438 THE YARD, PENDULUM, AND METRE. 



moon's. Now the mass of the moon is about one 88th 

 part of that of the earth, so that one 88th part of the force 

 that draws them together is due to the moon. By so 

 much then must the space fallen through be diminished, 

 to get that due to the earilis alone. Suppose, now, that 

 the moon's mass assumed should be in error by a 50th 

 part of its whole amount (and Laplace's estimate of it 

 differs by as much from that at present received) and 

 we shall find ourselves landed, from this cause of uncer- 

 tainty alone, in an error to the extent of nearly one 

 4000th of the quantity sought. 



(t8.) Lastly, our knowledge of the moon's mass is 

 mainly derived from its effect in producing the phceno- 

 menon of nutation, which it does through the medium of 

 the earth's ellipticity, so that not only the dimensions, 

 but the figure of the earth are thus mixed up in our 

 attempt to derive the length of the normal pendulum from 

 the moon's motion. 



(19.) I cannot but consider then that the uncertainty 

 of the one mode of obtaining the length of the normal 

 pendulum, and the non-independence of the other^ unfit 

 it for being received as the ultimate scientific basis of a 

 universal standard ; whatever merit it may possess in an 

 abstract and metaphysical point of view and that the 

 true and only practical use of the pendulum in relation 

 to such a standard is the ready, cheap, and perfectly un- 

 objectionable means its measurement, at a determinate 

 spot and under defined circumstances, affords of recover- 

 ing it when lost, by the recorded statement of its length 

 in terms of such standard. 



