CELESTIAL MEASURINGS AND WEIGHINGS. 207 



lation would land us) to our British standard ounce, of 

 which this our globe is equivalent to about 210 qiiad- 

 rillmis. * 



(32.) It is an elementary proposition in physical 

 astronomy that the time in which two masses so con- 

 nected into a system by their mutual attraction, revolve 

 about each other in ellii^tic orbits, depends only on the 

 su?n of their masses or weights, and on the length of the 

 elliptic relative orbit, and not at all on its breadth^ and is 

 therefore the same as if the orbit were circular, i.e., as 

 if the two masses were retained constantly at the same dis- 

 tance from each other, viz., that which we have called their 

 mean distance; and which mean distances, as we have seen 

 in the cases before us, are respectively in round numbers 

 fifteen and thirty times that of the sun from the earth. 



{;^,'}i^ It is an equally elementary conclusion from the 

 theory of gravitation, and was long since demonstrated 

 by Newton, that, so far as the time of revolution is con- 

 cerned, it is unimportant in what proportions the sum of 

 the masses or the entire ponderable matter of the system 

 is distributed between the two, the distance being un- 

 altered. That time, therefore, would remain unaltered, 



* Adopting that nomenclature which calls I followed by 6 ciphers 

 a million, by 12 a billion, by 18 a trillion, and by 24 a quadrillion. 

 For the weight of our globe in tons (5852 trillions), see Herschel's 

 "Physical Geography," 2d edit. sect. 5. The elastic forces with 

 which Mr Baile)', in his repetitions of the celebrated "' Cavendish 

 Experiment" (from which this estimate of the weight of our globe 

 is concluded) compared that weight, varied from less than one 

 29,000th part of a grain in some experiments to one 2500111 in 

 others ! The result, however, being corroborated in various ways, 

 is received without hesitation. 



