POPULAR SCIENTIFIC LECTURES. 



fV37 



ndden and violent, movements in the earth's 

 fctmosphere, becanse this is heated from tho 

 ground made hot by the sun and is cooled 

 above. With the far more colossal magnitude 

 mid temperature of the sun, its meteorologi- 

 cal processes are on a far larger scale, and are 

 fur more violent. 



We will now pass to the question of the 

 permanence of the present condition of our 

 wystein. For a long time the view was pretty 

 generally held that, in its chief features at 

 uny rate, it was unchangeable. This opinion 

 was based mainly on the conclusions at 

 which Laplace had arrived as the final results 

 of his long and laborious investigations, of 

 the influence of planetary disturbances. By 

 disturbances of the planetary motion astron- 

 omers understand, as I have already men- 

 tioned, those deviations from the purely 

 elliptical motion which are due to the attrac- 

 tion of various planets and satellites upon 

 each other. The attraction of the sun, as by 

 far the largest body of our system, is indeed 

 the chief and preponderating force which 

 produces the motion of the planets. If it 

 alone were operative, each of the planets 

 would move continuously in a constant ellipse 

 whose axes would retain the same direction 

 and the same magnitude, making the revolu- 

 tions always in the same length of time. 

 J;ut, in point of fact, in addition to the at- 

 traction of the sun there are the attractions 

 of all other planets, which, though umall, 

 yet, in long periods of time, do effect slow 

 changes in the plane, the direction, and the 

 magnitude of the axes of its elliptical orbit. 

 It has been asked whether these attractions 

 in the orbit of the planet could go BO far as 

 to cause two adjacent planets to encounter 

 each other, KO that individual ones fall into 

 the sun. Laplace was able to reply that this 

 could not be the case ; that all alterations in 

 the planetary orbits produced by this kind of 

 disturbance must periodically increase and 

 decrease, and again revert to a mean con- 

 dition. But it must not be forgotten that 

 this result of Laplace's investigation!! only 

 applies to disturbances due to the reciprocal 

 attraction of planets upon each other, and on 

 the assumption that no forces of other kinds 

 have any influence on their motions. 



On our earth we cannot produce such an 

 everlasting motion as that of the planets 

 seems to be ; for resisting forces are contin- 

 ually being opposed to all movements of ter- 

 restrial bodies. The best known of these nre 

 what we call friction, resistance of the air, 

 and inelastic impact. 



Henco the fundamental law of mechanics, 

 according to which every motion of a body 

 on which no force acts goes on in a straight 

 line forever with unchanged velocity, never 

 holds fully. 



Even if wo eliminate the influence of 

 gravity in n ball, for example, which rolls on 

 a plane surface, we see it go on for a while, 

 and the farther the smoother is tho path ; 

 but at tho same time wo hear tho rolling bail 

 xaake a clattering sound that is, it produces 



waves of Bound in the surrounding bodieg ; 

 there in friction even on tho smoothest sur- 

 face ; this set:i tho surrounding air in vibra- 

 tion, and imuarts to it some of its own 

 motion. Thus it happens that its velocity is 

 continually less and less until it finally 

 ceases. In like manner, even the most care- 

 fully constructed wheel which plays upon 

 fine points, once made to turn, goes on for a 

 quarter of an hour, or even more, but then 

 stops. For there is always some friction on 

 tho axles, and in addition there is the resist- 

 ance of the air, which resistance is mainly 

 due to that of the particles of air against each 

 other, due to their friction against tha 

 wheel. 



If we could once set n body in rotation, 

 and ki^ep it from falling, without its being 

 supported by another body, and if we could 

 transfer the whole arrangement to an abso- 

 lute vacuum, it would continue to move for- 

 ever with undiminished velocity. This case, 

 which cannot be realized on terrestrial 

 bodies, is apparently met with in tho planeU 

 with their satellites. They appear to move 

 in tho perfectly vacuous cosmical space, 

 without contact with any body which could 

 produce friction, and hence their motioij 

 seems to be one which never diminishes. 



You see, however, that the justification of 

 this conclusion depends on the question 

 whether cosmical space is really quito vacu- 

 ous. Is there nowhero any friction in th* 

 motion of the planets ? 



From the progress which the knowledge of 

 nature has made since tho timo of Laplaco, 

 we must now answer both questions in the 

 negative. 



Celestial space is not absolutely vacuous. 

 In the first place, it i < filled by that continu- 

 ous medium the agitation of which consti- 

 tutes light and radiant heat, and which phys- 

 icists know as th3 luminifertms ether. In 

 the second place, large and small fragments 

 of heavy mutter, from the size of huge stone* 

 to that of dust, are still everywhere) scat- 

 tered ; at any rate, in those parts of Hpaca 

 which our earth traverses. 



The existence of the luminiferous ether 

 cannot be considered doubtful. That light 

 and radiant heat are due to a motion whiub 

 spreads in all directions has been sufficiently 

 proved. For the transference of such a 

 motion through space there must bo some- 

 thing which can be moved. Indeed, from 

 the magnitude of the action of this motion, 

 or from that which the science of mechanic.! 

 calls its i:t. iui:a, we may indeed assign certain 

 limits for tho density of this medium. Such 

 a calculation has been made by Sir W. Thom- 

 son, the celebrated Glasgow physicist. II* 

 lias found that the density may possibly ba 

 far less than that of the air in tho most per- 

 fect exhaustion obtainable by A good air- 

 pump ; but that the mass of the cthor can- 

 not _bo_abjspluteVy_t : > <i>iiil to x.oro. A voinui't 



* This calculation would, however, lost it* bnm-a if 

 Mnxwcii'H hvpoi he-is wore cmitlrinrd. nixonling Jx 

 which li^ht tU-iiuida on electrical aiul raa^ueticAlcNi 



tUuuouj, 



