THE MESSENGERS OF THE GODS 273 



had in the meantime been discovered and its effect measured. 

 This is the comet which in all probability will return again 

 in 1910. 



Four others are now known which revolve in an elongated 

 ellipse like Halley's comet, with a period of between seventy 

 and eighty years ; about twenty-five are known with lesser 

 periods, and perhaps as many more with periods of over a 

 century. Altogether the paths of upwards of two hundred 

 comets have been plotted in whole or in part. So much, then, 

 is known, that they all obey the same laws as the planets ; 

 they are therefore material substances and under the domina- 

 tion of a simple mechanical law. So far from being a stumbling- 

 block towards the acceptance of the theory of attraction, they 

 have been, a help to show its universal application. Their 

 appearance is to-day no more of a mystery than the daily and 

 hourly flight of the earth through space. 



Whatever be4he substances which compose the comets, they 

 are in an exceedingly tenuous state. Their tails are often millions 

 upon millions of miles in length ; sometimes they reach clear 

 across the orbit of the earth. This implies necessarily an 

 enormous amount of matter of some kind ; nevertheless it is 

 so thinly distributed that very often on a clear night we may 

 look quite through these nebulous masses and see the stars 

 shining in their customary way. Even the slender flicker of 

 light from far-distant suns suffers little obstruction in passing 

 through them. 



Moreover, it is now known that the earth has twice shot 

 through a comet's tail ; so little disturbance did it create that 

 the fact was only made clear from calculations after the event. 

 Still again, the comet of 1843 swept so swiftly about the sun 

 that it had made the turn from one side to the other within a 

 couple of hours. In order to accomplish such a feat the speed 

 must have been something so enormous, that beside it the 

 earth, at nineteen miles per second, would seem to creep. And 

 this was simply for the head of the comet ; if the tail of the 

 comet, which was unusually long, followed the usual observed 

 actions of comets' tails and pointed away from the sun through- 

 out the curve described by the head, the rate at which its 

 particles moved would have been simply inconceivable. 



The obvious inference, therefore, is that the tail is not a 



s 



