Life, Researches, and Discoveries of F. W. Bessel. 315 



standard of length : for which purpose a fac-simile of the toise 

 of Peru was chosen, being a measure aux bouts which the prin- 

 ciple of construction of the apparatus rendered a necessary con- 

 dition. The actual vibrating pendulum was a ball suspended by 

 a wire, the suspending apparatus being made to rest alternately 

 on the upper end of the toise and on the flat support of its lower 

 end, the tangent plane of the lower surface of the ball being 

 brought to a constant level by the use of the lever of contact. 

 The series of experiments made with this apparatus was publish- 

 ed in the volume of Memoirs of the Berlin Academy for 1826, 

 though the date of their communication and reading was two 

 years later, the publication of the Memoirs being so much in 

 arrear. This investigation will always be considered as forming 

 an epoch in the history of pendulum experiments, on account of 

 the peculiar mode in which the subject of the resistance of the 

 air to the motion of the suspended body is taken into considera- 

 tion ; this renders it necessary to estimate as part of the mass set 

 in motion the weight of the air dragged along with it. The re- 

 searches of Mr. Baily and Col. Sabine have fully confirmed the ne- 

 cessity of taking into account this essential though small correction. 



The determination of the same important element for the then 

 newly erected observatory at Berlin followed in 1835. The 

 method employed, with some slight improvements, was the same 

 as that practiced at Konigsberg, and the whole process will be 

 found in the Memoirs of the Berlin Academy for 1835. 



The interim between these two determinations, was occupied 

 with a series of pendulum experiments of very especial physical 

 interest and importance— a rigorous inquiry, namely, into the 

 fundamental question whether gravity be really, in all kinds of 

 bodies, proportional to their inertia solely ? or in other words, 

 whether or not there be any thing specific or dependent on the 

 mumate nature or chemical constitution of a body which deter- 

 mines the energy of its gravitating power * the tnertta being 



gi^en? The experiments of Newton, though they preclude all 

 idea of any considerable or palpable amount of such specific de- 

 ference among bodies, could by no means be regarded as sufli- 

 ciently exact to settle a point of such vast importance with tnat 

 decision which modern science requires. All idea of such spe- 

 c »nc attraction is, however, completely done away with Dy tne 

 pult of the elaborate series of experiments set on foot by m. 

 kssel for this purpose, which form the subject of a Memoir pre- 



J It is so easy to misunderstand the true gist of this "^^j "J* ^ jjj 

 J !«" to state it o, hervvise. Suppose apound of gold . a pound ot lead an . pound 

 l[ *• to be formed into three spheres, and placed with their cen es M IM • WJJ 

 an S'esof an eauilateral trian«U' — will they or will they no .taking two Dy tun 

 a »^ct eaZnZ ^ vvth eouaTforces? The larth is here centered a 9 an .mparUal 



~i.wt uiticr wiin equai lui^cs. 



* lure of all elementary substances. 



