ON THE PROGRESS OF SCIENCE. V 



tern ; and if our sun moves, probably all the other stars or suns move 

 also, and the whole universe is in a perpetual state of motion through 

 space. 



" The second subject to which the attention of private observers has 

 been more particularly directed is that of double or multiple stars, or 

 those which, being situated very close to one another, appear single to 

 the naked eye, but when viewed through powerful telescopes are seen 

 to consist of two or more stars. The measuring the angles and dis- 

 tances from one another of the two or more component stars of these 

 systems has led to the discovery that many of these very close stars 

 are, in fact, acting as suns to one another, and revolving round their 

 common centre of gravity, each of them probably carrying with it a 

 whole system of planets and comets, and, perhaps, each carried for- 

 ward through space like our own sun. It became then a point of 

 great interest to determine whether bodies so far removed from us as 

 these systems observed Newton's law of gravity ; and to this end it was 

 necessary to observe the angles and distances of a great number of 

 these double stars, scattered everywhere through the heavens, for the 

 purpose of obtaining data to compute their orbits. This has been 

 done, and chiefly by private observers, and the result is that these 

 distant bodies are found to be obedient to the same laws that prevail 

 in our own system. 



" Of all the phenomena of the heavens, there are none which excite 

 more general interest than comets, and though the larger and brighter 

 comets naturally excite most general public interest, and are really val- 

 uable to astronomers, as exhibiting appearances which tend to throw light 

 on the internal structure of these bodies, and the nature of the forces 

 which must be in operation to produce the extraordinary phenomena 

 observed, yet some of the smaller telescopic comets are, perhaps, more 

 interesting in a physical point of view. Thus the six periodical comets, 

 the orbits of which have been determined with tolerable accuracy, and 

 w r hich return at stated intervals, are extremely useful, as being likely 

 to disclose the facts of which, but for them, we should possibly have 

 ever remained ignorant. Thus, for example, when the comet of Encke, 

 which performs its revolution in a period of a little more than three years, 

 was observed at each return, it disclosed the important and unexpected 

 fact that its motion was continually accelerated. At each successive 

 approach to the sun it arrives at its perihelion sooner and sooner ; and 

 there is no way of accounting for this so satisfactory as that of sup- 

 po^ing that the space in which the planetary and cometary motions are 

 performed is everywhere pervaded by a very rarefied atmosphere or 

 ether, so thin as to exercise no perceptible effect on the movements of 

 massive solid bodies, like the planets, but substantial enough to exert a 

 very important influence on more attenuated substances moving with 

 great velocity. The effect of the resistance of the ether is to retard the 

 tangential motion, and allow the attractive force of gravity to draw the 



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