ADVANCE OF ASTRONOMY. 187 



induced bj the encounters amid the swarms of me- 

 teoric stones. 



It is impressive to read how the comet of 1811 

 was assigned an orbit requiring 3065 years for its 

 completion, such that " when it last visited our 

 neighbourhood, Achilles may have gazed on its im- 

 posing train as he lay on the sands all night bewail- 

 ing the loss of Patroclus; and when it returns, it 

 will perhaps be to shine upon the ruins of empires 

 and civilisations still deep buried among the secrets 

 of the coming time." * It is impressive to note the 

 measurements of some of the great comets whose 

 highly rarefied emanations or " tails " may extend 

 for several millions of miles, but the behaviour of 

 the tail points to the conclusion that it is but " a 

 stream of matter driven off from the comet in some 

 way by the action of the sun,'' and the density must 

 be small indeed, since the earth has passed through 

 a tail at least twice in the century without the fact 

 being known until afterwards. Indeed the progress 

 of knowledge has robbed comets of some of their 

 dignity, for since the middle of the century it has 

 been generally recognised that, with the possible ex- 

 ception of the bright central " nucleus,'' a comet is 

 small in mass, and in a state of great tenuity, unable 

 to affect the motion of the planet it approaches, and 

 allowing the light of a star to pass even through its 

 " head." 



N'umerous interesting observations point to some 

 close connection between comets and meteors or 

 " shooting stars." Thus Biele's comet (with a period 

 of sixty-seven years), which scared the popular im- 

 agination in 1832, was first seen to become double, 

 and was afterwards lost altogether, while on two sub- 

 *A. M. Gierke, p. 131. 



