THE YARD, PENDULUM, AND METRE. 429 



that of an ideal seconds-pendulm supposed to be placed 

 at the extremity of the earth's polar axis. To this is 

 in effect equivalent, and derivable from it, as a mere 

 arithmetical conclusion, the space fallen through by a 

 heavy body on the same place by the earth's attraction 

 in a second of time. The modulus so obtained is there- 

 fore a measure of the earth's total attractive power (in- 

 dependent of centrifugal force arising from its rotation), 

 as that derived from the length of its diameter is of its 

 total bulk, and equally unalterable and universal. As 

 for the other two which depend on the nature of light, 

 the difficulty and delicacy of the processes they would 

 involve render all idea of resorting to either of them 

 purely visionary. 



(10.) The linear dimensions of the earth then, on the 

 one hand, and the linear measure of its attractive force 

 embodied in the pendulum on the other, are the two, 

 and, so far as we can see, the only two available sources 

 of the invariable and universal standard length which 

 we seek. And it is curious to observe that while the 

 French after considering both of them threw aside the 

 pendulum in favour of the metre (or ten millionth of the 

 meridian quadrant); the English on the other hand, by 

 the Act of Parliament in 1824, which repealed the old 

 statute already alluded to (and so threw aside the 

 principle of resorting to an organic type) did in effect, 

 at that time, adopt the pendulum as their ultimate resort. 

 For whije that act declares that a certain metallic bar 

 made by Bird in 1760 when at the temperature of 62 % 

 Fahr. should, without any further reference to its origin, 



