Conversation so far:
What was the disaster?
The cable was lost after 1,200 miles (1,900 km) had been laid and the expedition had to be abandoned.
In what year did this happen?
In July 1865.
What happened after the expedition was abandoned?
A joint committee of inquiry was established by the Board of Trade and the Atlantic Telegraph Company.

Question: What was the triumph?
Self-contained rewrite: What was the triumph of William Thomson?