148 Captain Sabine On the 



edges, one in the usual position of the centre of suspension, 

 the other near the lens of the pendulum : the vibrations of the 

 pendulum, when suspended from these two distant points, are 

 rendered isochronous, by a change in the position of a small 

 weight that slides along the pendulum-rod ; the point near the 

 lens is thus rendered the centre of oscillation, in consequence 

 of a property of the pendulum discovered by Huygens, who de- 

 monstrated that the centres of oscillation and suspension were 

 convertible points. The distance between the knife edges may 

 "be measured with great accuracy by means of microscopes 

 attached to accurate scales, giving thus the true length of the 

 experimental pendulum : the number of oscillations it performs 

 in any given time may be ascertained by comparison with the 

 pendulum of a well-regulated clock ; and hence the length of the 

 pendulum vibrating seconds at the place of experiment may be 

 .determined, by applying the well-known proposition that the 

 lengths of pendulums are inversely as the squares of the num- 

 bers of their respective vibrations in equal times. After the 

 length of the seconds pendulum has been thus determined in any 

 -one place, by experiments sufficiently multiplied to ensure against 

 any probable error, another pendulum of similar shape to the 

 first, with the exception of its having no moveable weight, and 

 but one knife edge, which is situated at the usual centre of sus- 

 pension, is employed. This pendulum may be hung up in front 

 of the clock, with which the original experiment was made, or of 

 some other whose rate is known, and which is placed in the 

 same apartment : its rate of oscillation may be thus known, and 

 its length calculated, upon the same principle as that which we 

 have stated as the mode in which the length of the seconds 

 pendulum was originally determined. The length of this last- 

 mentioned experimental pendulum being thus ascertained, and 

 with an accuracy equal to that of the fundamental experiment, it 

 may be carried from station to station ; the number of its vibra- 

 tions, in a given time, as shewn at each station by comparison 

 with an astronomical clock, will furnish data whence the length 

 of the pendulum, vibrating seconds at that place, may be calcu- 

 lated. This method is undoubtedly the best that has hitherto 

 heen proposed, and we are not prepared to say that it is sus- 

 ceptible of any material improvement in the theoretic part ; the 

 manner o£ construction or even of using the instrument may 

 perhaps undergo change, {the latter has undergone a very im- 

 portant change since its first employment by Captain Kater, in 

 the improved method suggested by Captain Sabine, and adopted 

 fry him, of observing the coincidences,) but we cannot fairly 

 anticipate that any principle more beautiful, or more readily 

 reduced to practice, is likely to be discovered. 



