( 4 ? ) . . 
ercury been a little fhorter , for it differed a little the 
contrary Way from the other Clock, going fafter with 
Heat, and flower with Cold j but I made no Alteration 
in the Length, to avoid an Interruption of the Obfer- 
vaiions. To confirm this Experiment the more, about 
the Beginning of July^ 1723, I took off the heavy 
Pendulum from the other Clock, and made another 
with Quickfilver, but with this Difference, that in- 
ftead of a Glafs Tube, I made ufe of Brafs, and ■varnilh- 
ed the infide, to fecure it from being injured by the 
Mercury. This Pendulum I have made ufe of ever 
fince, and find it about the fame Degree of Exadnefs 
as the other. The Reafon, why this kind of Pendulum 
is more exad than the common Sort, will be evident 
to any one, who confideis, that as Heat lengthens the 
Rod of the Pendulum, at the fame Time it incjeafes the 
Length of the Pillar of Qi-uckfilver, and its Center of 
Gravity is moved upwards : And when, by Cold, the 
Rod of the Pendulum is Ihortened, the Pillar of Quick- 
filver is likewife fliortened, and its Center of Gravity 
. carried downwards; by this Means, if the Column of 
Quickfilver be of a proper Length, the Diftance, be- 
tween the Point of Sufpenfion and the Center of Of- 
cillation of the Pendulum, will be always nearly the 
fame, upon which the exad Motion of a Clock princi- 
pally depends. VVere the Pendulum ofa Clock tore- 
main invariably of the fame Length, yet foine little 
Inequalities would appear in its Motion, from the Dif- 
ference of Fridion arifing from the Imperfections of 
the Materials, as well as different Degrees of Foul- 
nefs- upon which Account, the Force communicated 
to the Pendulum, would not be conftantly equal, 
which would caufe fomefmall Alteration. - But when 
the Pendulum is very heavy, and vibrates in a fmall 
Arch, and the Workmanfhip of all the Parts is well per- 
F 2 formed 
