Page:Proceedings of the Royal Society of London Vol 2.djvu/115

Rh and bones in a fossil state, creates a considerable degree of surprise; and by connecting the present animals with those that are extinct, adds a link to that chain of gradation which is the most interesting to the comparative anatomist and to the geologist.

It has long been a desideratum in science, to determine the precise length of a pendulum vibrating seconds in a given latitude. Most of those who have undertaken this inquiry have endeavoured to find the centre of oscillation; but as this depends upon the regular figure and uniform density of the body employed, it involves difficulties which may be considered as insurmountable. Despairing, therefore, of success in any attempt founded upon such principle, Captain Kater endeavoured to discover some other property of the pendulum less liable to objections; and was so fortunate as to perceive one which promised an unexceptionable result.

It is known that the centres of suspension and oscillation are reciprocal; or, in other words, if a body be suspended by its centre of oscillation, its former point of suspension then becomes the centre of oscillation, and the vibrations in both positions will be performed in equal times. Now as the distance of the centre of oscillation from the point of suspension depends upon the figure of the body employed, if the arrangement of its particles be changed, the place of the centre of oscillation will also suffer a change. Suppose, then, a body to be furnished with a point of suspension, and another point on which it may vibrate, to be fixed as nearly as can be estimated in the centre of oscillation, and in a line with the point of suspension and centre of gravity; if the vibrations in each position should not be equal in equal times, they may readily be made so, by shifting a moveable weight, with which the body is to be furnished, in a line between the centres of suspension and oscillation; when the distance between the two points about which the vibrations were performed, the length of a simple pendulum, and the time of its vibrations, will at once be known, uninfluenced by any irregularity of density or of figure. The mode of suspension which the author adopted was the knife-edge, of which the various advantages and disadvantages are pointed out, and the modes of overcoming the latter described.

The pendulum consisted of a thin bar of plate-brass, pierced with two triangular holes at the distance of 39·4 inches from each other; to admit the knife-edges, which were made of wootz, and finished to an angle of 120°, and firmly screwed to brass knee-pieces. The pendulum is prolonged at either extremity by a slip of deal, extending about twenty-two inches beyond the knife-edges. Three weights are employed for the adjustments. The great weight is immoveably fixed beyond the knife-edges; the second weight slides on the bar, Rh