198 PROGRESS OF SCIENCE IN THE CENTURY. 



perature rises and the nebula becomes more thor- 

 oughly gaseous, if it was not so at the start ; as the 

 outer parts cool they condense into the clouds of a 

 photosphere and the nebula becomes a sun; for a 

 time, as shrinkage increases, the temperature rises; 

 but the limits to this must be reached sooner or later 

 and the sun, passing the zenith of its splendour, grad- 

 ually sinks into dark coldness. 



" Fixed Starsy — One of the many instances of 

 the characteristic nineteenth century transition from 

 static to kinetic conceptions, may be found in the 

 hesitancy with which astronomers now speak of 

 " fixed stars/' In many cases it has been shown 

 that their positions relative to one another change in 

 the course of years, and the displacement, though ap- 

 parently very minute, indicates an enormous velocity 

 of movement. " Sirius drifts over the face of the sky 

 with such speed that in 1,400 years its position will 

 be removed from its present one by a distance that 

 would just be covered by the diameter of the full 

 moon. ... To do this it must travel athwart the 

 direction of vision with a speed of over ten miles per 

 second, more than one-half of that of the Earth in its 

 orbit; and this takes no account of any velocity the 

 star may possess in the direction of the line of vision, 

 a displacement in w^hich direction would obviously 

 not affect its position upon the face of the heavens." * 



Similarly, to take the most rapid known dis- 

 placement, a star in the Great Bear named Groom- 

 bridge, 1830, would move in 257 years over the 

 moon's diameter, and this at a distance of 2,300,000 

 times that of the sun implies a rate of 227 miles per 

 second. T^or should we forget here that the sun itself 

 is travelling in a line directed towards the star Vega, 

 * Fison, p. 46. 



