How do you find gravitational acceleration?

One elegant way to measure the acceleration of gravity at the place where you are

is to measure it with a swinging pendulum. You must be able to measure the length

of the pendulum, and the time of its swings, very accurately, and you must enclose it

in some sort of a can or a box in order to keep air currents away from it.

When you've measured its length ( ' l ' ) and the time of its swing ( ' T ' ), the acceleration

of gravity ( ' g ' ) is