38 FROM NEBULA TO NEBULA 



such a cable, and ask yourself whether it seems to you 

 reasonable that the unsupported moon, without cmy influx 

 of energy whatsoever from without, can, unenfeebled, 

 hold her own against it forever, as astronomers declare 

 she can? To the casual observer, the moon looks like a 

 silver wafer pasted upon the sky ; in reality it is a globe 

 of earthy matter 2160 miles in diameter (as far as from 

 New York to Santa Fe) with a mass nearly one-eightieth 

 that of the earth itself. So far as we know or have rea- 

 son to believe, it has remained exactly at its present mean 

 height for at least 3000 years, and possibly has been 

 there for as many hundreds of millions all the time 

 pulling against a strain of 240 trillions of untiring 

 horses ! The moon has no pillar to support it, no pulley 

 to suspend it, no atmosphere to buoy it, no breeze to waft 

 it, no screw to propel it, no engine to haul it, yet still it 

 stays up. Why? Whence comes the centrifugal force 

 to keep driving it away from the earth with identically 

 the same energy that the latter perpetually prompts it to 

 fall ? Where is the equivalent of that flinging motion of 

 your hand that supplied the centrifugal force to the little 

 metal ball of our experiment? To this query astron- 

 omers with one accord reply: "The moon's innate 

 momentum constitutes this sustaining power, " and then 

 they immediately fall mute. Let us see what merit lies 

 in this answer : 



To go back to our elastic string and metal ball ; sup- 

 pose you hold tight one end of the cord, while I pull the 

 ball away from you until we both sense the tension on the 

 string. Here your hand, say, represents the earth, the 

 tensed string the gravitational attraction, and the ball 

 the moon. Suppose, now, that I should let go the ball, 

 what will become of it? Will it fly toward your hand, or 

 will it, of itself, take on a tangential movement and re- 

 volve around you in a circle? Of course it will fly to 

 your hand! To do otherwise, we all should agree, were 

 unnatural. When astronomers, therefore, pretend that 

 the moon has a spontaneous tangential motion, they 

 know as well as we that such a motion is physically impos- 

 sible under the present order of nature, and that they 



