experiments made with an invariable pendulum. 5 
cessful trials, as it might have saved myself and others, much 
unnecessary labour. 
The clock used in these experiments was fitted with a 
gridiron pendulum, vibrating on knife edges in portions of 
hollow cylinders of agate, and belonged to the Royal Society. 
It was put in motion at Greenwich on the 17th of April, 
1824, three days previous to the commencement of the expe- 
riment, and its rate ascertained by comparisons with the 
transit clock of the observatory each day at noon, and also 
during the series, at the commencement and at the conclu- 
sion. In these essential observations, I was kindly assisted 
by Mr. T. Taylor, jun. of the Royal Observatory. 
In making the observation of the coincidences, the follow- 
ing mode was pursued. 
The pendulum being placed in the Y’s, was gently lowered 
until the knife edges rested on the agate planes ; and the 
sides of the diaphragm placed in the focus of the eye-piece 
of the small telescope, were made just to coincide with, or 
embrace those of the tail-piece of the pendulum ; and this 
adjustment was examined previous to every observation. 
The heights of the barometer, and of the thermometer sus- 
pended with its bulb about \ of the length of the pendulum 
below its point of suspension, and about J of an inch in front 
of the middle of the bar, were taken and registered at the 
beginning and end of each set of observations. The pendu- 
lum was set in motion, by drawing it gently on one side with 
a piece of twine fastened to one of the legs of its support, 
until the point at the end of the tail-piece, was about i°,2 
upon the arc ; and a little before the pendulum of the clock 
attained its highest ascent on that side, the twine was let go, 
and the pendulum allowed to vibrate freely. 
