of minute Forces on the Rate of the Seconds Pendulum, 24-9 



commenced by putting them in motion so as to vibrate exactly 

 with one another. This method was considered not only more 

 simple than that of coincidences, but fully as accurate. It is 

 also more suitable for an experimenter who cannot conveni- 

 ently have an assistant at all times ; nor indeed would the me- 

 thod of coincidences have answered so well for the various ex- 

 periments which I had in view. 



Though the preceding experiments occupied upwards of 12 

 months, during all my spare hours from business, either in 

 making the necessary apparatus or the experiments themselves, 

 yet considerable practical advantage was thereby obtained. 



The object of the following experiments was to ascertain 

 the difference in time between the vibrations of a pendulum 

 in air and in vacuo, &c. For the purpose of making experi- 

 ments in vacuo, or in an exhausted vessel, the following ap- 

 paratus was constructed. — An iron vessel was made, having 

 circular apertures about 6 inches in diameter, near the bottom, 

 at a proper distance for observing the extremity of the pen- 

 dulum rod. These apertures were glazed with strong plate- 

 glass, so as to be capable of bearing the great pressure con- 

 sequent on a high degree of rarefaction. The vessel was placed 

 close in front of the clock-case, which had sides made to with- 

 draw, so as to admit more readily of the adjustment of the 

 pendulum, &c. There was likewise in the vessel a contrivance 

 for putting the pendulum in motion at any degree of rarefac- 

 tion or exhaustion. 



In his experiments on the pendulum, Captain Kater appears 

 to consider that the buoyancy of the air causes the vibrations 

 of a pendulum to be slower. " Thus," says he, " the specific 

 gravity of water compared with that of air, may be known for 

 the temperature and altitude of the barometer at the time of 

 observation ; and multiplying this by the specific gravity of 

 the pendulum, the ratio of the weight of the pendulum com- 

 pared with that of air will be obtained. This ratio will ex- 

 press the diminution of the force of gravity arising from the 

 buoyancy of the atmosphere : and in order that the number 

 of vibrations may be the same in vacuo as in air, the length of 

 the pendulum must be increased in the proportion of this 

 ratio to 1, the lengths of pendulums vibrating in the same time, 

 varying directly as the force of gravity." 



Buoyancy is here considered equivalent to a diminution of 

 gravity, and proportional to the deduction of the weight of the 

 pendulum's bulk of air from its own weight; consequently, 

 it should appear as if a light pendulum would vibrate more 

 slowly than a heavy one. Not being of this opinion, I made 

 the following experiment to put it to the test. I mounted a 



Third Series. Vol. 2. No. 10. April 1833. 2 K 



