54< Memoir of the late Francis Baily, Esq.) F.R.S., 6fc. 



bodily dragged along by the pendulum in its motion, thus ad- 

 ding to its inertia without adding to its relative weight when 

 corrected for buoyancy ; and in this view, also, Mr. Baily re- 

 garded it. That this is not a complete and adequate view of 

 the subject is easily made a matter of ocular inspection, by 

 causing a pendulum to vibrate, or any body to move, near the 

 flame of a candle, when it will be at once evident that the 

 movement of the air consists in the continual transfer of a por- 

 tion of air from the front to the rear of the body, by perform- 

 ing a circuit half round it. Its hydrodynamical investigation, 

 therefore, is of an infinitely higher order of difficulty than the 

 ordinary problems of resistance, which turn upon a theory of 

 molecular impulse, simple indeed, but very far from satisfac- 

 tory. It properly refers itself to the theory of sound, and has, 

 in fact, been so investigated in an admirable memoir by Pois- 

 son*. 



But to return from this digression (which, however, will 

 not have been without its use, if it shall tend to diffuse clear 

 conceptions of the subject, and to disentangle from one another 

 corrections which seem to have got unduly mixed up together 

 in the minds of practical inquirers). No sooner were the ideas 

 of M. Bessel promulgated in England, than Captain Sabine, 

 whose attention was pointedly directed to a subject which had 

 occupied so large and active a portion of his life, resolved to 

 ascertain the true amount of this new, or newly mentioned, 

 correction, in the only way in which it could be effectually 

 done, viz. by vibrating the pendulum m vacuo^ which he ac- 

 cordingly effected by a series of highly interesting experiments, 

 carried on at the Royal Observatory at Greenwich, and re- 

 corded in the Philosophical Transactions, in a paper read 

 March 12, 1829. His result makes the total reduction to a 

 vacuum about one and two'thirds of that usually called " the 

 correction for buoyancy." It should, however, be borne care- 

 fully in mind that the particular correction now in question 

 has, in fact, nothing whatever to do with the buoyancy correc- 

 tion, either in its mode of production or its form of expression, 

 and ought, therefore, to be very studiously kept apart from it 



' * If this view of tlie subject be correct, as I am persuaded it is, it seems 

 not impossible that, by making a section of the pendulum coincident in form 

 with the " wave-formed outline " of Mr. Russel's ships, the resistance cor. 

 rection might be annihilated altogether, or so nearly as to render it quite 

 inappreciable. 



I trust that, in what is said above, I shall not be supposed to undervalue 

 M. Bessel's analytical treatment of this intricate problem, especially as it 

 conducts to results which, regarded as a first approximation, represent suf- 

 ficiently well the results of experience. 



