194 POPULAR SCIENCE MONTHLY. 



But in general, we can not suppose that the centers of gravity in 

 the various stellar systems are really occupied by actual physical bodies. 

 The center may be a mere mathematical point in space, situated among 

 the several bodies composing the system, but nevertheless endowed 

 with the same remarkable property of relative immobility. 



Having thus defined the center of gravity in its relation to the 

 constituent parts of any cosmic system, we can pass easily to its char- 

 acteristic properties in connection with the inter-relation of stellar 

 systems with one another. It can be proved mathematically that our 

 solar system will pull upon distant stars just as though the sun and 

 all the planets were concentrated into one vast sphere having its center 

 in the center of gravity of the whole. It is this property of the center 

 of gravity which makes it preeminently important in cosmic researches. 

 For, while we know that center to be at rest relatively to all the planets 

 in the system, it may, nevertheless, in its quality as a sort of con- 

 centrated essence of them all, be moving swiftly through space under 

 the pull of distant stars. In that case, the attendant bodies will go 

 with it — but they will pursue their evolutions within the system, all 

 unconscious that the center of gravity is carrying them on a far wider 

 circuit. 



What is the nature of that circuit? This question has been for 

 many years the subject of earnest study by the clearest minds among 

 astronomers. The greatest difficulty in the way is the comparatively 

 brief period during which men have been able to make astronomical 

 observations of precision. Space and time are two conceptions that 

 transcend the powers of definition possessed by any man. But we can 

 at least form a notion of how vast is the extent of time, if we re- 

 member that the period covered by man's written records is registered 

 but as a single moment upon the great revolving dial of heaven's dome. 

 One hundred and fifty years have elapsed since James Bradley built the 

 foundations of sidereal astronomy upon his masterly series of star- 

 observations at the Boyal Observatory of Greenwich, in England. Yet 

 so slowly do the movements of the stars unroll themselves upon the 

 firmament, that even to this day no one of them has been seen by men 

 to trace out more than an infinitesimal fraction of its destined path 

 through the voids of space. 



Travelers upon a railroad can not tell at any given moment whether 

 they are moving in a straight line, or whether the train is turning upon 

 some curve of huge size. The St. Gothard railway has several so-called 

 'corkscrew' tunnels, within which the rails make a complete turn in 

 a spiral, the train finally emerging from the tunnel at a point almost 

 vertically over the entrance. In this way the train is lifted to a higher 

 level. Passengers are wont to amuse themselves while in these tunnels 

 by watching the needle of an ordinary pocket compass. This needle, 



