288 * Captain Hall’s details of experiments , &c. 
raised by means of a screw, so that the knife edge was lifted clear of the agate 
planes on which it vibrated during the experiments. This screw being too 
small, or having some flaw in it, unexpectedly broke at San Bias before the 
experiments there were begun ; and although the knife edge was not raised more 
than the twentieth of an inch, yet, as the pendulum weighed more than 15 lbs., 
the fall might, he thought, have altered the form of so delicate an edge in a 
slight degree, and thus have virtually lessened the distance between the point 
of suspension and the centre of oscillation.* 
As the whole pendulum had acquired a coating of oxide, with the exception 
of the tail piece, which was lackered, I was desirous of ascertaining in what 
manner and to what degree its vibrations would be affected by this partial addition 
of weight ; and for this purpose the following experiments were made. The 
vibrations of the pendulum in its oxydized state having been determined, 10 
grains of weight were affixed at f of the length of the bar (measured through the 
ball), from the point of support, that being supposed to be near the centre of 
oscillation of the oxide. This had for its object to discover, before cleaning the 
pendulum, what would be the effect of an addition of weight at that place. On 
swinging it accordingly, the number of vibrations was increased 0,83 in 24 hours. 
It was then taken to the Mint, and the weight, cai'efully determined by Mr. 
Barton in one of his delicate balances, was found to be 15 lb. 10 oz. 14 dwt. 
I2f grs. It was next cleaned by Captain Kater, by means of diluted sulphuric 
acid, and afterwards washed with a solution of soda in water, and being effec- 
tually dried, was again weighed, when it was found to have lost exactly 24f 
grains. Coincidences were now taken on three succeeding days, and the number 
of vibrations of the pendulum in its clean state proved to be fewer than when it 
was coated with oxide by only 0,73 of a vibration. Since no more than 3 - part of 
the oxide removed could be oxygen, only \ of the above difference between its 
vibrations when clean and when coated, or 0,14, can be ascribed to additional 
weight since it was formerly swung in 1820 ; the real difference, however, to be 
accounted for being 0,97, this cause is manifestly inadequate to the effect. I 
have therefore thought it right, after attentively considering every other possible 
manner in which the pendulum could have been altered, to adopt the idea which 
had been suggested, and which was eventually proved to be correct, since the 
knife edge, upon removal after the expei’iments were over, was found to be dis- 
tinctly 7 rounded. To obtain the most correct results, I have accordingly used the 
vibrations made in London in 1820, to compare with the experiments made be- 
fore the accident, and the vibrations recently determined in London for com- 
paring with those made after it; an arrangement rendering the resulting 
ellipticities entirely independent of that circumstance. 
* If the knife edge be supposed to have become cylindrical, the virtual point of suspension, 
it has been demonstrated, would be at the distance of the radius of curvature of this cylindrical 
portion below its surface, and the number of vibrations of course be greater than before. 
