454 EDITOR'S NOTES. 



But before such a combination as is here made can 

 be safely employed in the deduction of the ellipticity 

 of the whole quadrant, and still more of separate por- 

 tions of the quadrant, it is of the first importance to 

 be assured of the strict intercomparability of results 

 obtained in the different parts of the hemisphere by dif- 

 ferent methods and by very dissimilar apparatus. 



The pendulum experiments of the French philosophers 

 in the middle latitudes were made, as is well known, by 

 the method and with the apparatus (slightly modified) 

 of Borda. Mine, both in the neighbourhood of the 

 equator and in the high latitudes, with the very different 

 apparatus known as the Invariable Pendulum of Kater. 

 The object which is sought by the experiment is also 

 different in the two cases. With the apparatus of Borda, 

 the object sought, and the result obtained, is the absolute 

 length of the seconds pendulum at the place at which 

 the experiment is made. The object sought, and the 

 result obtained, by the invariable pendulum is a far 

 more simple one, viz. the acceleration of the pendulum 

 in the different latitudes to which the pendulum is suc- 

 cessively conveyed. The acceleration is a function of 

 the ellipticity ; and the ellipticity is deduced from the 

 acceleration, quite independently of the absolute length 

 of the seconds pendulum at any one of the stations 

 of experiment ; which length is neither determined 

 nor determinable by means of the invariable pendu- 

 lum, nor does it need to have been determined at any 

 station whatsoever. To bring, consequently, a series 

 of experiments with an invariable pendulum (which 

 furnish simply the acceleration, or the difference in 

 the number of vibrations which a pendulum media- 



