422 An Account of Experiments for determining the Length of 
Hence, 39°44085 inches may be taken as the distance be- 
tween the knife edges at the temperature of 62 degrees. 
Using the vibrations when the great weight was Lelow, as be- 
ing nearer to the truth than in the other position of the pendu- 
lum, we obtain the following results. 
Length of 
Corr. 
Length of | Diflerence 
= ate 
& |Temp .|Barom. Lie pou the seconds |for the at-|the seconds | from the 
ioe pen. in air. |mosphere |pen.invacuo| mean. 
A | 68-7 | 29-76 | 86059-42 | 39-13313 -00544 | 39-13857 | 4+-00028 
B | 71-3 | 29-86 | 86059:93 | 39-13278 *00544 39-19822 | —-00007 
C | 71-4 |} 29-86 | 86057-70 | 39°18269 00544 1359-15804 | —.00025 
‘D | 73-1 | 29-95 | 86056-97 | 39-13259 “00544 39-13803 | —-00026 
E | 69:3 | 29-70 | 86058-94 |, 39-1329% “00544 39-13837 | +-00008 
F | 69:3 | 29-70 | 86059-01 | 39:13298 *00544 39-13842 | +-00013 
G | 68-5 | 29-70 | 86059-22 | 39-13286 “00545 39-13831 +:00002 
H | 68-7 | 29-70 | 86059 25 | 39-13296 *00544 39:13840 | +-08011 
I | 69-3 | 29°70 | 86058-93_ | 39-13291 “00544 39:13834 |+-00005 
K | 69-3 | 29-70 | 86058-83 |} 39-15282 -00544 | $9-15825 |— (0003! 
L | 681 | 29-90 | 86059-22 | 39-1327] -00548 39-13819 |—-00009 
M | 68-4 | 29-90 | 86059-21 | 39-13281 “(0548 39-13829 }|—-Q0000 
Mean ! 39-13829 
The length of the pendulum thus obtained requires yet an- 
other 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 pen- 
dulum 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 increasing inversely as the square of the distance 
from the earth’s centre, the length of the pendulum must be in- 
creased 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 
<aissath of the whole 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 can- 
not be very distant from the trnth. 
The length here given, is that required to perform one vibra- 
tion in —,1,,dth part of a mean solar day, this being the mea- 
sure of time usually employed for the purpose ; but I am at a loss 
to 
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