This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
When a cheap clock, such as an American spring clock without a fusee, is first wound up, the motive power is very great, and when the sane clock is nearly run down, the power has diminished to perhaps less than half. The effect of this with a recoil escapement (one in which the 'scape-wheel recoils at each beat) and a light pendulum is to make the clock go gradually slower a-! it runs down. With a heavy pendulum the error is less. A dead-beat escapement (one in which the 'scape-wheel remains perfectly still between each beat) has a very small error in the opposite direction, and the same clock fitted with it would gradually gain as it ran down. Therefore, to keep correct time, the escapement must not have much recoil, nor must it be perfectly "dead."
A cheap clock with a light pendulum should have an escapement with a moderate recoil only, and a good clock with a heavy pendulum should have a nearly dead-beat escapement, or what is known as a "half dead," i.e. a dead-beat with a very slight amount of recoil on the resting surfaces, but hardly perceptible. The amount of recoil is determined by the shape of the pallets.