128 A CENTURY'S PROGRESS IN ASTRONOMY. 



class of celestial bodies, distinguished as comets 

 of short period." 



In 1825 the comet was again observed by Valz, 

 passing perihelion on September 16, and in 1828 

 it was seen by Struve. Encke now made a very 

 remarkable discovery. Determining its period 

 with great accuracy, in 1832 he found that his 

 comet returned to perihelion two and a half 

 hours before the predicted time. As this re- 

 peatedly happened, Encke put forward the theory 

 that the acceleration was due to the existence 

 of a resisting medium in the neighbourhood of 

 the Sun, too rarefied to retard the planetary 

 motions, but quite dense enough to make the 

 comet's path smaller, and to eventually precipi- 

 tate it on the Sun. The theory was widely 

 accepted, but after 1868 the acceleration began 

 to decrease, diminishing by one-half; besides, 

 no other comet is thus accelerated, and the 

 hypothesis has accordingly been abandoned. 



The second comet recognised as periodic was 

 that discovered on February 27, 1826, by an 

 Austrian officer, Wilhelm von Biela (1782-1856), 

 and ten days later by the French observer, 

 Gambart (1800-1836), both of whom, in com- 

 puting its orbit, noticed a remarkable similarity 

 to the orbits of comets which appeared in 1772 

 and 1805. Accordingly, they concluded it to 



