|Christiaan Huygens, Gaspard de Prony, Henry Kater kinematics|
Calculating the precise length of a pendulum is difficult because a pendulum is constructed of multiple materials and the masses of its parts are not uniform. But Christaan Huygens proved that a pendulum’s center of oscillation is interchangeable with its pivot point; that is, if you interchange their positions, the pendulum will have the same period. Huygens also gave the formula that relates the length and period of a pendulum to the force of gravity, so Gaspard de Prony realized you could use this formula and the length of a pendulum to calculate the local force of gravity. Independently, Henry Kater had the same realization but also built and refined the reversable pendulum with reversable pivot blades and showed how to use it to calculate the local force of gravity.
It turns out that local gravity is affected by the bulging of the globe at the equator, by centrifugal force of Earth’s rotation, by the gravitational pull of the sun and moon, by the atmospheric pressure and your altitude, and by the thickness and density of the earth beneath you, not necessarily in that order. Gravitational anomalies beacon the tourist to watch water flow uphill or otherwise indulge in fantasies related to visual illusions. Very small differences are imperceptable except to the skeptical and refined.
After your first drink, you feel lighter; after your last drink, heavier. Individual gravitas may differ. In free-fall, you feel no gravity at all. Between individuals, differences are greater than between groups. Trying harder may harden the effect.