As has already been said, the iambus is the common foot of English verse.
The second pda of this stanza is wanting an iambus in its middle part.
In place of the Iambus, a Tribrach ( ) may stand in any foot but the last.
Such a foot is called an iambus (plural, iambuses, or the Latin iambi), and the form of verse is called iambic.
He could make Greek iambics, and doubted whether ...