2G2 ELEMENTS OF GENERAL SCIENCE 



When a comet approaches the sun its tail streams behind 

 like the smoke from a locomotive, but when it again recedes 

 from the sun the tail goes out ahead like the rays from the 

 headlight on a misty night. There is much speculation as 

 to the causes of the phenomena observed in comets. It is 

 supposed that there is some sort of an electrical repulsion, 



FIG. 130. Halley's comet 



Note that the tail of the comet extends and slowly disappears at -a great distance 



across the sky. The light spots are photographs of stars, which appear elongated 



due to changes of position while the picture was heing taken. Photograph hy the 



Yerkes Observatory 



and probably also pressure of the light radiated by the sun, 

 which drives off the minute particles of which the tails of the 

 comets are composed and makes them stream out ahead in 

 the direction opposite to the sun. 



265. Visible comets. While most comets are visible only 

 through a telescope, some occasionally appear in great 

 splendor. One of these, known as Halley's comet, returns 

 after periods of about seventy-five years. Its orbit was first 

 computed by Halley, who showed that the comet which 

 appeared in 1682 ought to return again about 1757. He 



