88 Capt. Kater’s experiments for determining the 
The length of the pendulum thus obtained requires yet 
another correction to reduce it to what it would have been at 
the level of the sea. The elevation of the apartments of the 
Royal Society at Somerset House above low- water mark, is 
known to be 81 feet; and by several careful observations 
with an excellent mountain barometer by Ramsden, I found 
the room in Portland Place, in which the experiments were 
made, to be two feet below the Royal Society’s apartments ; 
and as the height of the pendulum above the floor was four 
feet, we obtain 83 feet for the elevation of the pendulum 
above the level of the sea. Now the force of gravity in- 
creasing inversely as the square of the distance from the 
earth’s centre, the length of the pendulum must be increased 
in the same proportion, and taking the radius of the earth for 
the latitude of Portland Place to be 3954,583 miles, we have 
39,1386 inches for the length of the pendulum vibrating 
seconds at the level of the sea. 
It may be remarked that the greatest difference between 
the mean result and that of any one of the twelve sets of 
experiments contained in the preceding table, is only ,00028 
of an inch, or T-jfwT'? t ^ ie w hole length of the pendulum; 
and as seven of the twelve sets are within one ten thousandth 
of" an inch of the mean result, it may be inferred that the 
above determination cannot be very distant from the truth. 
The length here given, is that required to perform one 
vibration in Te j Q Q part of a mean solar day, this being the 
measure of time usually employed for the purpose ; but I am 
at a loss to conjecture why this is preferred to the sidereal 
day, a measure of time which marks a complete revolution 
