shown that we ought never 

 to see the sun. For the rays which set out from him 

 directly towards an observer on earth are refracted entirely 

 out of their course, and never reach the observer at all. 

 Precisely, however, as such an observer sees the sun by 

 rays which but for our atmosphere could ne\er have 

 reached him, so solar rays pass through the atmosphere of 

 Venus in transit to an observer on eaith, which would 

 never have reached him but for that atmosphere. In fact, 

 the solar light seen close to the black disc of Venus in 

 transit does not come from the part of the sun immediately 

 l)eyond Venus, but from other parts, or, to speak more 

 correctl}-, from e\erv other part of the sun's disc. 



Professor Newcomb disposes rather too summarily of the 

 interesting discovei-y made by Kirk wood, that there are 

 gaps in the mean distances of the small planets from the 

 sun, none travelling at .1, ^, 2-5ths, 3-7ths, &c., of the 

 mean distance of .1 upitei-. " Whether these gaps are really 

 due to the relations of the periodic times, or are simply 

 the result of chance, cannot yet be settled." He says, 

 " the fact that (juite a number of the small planets have a 

 period very nearly three-eighths that of Jupiter may lead 

 us to wait for further evidence before concluding that we 

 have to deal with a real law of nature in the cases pointed 

 out by Professor Kirkwood.' The law involved is most 

 important in its consequences, leading almost inevitably to 

 the rejection of the simple nebular hypothesis of the solar 

 system, and to the adoption of a theory of the develop- 

 ment of the system in large part by meteoric accretion. 

 We may be excused, therefore, for dwelling upon a 

 point which, at a tirst view, may seem to belong rather 

 to the smaller details of astronomical research than to 

 the broad facts in which the general public take in- 

 terest. If Professor Newcomb had constructed a graphical 

 idelineation of the distances of the smaller planets, he 

 would have rejected at once, w'e believe, the idea that 

 chance has anj'thing to do with the relation in question. 

 Such a delineation we have before us as wo write, and we 

 find the smaller planets most markedly divided into five 

 principal families, the spaces separating which correspond 

 to the following periods, or sets of periods (Jupiter's period 

 being taken as 1) : — (i.) |, 2-7ths, and 3-lOths; (ii.) ^ ; 

 ,(iii.) 2-5ths ; (iv.) 3-7ths ; (v.) i ; and (vi.) .'i-oths. Now 

 the distance corresponding to the period | falls in the very 

 heart of the richest of all these sub-families. Here we 

 could hardly expect to find a gap, especially when we 

 iiiiiember that the perturbing action due to the corre- 

 -pondence between three periods of Jupiter and eight 

 1'1'riods of a small planet would be very small compared 

 \vith the disturbance due to simpler relations — as where 

 four, three, or t\v'o periods of a minor planet correspond 

 with one period of Jupiter, or tive periods of a minor 

 planet with two or with three periods of Jupiter. JJut 

 in reality, the richest sub-family of small planets does 

 open out unmistakably at the distance corresponding 

 to a period equal to 3-8ths of Jupiter's. This distance 

 would be 2-70.55, the earth's being unity. Now, there are 

 six of the small planets whose distances lie between 2 686 



and 2-701, and six whose distances lie between 2-708 and 

 2-721 ; but there is not one whose distance lies between 

 2-701 and 2-70S. In other words, whereas the average 

 interval between successive distances amounts only to 

 0-0025 in the groups on either side of the critical distance 

 we are considering, there is a gap of 0-007, or nearly three 

 times as great, between these groups. 



We have left to ourselves ho space to comment on the 

 more speculative opinions expres.sed by Professor New- 

 comb. On the whole, he seems to us to show at once more 

 caution and more daring than most of his contemporaries — 

 more caution, inasmuch as he docs not accept old-fashioned 

 views without carefully inquiring into them ; more daring, 

 inasmuch as he is not afraid to accept ;iew >iews when he 

 finds that there is strong evidence in their favour. Here 

 and there, as where he ascribes the darkness of certain 

 zones of Satu:-n's rings rather to the blackness of their 

 component satellites than to sparseness of distribution, we 

 are unalile to agree with him. But it is refreshing to find 

 an official astronomer, and especially one in Newcomb's 

 high position, prepared to analyse and weigh evidence, 

 instead of merely recording what has been observed. In 

 fine, though we have thought it our duty to point out some 

 errors w-hich might mislead the general reader, we can 

 cordially recommend Professor Newcomb's " Popular As- 

 tronomy " as the finest general treatise on the subject since 

 Sir J. Herschel published his celebrated " Outlines of 

 Astronomy." 



THE CRYSTAL PALACE ELECTRICAL 

 EXHIBITION. 



Sixth Nctice. 



"TTTE commence this week a description of the various 

 Vt systems of incandescent lighting now being ex- 

 hibited at the Crystal Palace. Before describing the 

 difl'erent lamps, a word or two on their general principles 

 may not be out of place. In. the first instance, we must 

 understand that a current of electricity, in passing through 

 a substance, tends to heat it in exact proportion to 

 the difiiculty experienced by the current in so passing, 

 just as a flow of water in traversing a pipe brings 

 about a greater or less exaltation of temperature. 

 This arises from a never-failing law that, where motive 

 power or force is hindered or opposed, that force 

 is converted into another force which wo know as 

 heat. Theoretically, no substance allows electricity to 

 pass through it unimpeded, any more than water can pass 

 through a pipe without having more or less friction to 

 overcome. In practice, we allow a pipe large enough to 

 carry the water, without having to resort to any undue 

 pressure ; so also in sending an electric current from place 

 to place, we provide a good conductor of electricity for the 

 purpose. Furthermore, suppose, in the case of the water- 

 flow, that the pipe is not uniformly wide or smooth, or 

 that here and there quantities of sand, A-c, have accunm- 

 lated, then at these particular places the flow will 

 be impeded, and heat produced, while the general 

 rate of progress can only be maintained by applying 

 extra force to push the water forward. Could we 

 measure the heat produced, we should find that it 

 just equals the extra force necessitated by the oppo- 

 sition. This has also its analogy in electricity. Let 

 our large smooth pipe be represented by the good con- 

 ductor of comparatively thick copper wire, and let the 

 obstacle in the pipe ha\e its counterpart in the electrical 

 circuit V)y inserting a very thin piece of conducting sub- 



