Astronomical and Nautical Collections. 



of the pendulum and an astronomical clock nearly in the manner 

 employed by Captain Kater, but with some little improvement in 

 the mode of determining the instants of perfect coincidence. They 

 were repeated in most cases with two different pendulums of the 

 same form, and the greatest care was taken to preserve the identity 

 of the circumstances in all the experiments ; it was even found that 

 the changing the pieces of agate, which supported the knife-edges, 

 affected one of the two pendulums a little differently from the 

 other ; and allowance was made for this diversity when required. 

 Perhaps, however, all such irregularities might have been more 

 easily avoided by using a blunt point on each side rather than a 

 knife-edge ; the knife-edge was required in Captain Kater's expe- 

 riments on the length of the pendulum, for the purpose of obtaining 

 a precise line from which the length could be measured ; but there 

 was no such necessity where the comparative frequency only of 

 the vibrations was concerned. 



With the most laudable zeal to omit no confirmation that could 

 be obtained for the accuracy of his results, Captain Sabine repeated 

 his experiments at all the stations, in a manner totally different, 

 with pendulums forming parts of actual clocks, but resting like 

 the others on knife-edges, instead of being suspended by springs. 

 The results agreed with those of the detached pendulum as nearly 

 as could possibly be wished, the irregularity never amounting to 

 more than two vibrations in a day for any one station ; and the 

 mean inference, with respect to the figure of the earth, being al- 

 most exactly the same for both series. 



The grand ultimate result of all the operations at the thirteen 

 stations, deduced by the modern method of computing the proba- 

 bility of least error, as well as from the most natural mode of 

 grouping the observations, affords us an ellipticity equal to j-^ 

 of the earth's-axis. 



Taking it for "granted that this is the true measure of the earth's 

 compression, we may proceed to inquire what are the inferences 

 from the determination, respecting the central and superficial den- 

 sity of the earth. It will then appear from Dr. Young's calcula- 

 tion of the effects of compression, inserted in this Journal, Vol. IX. 



