2 Lieutenant Foster's account of 
to entrust me with an invariable pendulum ; and the details 
of the observations made with this instrument, together with 
a statement of all the attendant circumstances, are given in 
the following pages. 
The first set of experiments, which are marked (No. I.), 
were made at the Royal Observatory at Greenwich, in an 
apartment to the S. W. of the Transit Room, originally in- 
tended, I believe, for the observations of the eclipses of 
Jupiter's satellites, but upon this occasion kindly appropri- 
ated by Mr. Pond to my use. This room has a solid stone 
floor, on which the triangular supports for the pendulum and 
clock were placed. The roof is low, and being composed of 
wooden panels, the temperature of the room was materially 
affected by the state of the weather ; on one occasion the 
thermometer ranged four degrees during the observations, 
although the light was admitted by a window on the north 
side. 
In the adjustments of the instruments employed in the 
experiments, I strictly adhered to the mode described by 
Captain Kater, in his paper read before the Royal Society in 
June, 1819. The intervals between the coincidences were 
determined by the disappearance of the white disk on the 
pendulum of the clock behind the tail-piece of the pendulum, 
and also by the mean of its disappearance and re-appearance. 
I was induced to take this additional trouble, in order to 
remove all possible objections which might be raised as to 
the accuracy of the result ; and partly that I might, by actual 
trials, furnish materials for putting at rest the controversy on 
this subject. The method of disappearances has been fol- 
lowed by Captain Kater, and more lately by Captain Basil, 
