Page:Proceedings of the Royal Society of London Vol 2.djvu/116

Rh near the knife-edge, at the opposite end, and may be fixed at pleasure ; the third weight is a small slider, intended to move near the centre of the bar, upon which are engraved divisions of one twentieth of an inch, seen through an opening in the slider. The support of the pendulum consisted of agate planes bedded in bell-metal.

In proceeding to the details of the experiments, the author acknowledges his obligations to Henry Browne, Esq. F.R.S., who permitted him to use his house in Portland-place, and his excellent clocks, for the purposes of the investigation. The greatest daily variation of the clock used as a standard of comparison did not exceed three tenths of a second between the months of February and July.

By the method of coincidences which Captain Kater employed, the number of vibrations made by the pendulum in twenty-four hours might be obtained in the space of eight minutes to within half a second of the truth ; and the usual correction was applied for the extent of the arc of vibration.

The pendulum being suspended with the great weight above, the number of vibrations in twenty-four hours was determined ; and if it differed when the pendulum was inverted, it was equalized by moving the second weight, and finally adjusted by the slider, every allowance being made for the temperature, and the height of the barometer being noted. Thus the number of vibrations in twenty- four hours, of a pendulum equal in length to the distance between the knife-edges at a given temperature and barometrical height, was ascertained.

The next sections of Captain Rater's communication refer to the apparatus and methods employed for the measurement of the distance between the knife-edges ; for the comparison of the British standard measures of the highest authority ; and to the expansion of the pendulum, which was found to be .00000996 of its length for each degree of Fahrenheit's thermometer.

After describing the methods of deducing the length of the pendulum vibrating seconds, and the corrections for the buoyancy of the atmosphere, the author makes it appear, that the distance of the knife-edges, at the temperature of 62 Fahr., by the mean of three several sets of measurements, the greatest difference between any two of which did not amount to T-r.W-o-th of an inch, was, upon Sir George Shuckburgh's scale, 39'44085 inches. From a table inserted in this paper of twelve sets of experiments, each set consisting of four, from which, and from the preceding measurements, the length of the seconds' pendulum in vacua is calculated, it appears that seven of these sets are within T ^Vmrth of an inch of the mean result ; two a little exceeding -ra-.-o-o-o-th of an inch ; and of the remaining three, the greatest difference is less than -ro-.-S-o-Tj-th of an inch ; so that the mean result must, it is presumed, be very near the truth.

To the length thus found, the, author next applies a correction for the height of the place of observation above the level of the sea. The advantages of his different methods are then explained ; and the conclusion of the whole is, that the length of the pendulum vibrating