What is that clock like device on the mast of old battleships?

Answer 1

Those are ranging clocks for the fire control platform (whose spotters were initially stationed above the clocks). They're mounted on the lattice masts to improve visibility under combat conditions (e.g., cordite discharges).

Answer 2

it was as a system to communicated between the ships .
With putting the 2 arms of the clock in different positions it was possible to send messengers