HISTOEY OF PHYSICAL ASTRONOMY.

] 56

The researches of the astronomer just mentioned, being mixed up withtheoretical views which were not clearly established, the Board of Longi-tude of this country resolved to submit the question to the issue of directexperiment, by swinging the same pendulum both in air and in a vacuum.The operations for this purpose were executed with great care by ColonelSabine, at the Royal Observatory of Greenwich , in the year 1828. Themean of his results gave 10.36 as the excess of the number of oscillationsin a mean solar day, made by a pendulum swinging in vacuo over thecorresponding number of oscillations made by the same pendulum swingingin air. The old correction for the buoyancy of the atmosphere gave only6.26 oscillations. It followed then that the coefficient for the reduc-tion to a vacuum should be increased in the proportion of 1.655 to I*.Mr. Baily shortly afterwards repeated these experiments with a greatvariety of pendulums and arrived at results of a similar character j% Theamount of retardation differed with the pendulum employed in theexperiment, and was found to depend mainly on its form and extent ofsurface. It appeared also that the air was dragged chiefly by the parts ofthe surface which were perpendicular to the plane of motion, and that onlya small quantity adhered to the sides. Mr. Baily clearly shev'ed that thetrue correction can only be determined with precision by means of directexperiments with each particular pendulum j.

The usual correction for reducing the oscillations of a pendulum to avacuum having thus been demonstrated, by evidence of the most indis-putable character, to be too small, it followed that all the results which hadbeen derived from experiments hitherto performed were more or lesserroneous. When the question relates to the determination of the figureof the earth, the error depending on the imperfect reduction is indeedsmall; since in this case the variation of gravity at the surface of theterrestrial spheroid, and not its absolute intensity, forms the subject ofresearch; but, as in delicate inquiries of this nature the most minutecircumstance must be taken into account, the true correction has beenemployed in all subsequent reductions.

The most extensive series of experiments with the pendulum whichhave been made by any individual are due to the late Captain Foster, whocommanded a scientific expedition which was sent out to the South Seasby the Government of this country in the year 1829. This talentedofficer having unfortunately been drowned a little before the time appointedfor the return of the expedition, his papers, upon being brought toEngland, were put into the hands of the late Mr. Baily, who drew up adetailed account of them, which is contained in the seventh volume of theMemoirs of the Astronomical Society. The experiments were made at four-

* Phil. Trans. 1829. + Phil. Trans. 1832.

I Poisson has adopted a different opinion respecting the physical origin of thiscorrection. He considers that the successive impulses of the pendulum would give riseto alternate contractions and expansions of the surrounding medium, whence the particlesof the latter, by their reaction, could not fail to produce a sensible effect on the rate ofoscillation. The correction is thus made to depend on the resistance offered to the pendu-lum by the aerial medium, the mode of action being considered in its most absolutecharacter, and independently of any assumed relation between the resistance and thevelocity. Proceeding upon this principle, Poisson has investigated the correction by aprofound analysis, and has arrived at results which, in some instances, are confirmed bydirect experiment. See Mem. de I'lnstitut, tome xi., or the Connaissance des Temps for1834. This view of the subject has also been adopted by Plana, Sir John Herschel , andseveral other eminent philosophers.