138 



POPULAR SCIENTIFIC MATURES. 



equal to that of tho earth cannot, contain less 

 than 2775 pounds of luminous ether.* 



The phenomena in celestial space are in 

 conformity with this. Just as a heavy stone 

 flung through the air shows scarcely any in- 

 fluence of the resistance of the air, while a 

 light feather is appreciably hindered ; in 

 like manner tho medium which fills space is 

 far too attenuated for any diminution to 

 have been perceived in tins notion o lLo 

 planets since the time in which we possess 

 astronomical observations of their path. It 

 is different with the smaller bodies of 

 our system. Encke in particular has 

 Bhown, with reference to the well-known 

 small comet which bears his name, that it 

 circulates round the sun in ever-diminishing 

 orbits and in ever shorter periods of revolu- 

 tion. Its motion is similar to that of the 

 circular pendulum which wo have men- 

 tioned, and which, having its velocity grad- 

 ually delayed by the resistance of the inr, de- 

 scribes circles about its centre of attraction, 

 which continually become smaller and 

 smaller. The reason for this phenomenon is 

 the following : The force which offers a re- 

 sistance to the attraction of the sun on all 

 comets and planets, and which prevents them 

 from getting continually nearer to the sun, is 

 what is called the centrifugal force that is, 

 the tendency to continue their motion in a 

 straight line in the direction of their path. 

 As'the force of their motion diminishes, they 

 yield by a corresponding amount to the at- 

 traction of tho sun, and get nearer to it. If 

 the resistance continues, they will continue 

 to get nearer the sun until they fall into it. 

 Encke' s comet is no doubt in this condition. 

 But tho resistance whoso presence in spaco 

 is hereby indicated, must act, and has long 

 continued to act, in the same manner on tho 

 far larger masses of the planets. 



Tho presence of partly fine and partly 

 coarse heavy masses diffused in cosinical 

 space is more distinctly revealed by tho phe- 

 nomena of asteroids and of meteorites. Wo 

 know now that these are bodies which ranged 

 about in cosmical space, before they caiue 

 within the region of our terrestrial atmos- 

 phere. In the more strongly resisting medi- 

 um which this atmosphere offers they are 

 delayed in their motion, and at the same time 

 are heated by the corresponding friction. 

 Many of them may still find an escape from 

 the terrestrial atmosphere, and continue 

 their path through space with an altered and 

 retarded motion. Others fall to the earth ; 

 the larger ones as meteorites, while tho 

 Hmaller ones arc probably resolved into dust 

 by the heat, and iw such fall without being 

 Keen. According to A2"\.r.der Ilerschel'-s 

 estimate, we may figure shooting-stars a.-< be- 

 ing on an average of tho samo>i/e as paving- 

 Ktuncs. Their incandescence mostly occurs 

 in the higher and most attenuated regions of 

 the atmosphere, eighteen miles and more 

 above the surface of tho earth. As they 

 move in spaco under tho influence of tho 

 name laws as the planets and comets, they 

 possess a planetary velocity of from eighteen 



to forty miles In a second. By this, aTeo, 

 wo observe that they aro in fact steUe caiientv, 

 falling stars, as they have long been culled 

 by poets. 



This enormous velocity with which they" 

 enter our atmosphere is undoubtedly t)i 3 

 cnuso of their becoming heated. You nil 

 know that friction heats the bodies rubbed. 

 Every match that we ignite, every badly 

 greased coach-wheel, every auger which we 

 work in hard wood, teaches this. The air, 

 like solid bodies, not only becomes heated 

 by friction, but also by the work consumed 

 in its compression. One of tho most im- 

 portant results of modern physics, the actual 

 proof of which is mainly due to tho English- 

 man Joule, is that, in such a case, tho heat 

 developed is exactly proportional to the 

 work expended. If, like tho mechanicians, 

 wo measure tho work done by tho weight 

 which would be necessary to produce it, mul- 

 tiplied by the height from which it must fall, 

 Joule has shown that tho work, produced by 

 a given weight of water falling through .1 

 height of 425 metres, would be just sufficient 

 to raise tho same weight of water through 

 ono degree Centigrade. The equivalent i:i 

 work of a velocity of eighteen to twenty -four 

 miles in a second may bo easily calculated 

 from known mechanical laws ; and this, 

 transformed into heat, would bo sufficient to 

 raiso the temperature of a picco of r.iettorio 

 iron to 900,000 to 2,500,000 degrees Cen- 

 tigrade, provided that all tho heat were re- 

 tained by the iron, and did not, us it un- 

 doubtedly does, mainly pass into tLo air. 

 This calculation shows, at any rate, that tho 

 velocity of the shooting-stars is perfectly ad- 

 equate to raiso them to tho most violent in- 

 candescence. Tho temperatures attainable 

 by terrestrial means scarcely exceed 2000 de- 

 grees. In fact, the outer crusts of meteoric 

 stones generally show truces of incipient 

 fusion ; and in cases in which observers ex- 

 amined with sufficient promptitude tho 

 stones which had fallen they found 1 hem hot 

 on tho surface, while the interior of detached 

 pieces seemed to show tho intense cold of 

 cosmical space. 



To the individual observer who casually 

 looks toward tho starry bky the meteorites 

 appear as u rare and exceptional phenome- 

 non. If, however, they are continuously 

 observed, they aro seen with tolerable regxi- 

 larity, especially toward morning, when they 

 usually fall. But a single observer only 

 views but u. small part of the atmosphere ; 

 and if they are calculated for the entire sur- 

 face of the earth, it results that about seien 

 and a half millions fall every day. In our re- 

 gions of space, they are somewhat sparse and 

 distant from each other. According to Alex- 

 ander Herschel's estimates, each Ktono is, on 

 an average, at a distance of 450 miles from 

 its neighbors. But tho earth moves through 

 18 miles every second, and has a diameter of 

 7820 miles, and therefore sweeps through 87' 

 millions of cubic miles of space every second, 

 and curries with it whatever stones aro con- 

 tained therein. 



LIuny groups are irregularly distributed 



