POPULAR SCIENTIFIC; LECTURES. 



823 



vations, that the force which attracts the 

 moon toward the earth, bears toward the 

 gravity of a terrestrial body the ratio required 

 by the above law. 



In the course of the eighteenth century tho 

 power of mathematical analysis, and tho 

 methods of astronomical observation, in- 

 creased BO far that all the complicated 

 actions, which take place between all tho 

 planets, and all their satellites, in conse- 

 quence of the mutual action of each upon 

 t-ach,and which astronomers call disturbances 

 - -disturbance, that is to say, of the simpler 

 elliptical motions about the sun, which each 

 one would produce if the others were absent 

 that all these could be theoretically pre- 

 dicted from Newton' s law, and be accurately 

 compared with what actually takes place in 

 the heavens. The development of this theory 

 of planetary motion in detail was, as has 

 been said, the merit of Laplace. The agree- 

 ment between this theory, which was de- 

 veloped from the simple law of gravitation 

 and the extremely complicated and manifold 

 phenomena which follow therefrom, was BO 

 complete and so accurate, as had never pre- 

 viously been attained in any other branch of 

 human knowledge. Emboldened by thia 

 agreement, the next step was to conclude 

 that where slight defects wer.e still constantly 

 found, unknown causes must be at work. 

 Thus, from Besscl's calculation of the dis- 

 crepancy between the actual and the calculat- 

 ed motion of Uranus, it was inferred that 

 there must be another planet. The position 

 of this planet was calculated by Leverrier 

 and Adams, and thus Neptune, the most dis- 

 tant of all known at that time, was discovered. 

 But it was not merely in the region of the 

 xttraction of our sun that the law of gravita- 

 ion was found to hold. With regard to the 

 led stars, it was found that double stars 

 noved about each other in elliptical paths, 

 jjid that therefore the same law of gravitation 

 must hold for them as for our planetary sys- 

 tem. The distance of some of them could be 

 calculated. The nearest of them, a, in the 

 constellation of the Centaur, is 1,039.600 

 miles farther from (he sun than the earth. 

 Light, which has a velocity of 186,000 

 miles a second, which traverses the dis- 

 tance from tli sun to the earth in eight 

 minutes, would take three years to travel 

 from a Centauri lo us. The more deli- 

 cate methods of modern astronomy havu 

 made it possible to determine distances 

 which light would take thirty-five years 

 to traverse ; as, for instance, the Pole 

 Star ; but the law of gravitation is seen to 

 hold, ruling the motion of the double stars, 

 at distances in the heavens which all the 

 means we possess have hitherto utterly failed 

 to measure. 



The knowledge of the law of gravitation 

 has here also led to the discovery of new 

 bodies, as in tho case of Neptune. Peters of 

 Altona found, confirming therein a conjee- 

 tare of BeKsel, that Sirius, the most brilliant 

 of tho fixed Ktars, moves in an elliptical path 

 about an invisible centre. This must have 



been due to an unseen companion, and wheo 

 the excellent and powerful telescope of .tho 

 University of Cambridge, in the United 

 States, had been set up, this was discovered. 

 It is not quite dark, but its light is so feeble 

 that it can only be seen by the most perfect 

 instruments. The mass of Sirius is found to 

 be 13-76, and that of its satellite 6-71, timm 

 the mass of the sun ; their mutual distance is 

 equal to thirty-seven times the radius of th; 

 earth's orbit, and is therefore somewhat large? 

 than the distance of Nf une from the sun. 



Another fixed star, P\ :yon, is in tho sani* 

 case as Sirius, but its ; tellite has not y?i 

 been discovered. 



You thus see that in gravitation we havo 

 discovered a property common to an matte*, 

 which is not confined to bodies in our syn- 

 tem, but extends, as far in the celestial spac^ 

 as our means of observation have hitherto 

 been able to penetrate. 



Bat not merely is this universal property 

 of'all mass shared by the most distant celes- 

 tial bodies, as well as by terrestrial ones , 

 but spectmm analysis has taught us that ft 

 number of well-known terrestrial element^ 

 are met with in the atmospheres of the fixed 

 stars, and even of the nebuls9. 



You all Imow that a fine bright line ot 

 light, seen through a glass prism, appears ;* 

 a colored band, red and yellow at one edge, 

 blue and violet at the other, and green in tho 

 middle. Such a colored image is called n 

 spectrum the rainbow is such a one, pro- 

 duced by the refraction of light, though not 

 exactly by a prism ; and it exhibits therefore 

 the series of colors into which white sunlight 

 can thus be decomposed. The formation of 

 the prismatic spectrum depends on the fact 

 that the sun's light, and that of most ignited 

 bodies, is made up of various kinds of lights 

 which appear of different colors to our eye*, 

 and the rays of which are separated from 

 each other when refracted by a prism. 



Now if a solid or a liquid is heated to such 

 an extent that it becomes incandescent, the 

 spectrum which its light gives is, like the 

 rainbow, a broad colored band without any 

 breaks, with the well-known series of colors, 

 red, yellow, green, blue, and violet, and in no 

 wise characteristic of tho nature of the body 

 which emits the light. 



The case is different if the light is emitted 

 by an ignited gas, or by an ignited vapor- 

 that is, a substance vaporized by heat. The 

 spectrum of such a body consists, then, of 

 one or more, and sometimes even fr great 

 number, of entirely distinct bright lines, 

 whose, position and arrangement ia the spec- 

 trum is characteristic for the substances of 

 which the gas or vapor consists, ao that it 

 can be ascertained, by means of spectrum 

 analysis, what is tho chemical constitution of 

 the ignited gaseous, body. Gaseous spectra 

 of this kind are shown in the heavenly space 

 by many nebulro ; for the most part they are 

 spectra which show tho bright line of ignited 

 hydrogen and oxygen, and along with it a 

 lino which, as yet, has never been again 

 found in the spectrum of any terrestrial el&- 



