Computer:

The tide-predicting machine invented by Sir William Thomson in 1872 was of great utility to navigation in shallow waters. It used a system of pulleys and wires to automatically calculate predicted tide levels for a set period at a particular location.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What did Sir William Thomson's tide-predicting machine use to function?
system of pulleys and wires