33G 



KNOWLEDGE 



[FEa 17, 1882. 



nortimlly toucliing it, is another button, M, in connection 

 with the tliick win; attached to tlic wire leading to Y. W 

 is a waslier or dutch, consisting of a. small circular disc of 

 l>riis8 fitting loosely on to tlu; cnrhon-holdcr, so that, if 

 slightlj- tilted, it takes a grip on the holder on both sides, 

 ond prevents the carbon-rod falling. When the current 

 cnt<'rs, the rods being in contact, a circuit is formed. The 

 current divides between the thick and thin wires on ////', 

 only 1 per cent, however, going through the thin wire, its 

 resistance being ■150 ohms, wliile that of the thick wire i.s 

 only 1 ■.") ohms. The current going through the thick wire 

 draws up the soft iron plungers, .W, and with them the 

 washer, 11', which pulls the carbon-holder up a .sliort 

 distance, and so creates an arc. When the arc gets too 

 long, the resistance of the thick wire circuit is con- 

 siderably increased, while that of the thin wire cir- 

 cuit is relatively deci-eased. Consequently, the upward 

 tendency imparted by the thick wire is count<'racted 

 by the greater amount of current passing through 

 the thin coils, and the holder falls until it is again 

 restrained by tlie clutch, t^hould it so happen that the 

 carbon becomes broken, or that by any means an arc 

 cannot be formed, the whole of the current passes through 

 the thin circuity and in passing through T is sufficiently- 

 powerful to attract A, thereby putting .1/and J/' in contact. 

 The current then flows from X along lever BA through 

 M' to M, through the small, thick wire, and away to )', 

 thus cutting the carbon-rod completely out of the circuit. 

 This " cut out " allows the lamps to be joined up in one 

 circuit, without the risk of all of them going out in the 

 event of one being faulty, a feature belonging only to this 

 and one other form of lujup. 



Preparations for further lighting are being made on a 

 magnificent scale; a 30-horse-power engine is to drive one 

 of the largestrsized machines, which, it is said, wiU main- 

 tain one light of 1.50,000 candle-power, and from another 

 machine a light of 50,000 candle-power is to be directed 

 towards the llouses of Parliament. 



A magnificent display is being rapidly prepared in the 

 Alhanibra Com-t, wliich promises to outdo everything else 

 in the Exhibition. We must, however, defer any further 

 remarks on this company's exhibit until a future occasion. 



POPULAR ASTRONOMY 

 Bv TUB Chjsb 05 AQ^xire ,J;ifAXKHJAL. Observatoky.* 



SEVERAL coiTespondents, in the course of their 

 inquiries as to books on astronomy, have asked us 

 whether we can recommend the treatise written by Pro- 

 fessor Simon Newcomb. Although the book has been 

 more than two years before the public in this counti-y, as 

 well as in America, we deem it well, in response to these 

 •lueries, to give an account of it, as manifestly many in 

 England are unaware of its merits. 



We do not, as a rule, much admire the way in which 

 soi-disanU professional astronomers treat the wonders of the 

 heavens and the grand problems presented by the move- 

 ments of the celestial orbs. Too often they discuss these 

 as a mere land surveyor might discuss the teachings of the 

 earth's crust Methods and instruments of observation 

 are of much more interest in their eyes tlian the 

 lessons to be learned from observations after these have 

 been carefully made. Tliey seem, at times, even disposed 

 to be angry, as Flarasteed was with Newton, when the raw 



•"Popular Astronoiny." By Simoq Newcomb, LL.D., United 

 States Naval Obscn-alory. (London : Macmillan & Co.) 



mat^'Hals which they have gathered together are worked 

 into the manufactured article — Knowledge. 



It is a plea-Hant di.-iappointment, then, to find that Pro- 

 fessor Newcomb, the chief official astronomer of the United 

 Htat<'s, does not take so limited a view of popular astronomy. 

 In his "Popular Astronomy," he deals more or less fully 

 w ith every part of the .science which could pos.sibly be of 

 intcrtat to tht; general reader. He gives a full, though 

 condensed, view of the histoiy, principles, methods, and 

 results of astronomical research. The lii.storic and philo- 

 sophic sid<ts of his subject are treated with more fulness 

 than is usual in works of this kind. The purely tf'chnical 

 side has been somewhat condensed, but i.t dealt with as 

 fully as is neces.sary. 



Having briefly treated in the first chapter of the ap- 

 parent motions of the heavenly lx)dies, and described the 

 Ptolemaic system. Professor Newcomb jiroceeds in the 

 second and third to descril>e the work of Copemicns, 

 Kepler, and Newton, in establishing, step Viy step, the true 

 system of the universe. These three chaptei-s form the 

 first part of th(t work. The second part deals with prac- 

 tical astronomy. The picture of Bianchini's monstrous 

 tube, as mounted in the grounds of the Barberini Palace 

 at Rome in the sevent<;enth century, gives a good idea of 

 the difficulties with which the earlier observers had to con- 

 tend. A telescope that a child could handle would, in 

 our day, give better views of the heavenly liodies than 

 this unwieldy instrument, the moving of which required 

 the eflTorts of several men. After a description of 

 the modem achromatic telescopes and reflectore, there 

 follows a section on the magnifying powers of telescopes, 

 wliich we would recommend to the especial study, iiot only 

 of obser\-ers, but of those Nvho have occasion to discuss 

 observations. In particular, this section will serve to 

 correct the, common error that large telescopes increase 

 the apparent brightness of objects which present a visible 

 surface, as distinguished from olijeots like the stais, which 

 even under the highest te!escopi<^ powers appear as mere 

 points. We could have wished, however, tbot this error 

 had not been corrected mei'«ly by an €^: cathedrd state- 

 ment, but that the reasoning establishing the time relations 

 between magnifying and illuminating power had been 

 gi\en in full. The second part includes ulso an account 

 of the application of the telescope to celestial measure! 

 ment":, a most interesting and valuable chapter on measur- 

 ing the distances of the heavenly Viodies. and chapters on 

 the motion of light and on the s]>ectroseope. 



The third and fourth parts of the work are devoted <» 

 descriptive astronomy. In the former, after a chapter on 

 the general structure of the solar system, we have a long 

 and very interesting chapter on the sun, and chapters OB 

 the inner group of planets (including the earth and moon), 

 on the outer group or family of giant planets and on 

 comets and meteors. The fourth part deals with the stars 

 — first, as they are seen witli the naked eye and with the 

 telescope : secondly, as they are probably arranged in 

 space ; and, thirdly, as each lias probably been formed 

 The third chapter discusses questions of extreme interest, 

 such as the sources of our sun's heat, the secular cooling of 

 the earth, the plurality of worlds, and tlie nebular hypo- 

 thesis. Lastly, we have lists of the principal telescopes of 

 the world, and catiilogues of doul>le stars, nebula*, star 

 clustei*s, and comets ; tlie usual tables of elements (but 

 greatly improved in many respects), a useful glossary, and 

 a set of star maps. 



One general characteristic of these \arious chaptei-s may 

 be broadly indicated. There is scarcely one statement, 

 properly so called, in the whole work which is not strictly ' 

 accurate, or rather, which does not present with strict j 



