length of the pendulum vibrating seconds. 89 
of the earth, and is readily obtained, being the interval 
between the returns of any fixed star to the meridian. 
I shall now proceed to notice the sources of error which 
may be supposed to have affected the results of the preceding 
experiments. 
These may be classed under the following heads. 
1 . The measurement of the distance of the knife edges. 
2. The number of vibrations in 24 hours. 
3. The temperature, and 
4. The form of the knife edges. 
On the first, it is scarcely necessary to offer any remark. 
Since the mean results of three several sets of measurements 
are within one ten-thousandth of an inch of each other, and 
the different methods employed, preclude, it may be pre- 
sumed, any accidental coincidence, we may with confidence 
infer that the error in the distance of the knife edges, cannot 
amount to one ten-thousandth of an inch. 
Among the number of vibrations in 24 hours given in the 
various sets of experiments, there appear to be differences 
which amount in some instances to 1 , 6 . These differences 
however do not influence the truth of the result, beyond a 
certain minute quantity, the extent and origin of which I 
shall proceed to explain. 
In order to determine the vibrations in 24 hours, it is neces- 
sary to ascertain the number of vibrations and parts of a vibra- 
tion made by the brass pendulum during a certain number of 
complete seconds ; but the moment of observation being limited 
to that when the brass pendulum is at the lowest part of the 
arc, the process is of necessity reversed, and the brass pendulum 
is observed to make a certain number of complete vibrations, 
MDCCCXVI II. N 
