What does " 1700s " and " 1940's " mean in the context? Why is the fisrt one followed by " s ", and the second one followed by " 's " ? I do not know what is the difference between them.
Thanks for your help.
1.Well, you can image that there was some interest in why the 2.8 spot was skipped, but there wasn't anything obivous there, in the early telescopes. Then what happened in the late 1700s? The discovery of...?
2.In fact, by the early 1940's, even though steel cables were available, most ships in the United States Navy were not moored with steel cables, they were moored with Manila hemp ropes.