The magnetic compass, perfected slowly over years of experimentation, trial and scientific endeavor, became the sailor's most common and most reliable direction-indicating aid.
(The magnetic compass; perfected; T:slowly over years of experimentation, trial and scientific endeavor) (The magnetic compass, perfected slowly over years of experimentation, trial and scientific endeavor; became; the sailor's most common and most reliable direction-indicating aid)
The magnetic compass The magnetic compass, perfected slowly over years of experimentation, trial and scientific endeavor
perfected became
T:slowly over years of experimentation, trial and scientific endeavor
the sailor's most common and most reliable direction-indicating aid