286 PHYSICAL SCIENCE BK. vn 



certain than the limits of the space which they may 

 not exceed. 



XIII 



1 IN reply to arguments like mine it is urged by 

 Artemidorus that the five planets are not the only 

 stars with erratic courses, but merely the only ones 

 of the class that have been observed. But in- 

 numerable others revolve in secret, unknown to us 

 either by reason of the faintness of their light, or 

 the situation of their orbit being such that they 

 become visible only when they reach its extremities. 



2 It is thus, he says, that certain new stars enter our 

 field of vision, mingling their light with the fixed 

 stars, but displaying a brightness greater than is 

 usual in stars. This is the least serious of his lies : 

 his account of the universe is from end to end a 

 shameless tissue of lies. For instance, if we are to 

 believe him, the upper regions of heaven are 

 perfectly solid a lofty thick vault, as hard as the 

 roof of a house, formed by the accumulation of 

 masses of atoms. The surface immediately above 

 it is of fire so compact that it cannot be broken up 



3 or altered. Nevertheless, it has certain ventilators, 

 and, as it were, windows through which portions of 

 the fire stream from the outer part of the universe, 

 but not so large as to cause commotion in the inner ; 

 and again the fires pass from the world back into 

 the outer spaces. These extraordinary appearances, 

 therefore, Artemidorus supposes, have streamed in 

 from that mass of matter which lies outside the 

 world. To set about disproving such a theory is 

 nothing short of beating the air for the sake of 

 exercising the muscles ! 



