Observations made with the invariable pendulum (No. 4. Jones), at the royal observatory, cape of good hope, for the purpose of determining the compression of the earth

Abstract

Of the two methods employed for determining the figure of the earth, namely, the direct measurements of arcs of the meridian, or of ascertaining the variations in the length of the seconds pendulum in different places, the author remarks that the former is attended with the collateral benefit of fixing the geographical position of certain stations in the country surveyed; but the latter possesses the advantage of enabling the observer to concentrate, under his own immediate eye, the results of his inquiries. The Observatory at the Cape of Good Hope having been furnished by the Lords Commissioners of the Admiralty with the invariable pendulum of Jones, which had for several years been strictly examined by Capt. Sabine, the author was anxious to begin a series of experiments with it; and as it was not likely that the observatory would be completed for a considerable time, he caused a strong brick pier to be built in an adjoining outhouse for the support of a transit instrument, the same which he had used in forming his catalogue of southern stars. He gives a detailed account of his mode of fitting up the clock, and other parts of the apparatus necessary for the pendulum experiments. He was ably assisted by Capt. Ronald and Lieut. Johnson, who took an active part in all the observations. He remarks, that the near agreement of the three independent series of observations, made by himself and these two gentlemen, and which accompany the paper, is no small argument in favour of their accuracy. The difference in the number of vibrations of the seconds pendulum at the Cape, from that in London, in a mean solar day, he finds to be 67·12, from which it results that the compression of the earth is 1/288.5. The author is of opinion that the invariable pendulum ought to be a standard instrument in every observatory; that it should be swung at all seasons of the year, and occasionally transferred to various fixed observatories in both hemispheres, and returned again to its original station, where it should undergo a renewed and rigid examination before it is sent round on a fresh circuit of these stations.