572 



NATURE 



[October 15, 1891 



stable forms, so that energy is dissipated whenever these 

 are broken up and rearranged. When we strain a solid 

 body beyond its limit of elasticity, we expend work irre- 

 coverably in overcoming, as it were, internal friction. 

 What is this internal friction due to but the breaking and 

 making of molecular ties ? And if internal friction, why 

 iiot also the surface friction which causes work to be 

 spent when one body rubs upon another ? In a highly 

 suggestive passage of one of his writings,^ Clerk Maxwell 

 threw out the hint that many of the irreversible processes 

 of physics are due to the breaking up and reconstruction 

 of molecular groups. The models help us to realize 

 Maxwell's notion, and, in studying them to-night, I think 

 we may claim to have been going a step or two forward 

 where that great leader pointed the way. 



THE SUN'S MOTION IN SPACE. 



SCIENCE needed two thousand years to disentangle 

 the earth's orbital movement from the revolutions 

 of the other planets, and the incomparably more arduous 

 problem of distinguishing the solar share in the confused 

 multitude of stellar displacements first presented itself as 

 possibly tractable little more than a century ago. In the 

 lack for it as yet of a definite solution there is, then, no 

 ground for surprise, but much for satisfaction in the large 

 measure of success attending the strenuous attacks of 

 which it has so often been made the object. 



Approxirnately correct knowledge as to the direction 

 and velocity of the sun's translation is indispensable to a 

 profitable study of sidereal construction ; but apart from 

 some acquaintance with the nature of sidereal construc- 

 tion, it is difficult, if not impossible, of attainment. One, 

 in fact, presupposes the other. To separate a common 

 element of motion from the heterogeneous shiftings upon 

 the sphere of three or four thousand stars is a task 

 practicable only under certain conditions. To begin 

 with, the proper motions investigated must be established 

 with general exactitude, The errors inevitably affecting 

 them must be such as pretty nearly, in the total upshot, 

 to neutralize one another. For should they run mainly 

 in one direction, the result will be falsified in a degree 

 enormously disproportionate to their magnitude. The 

 adoption, for instance, of a system of declinations as 

 much as i" of arc astray, might displace to the extent 

 of 10° north or south the point fixed upon as the 

 apex of the sun's way (see L. Boss, Astr. Jour., No. 213). 

 Risks on this score, however, will become less formidable 

 with the further advance of practical astronomy along a 

 track definable as an asymptote to the curve of ideal 

 perfection. 



Besides this obstacle to be overcome, there is another 

 which it will soon be possible to evade. Hitherto, in- 

 quiries into the solar movement have been hampered 

 by the necessity for preliminary assumptions of some 

 kind as to the relative distances of classes of stars. But 

 all such assumptions, especially when applied to selected 

 lists, are highly insecure ; and any fabric reared upon them 

 must be considered to stand upon treacherous ground. 

 The spectrographic method, however, here fortunately 

 comes into play. "Proper motions" are only angular 

 velocities. They tell nothing as to the value of the per- 

 spective element they may be supposed to include, or as 

 to the real rate of going of the bodies they are attributed 

 to, until the size of the sphere upon which they are 

 measured has been otherwise ascertained. But the dis- 

 placements of lines in stellar spectra give directly the 

 actual velocities relative to the earth of the observed 

 stars. The question of their distances is, therefore, at 

 once eliminated. Now the radial component of stellar 

 motion is mixed up, precisely in the same way as the 



" "Encyc. Brit.," Art. " Constitution of Bodies." 



NO. 1 146, VOL. 44] 



tangential component, with the solar movement ; and 

 since complete knowledge of it, in a sufficient number of 

 cases, is rapidly becoming accessible, while knowledge of 

 tangential velocity must for a long time remain partial or 

 uncertain, the advantage of replacing the discussion of 

 proper motions by that of motions in line of sight is 

 obvious and immediate. And the admirable work carried 

 on at Potsdam during the last three years will soon afford 

 the means of doing so in the first, if only a preliminary 

 investigation of the solar translation based upon measure- 

 ments of photographed stellar spectra. 



The difficulties, then, caused either by inaccuracies in 

 star-catalogues or by ignorance of star-distances, may be 

 overcome ; but there is a third, impossible at present to 

 be surmounted, and not without misgiving to be passed 

 by. All inquiries upon the subject of the advance of our 

 system through space start with an hypothesis most un- 

 likely to be true. The method uniformly adopted in them 

 — and no other is available — is to treat the inherent 

 motions of the stars (their so-called viotus peculiares) 

 as pursued indifferently in all directions. The steady 

 drift extricable from them by rules founded upon the 

 science of probabilities is presumed to be solar motion 

 visually transferred to them in proportions varying with 

 their remoteness in space, and their situations on the 

 sphere. If this presumption be in any degree baseless, 

 the result of the inquiry \% pro tanto falsified. Unless the 

 deviations from the parallactic line of the stellar motions 

 balance one another on the whole, their discussion may 

 easily be as fruitless as that of observations tainted with 

 systematic errors. It is scarcely, however, doubtful that 

 law, and not chance, governs the sidereal revolutions. 

 The point open to question is whether the workings of 

 law may not be so exceedingly intricate as to produce a 

 grand sum-total of results which, from the geometrical 

 side, may justifiably be regarded as casual. 



The search for evidence of a general plan in the wan- 

 derings cf the stars over the face of the sky has so far 

 proved fruitless. Local concert can be traced, but no 

 widely-difiused preference for one direction over any 

 other makes itself definitely felt. Some regard, never- 

 theless, must be paid by them to the plane of the Milky 

 Way ; since it is altogether incredible that the actual con- 

 struction of the heavens is without dependence upon the 

 method of their revolutions. 



The apparent anomaly vanishes upon the consideration 

 of the profundities of space and time in which the fun- 

 damental design of the sidereal universe lies buried. Its 

 composition out of an indefinite number of partial systems 

 is more than probable : but the inconceivable leisureliness 

 with which their mutual relations develop renders the 

 harmony of those relations inappreciable by short-lived 

 terrestrial denizens. " Proper motions," if this be so, are 

 of a subordinate kind ; they are indexes simply to the 

 mechanism of particular aggregations, and have no de- 

 finable connection with the mechanism of the whole. No 

 considerable error may then be involved in treating them, 

 for purposes of calculation, as indifferently directed ; and 

 the elicited solar movement may genuinely represent the 

 displacement of our system relative to its more immediate 

 stellar environment. This is perhaps the utmost to be 

 hoped for until sidereal astronomy has reached another 

 stadium of progress. 



Unless, indeed, effect should be given to Clerk Max- 

 well's suggestion for deriving the absolute longitude of 

 the solar apex from observations of the eclipses of 

 Jupiter's satellites (Proc. Roy. Soc, vol. xxx. p. 109). 

 But this is far from likely. In the first place, the revo- 

 lutions of the Jovian system cannot be predicted with 

 anything like the required accuracy. In the second 

 place, there is no certainty that the postulated pheno- 

 mena have any real existence. If, however, it be safe 

 to assume that the solar system, cutting its way through 

 space, virtually raises an ethereal counter-current, and if 



