How does it tick when the pendulum is shortened?

It ticks more rapidly.

How does it tick when the pendulum is lengthened?

It ticks more slowly.

How then does cold affect a clock?

Cold makes the clock go faster by shortening the pendulum.

How does heat affect a clock?

Heat makes the clock go more slowly by lengthening the pendulum.

How may the pendulum always be kept of the same length?

By having the weight at the bottom of the pendulum, so that it may be raised or lowered by means of a screw.