Memoir of the late Francis Baily, Esq.^ F.R.S.^ S^c, 55 



in all theoretical views, though of course they must be nume- 

 rically amalgamated in the " reduction to a vacuum." 



Meanwhile the attention of Mr. Baily had, about the same 

 time, been called to the pendulum, in consequence of the con- 

 templated expedition about to sail under the command of 

 Captain Foster, on that memorable and most unfortunate ex- 

 pedition which cost him his life. It was on this occasion, and 

 with a view to the use of this expedition, that Mr. Baily (still 

 acting for the Astronomical Society, whose aid had been re- 

 quested in suggesting useful objects of inquiry) devised that 

 capital improvement in the system of itinerant pendulum ob- 

 servation, which consists in making each transferable pendu- 

 lum a convertible one, by the simple addition of another knife- 

 edge, and in doing away with extra apparatus of tail-pieces, 

 sliders, Sec, by the initial adjustments of the instrument. And 

 I may here incidentally remark, that the general principles of 

 reducing, as far as possible, the number of moveable parts in 

 every instrument intended for standard determinations of 

 whatever kind, is one which cannot be too strongly recom- 

 mended, and has been successfully acted on by the present 

 Astronomer Royal in more than one recent construction. 

 Two pendulums, a copper and an iron one, on Mr. Baily's 

 principle, were furnished by the Society for this expedition, an 

 account of which may be found in the Notices of the Society 

 for June 13, 1828. 



The adjustment and trial of these pendulums previous to the 

 sailing of the expedition, were performed by Mr. Baily at his 

 own house, and, thus engaged in actual experiment, he at 

 once became led on into a minute examination of all the pos- 

 sible sources of practical error in the experiments and conse- 

 quent uncertainty in the important results of which they had 

 become the basis. It was in this stage of his experience that 

 he became acquainted with Prof. Bessel's results, which deter- 

 mined him (as it had already done Captain Sabine) to go into 

 the whole subject of the new correction by experiments per- 

 formed in vacuo. But not content with assuming any fixed 

 proportionality between it and the buoyancy correction, he 

 resolved so to vary the form, magnitude, and materials of the 

 vibrating masses, as to make its true nature and amount an 

 object of inductive experimental inquiry : thus, though adopt- 

 ing the language of Buat and Bessel, disengaging himself in 

 effect from any theoretical view of the niodiis operaJuU or me- 

 chanical process by which the effect was produced. 



The result of these inquiries was a very elaborate and mas- 

 terly paper read to the Royal Society, on the 31st of May, 

 1832, containing the results of experiments in air and m vacuOf 



