Mechanical device and Weather vane

Mechanical device noun - Mechanism consisting of a device that works on mechanical principles.

Weather vane and mechanical device are semantically related. In some cases you can replace phrase "Weather vane" with "Mechanical device", this phrases are similar.

Weather vane noun - Mechanical device attached to an elevated structure; rotates freely to show the direction of the wind.

Mechanical device and weather vane are semantically related. You can use "Mechanical device" instead a noun phrase "Weather vane".