118 MOTION, LIGHT, AND HEAT. 



thread which the little spider spins from stubble 

 to stubble in the autumnal field ; and it is as silent 

 as if the mighty careering mass were in a state of 

 perfect repose. 



What effect the rapid motions of the earth may 

 have upon light and heat is quite another matter ; 

 but it is a matter so exquisitely nice and delicate 

 that it will not come at all within the range of our 

 observation. If the earth will not pause in its 

 path round the sun until we can find out the 

 general influence which its motions have upon the 

 creatures on its surface, and their phenomena, 

 much less can we hope to question the march of the 

 sun-beams, which actually speed in about twenty- 

 four seconds of time, over a distance equal in length 

 to the earth's annual path. Instead, therefore, of 

 being able to question light so as to know whether 

 it be substance, or merely motion, we cannot divide 

 time so minutely as to take the slightest note of 

 the duration that it requires to pass from one pole 

 of the earth to the other ; and before we can think 

 of the gleam that shoots past us it is millions of 

 miles into the regions of space, shedding its be- 

 nignant influence upon other and distant worlds. 

 The matter of the earth and also that of the atmo- 

 sphere, moving so rapidly as they do, and in the 

 direction across the path of the sunbeam, must 

 produce an effect different from that which would 

 result if the earth were at rest and the beam only 

 in motion ; but as we know nothing of either of 

 the two we cannot compare them or state what 

 phenomena of nature result from the compound 

 motion. There is no doubt, however, that the 

 action is greater than it would be if there were 

 only one motion, because we find that to be a 



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