HISTOBY OF PHYSICAL ASTBONOMY.
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Indian Ocean , almost under the equator; by Hall*, at various places in thePacific Ocean; by Sir Thomas Brisbanef, at New South Wales ; by Foster J,
; at Port Bowen, in the North Seas; by Fallows §, at the Cape of Good Hope ;land by Leutkej|, an officer in the Russian navy, at several places in bothhemispheres. In 1826, Bessel determined the length of the seconds’ pendu-lum with great accuracy by a new method. Having fixed two points of sus-pension in a vertical position with respect to each other, he suspended fromeach of them a spherical ball by a fine wire. The distance between the twopoints of suspension was entirely arbitrary. Bessel made it exactly equal tothe Peruvian toise. The two pendulums when in a vertical position wereequally depressed, and therefore the difference of their lengths wasmeasured by the distance between the points of suspension. The depres-sion of the lowest parts of the surfaces of the balls to the same level waseffected by means of a delicate apparatus, termed the lever of contact.From an approximate knowledge of the lengths of the pendulums, it waseasy to derive the correction due in each case to the spherical form of theball. Hence, the difference of the lengths and the difference of the cor-rections being given, the difference of the lengths of the simple pendulumscorresponding to the two points of suspension was readily assignable.Again, by counting the number of oscillations performed in a given timeby each pendulum, the ratio of the lengths of the simple pendulums wasalso ascertained. By means of these two data, Bessel was enabled to com-pute the absolute length of each of the simple pendulums. This methodof determining the length of the seconds’ pendulum is generally consideredto be susceptible of greater precision than any other which has beendevised for the same purpose.
While Bessel was engaged in the experiments just referred to, he dis-covered that the usual correction for reducing the oscillations of a pendulumto a vacuum were erroneous to a considerable extent. This important factwas first announced by him in a memoir which appeared in the volume ofthe Berlin Academy of Sciences for 1826. He established its realitybeyond dispute; first, by swinging two pendulums in air, one of brass andthe other of ivory, and then by swinging the same brass pendulum in airand in water. It appeared from both experiments that the retardation ofa pendulum oscillating in a fluid medium was greater than had beenhitherto imagined. In the one case he found that the true correction forreducing the oscillations to a vacuum exceeded the correction usually em-ployed in the proportion of 1.956 to 1; and in the other case, in the pro-portion of 1.625 to 1. He attempted to account for this increase in theamount of retardation by ascribing it to the influence of a coating of air,which, according to him, adheres to the pendulum, and is dragged alongwith it during its motion. The mass of the pendulum being enlarged bythis addition, while the moving power remains the same, it is clear thatthe accelerative force of each particle will be less intense than it -would bein a vacuum, and that a diminution of motion will ensue as a necessaryconsequence. It is singular that, as early as the year 1780, Du Buat, aFrench experimentalist, recognised the existence of this retardation, andeven endeavoured to give an explanation of its physical origin; but itappears to have entirely escaped the notice of all philosophers until theperiod of its rediscovery by Bessel.
* Phil, Trans. 1823. 1 Ibid. 1823. J Ibid. 1826. § Ibid. 1830.
|| Mem. Acad, des Sciences de St. Petersburg; see also Phil. Mag. 1832.