276 THE WORLD MACHINE 



The swarm is enormous in extent, almost countless in 

 number. The display lasts for four or five hours ; this means 

 that the earth, shooting along at 70,000 miles per hour, requires 

 this length of time to cover it from side to side. Its velocity 

 of motion has been calculated ; it comes out, at least in the 

 earth's atmosphere, at twenty-five miles a second ; yet it takes 

 nearly three years for the whole swarm to pass a given point. 

 So, in a rough way, we may compute its size. The breadth of 

 the swarm is greater than the distance from the earth to the 

 moon ; if its speed in space is the same as near the earth, its 

 length is many times the distance of the earth to the sun. 

 Perhaps if we conceive a shoal of fish, some large, some small, 

 some not so big as tadpoles, rushing along at twenty miles per 

 second a shoal perhaps several hundreds of millions of miles 

 in length we may form some picture of how they might look, 

 could we see them as a whole. We cross the orbit of this swarm 

 every year, yet it is so tilted to that of the earth that it is only 

 once in thirty- three years that we encounter this enormous mass. 



When the orbit of the Leonids was laid down in a diagram, 

 it was found to bear a close resemblance, in its general form, 

 to that of comets like Halley's, which move in a very long 

 ellipse. The instant conclusion was that there is an intimate 

 relation between these meteoric swarms and the comets. This 

 became a practical certainty when the paths of four different 

 swarms were computed and found to agree closely with the 

 paths of known comets. 



One of these comets had an extraordinary history. This 

 was Biela's, a carefully observed telescopic apparition, making 

 its round of the sun once in six years. It was duly observed 

 on several successive returns ; but along about 1846 it was 

 calculated that, at its next revolution, it would come very near 

 to Jupiter. Naturally there was a great deal of wonderment 

 as to what would happen to Jupiter, or the comet. It was the 

 idea that we might see, as on a stage, the representation of 

 what would happen, in case, as the old-time terror had so long 

 anticipated, a comet should strike the earth. 



When it came again, it was seen that Jupiter was not de- 

 stroyed ; but the comet itself seemed to have been cut in two. 

 Both portions of the split-up comet appeared again in 1852 ; 

 but in 1858 it was nowhere to be found. The astronomical 

 authorities duly sent out notices that kindly disposed individuals 



