There is not an overabundance of figures of speech in William Cowper's humorous poem "The Diverting History of John Gilpin." It is mostly written in literal language, so that when Gilpin's wife says "Though wedded we have been / These twice ten tedious years..," she means literally just that. Literal language like this is language in which words mean exactly what they are defined as meaning. The opposite of literal language is figurative, which means something other than what it is defined as meaning.
As an example of figurative speech--speech that means something other than what it is defined as meaning--look at a line in the conversation between Gilpin and the calender who has just asked Gilpin why he has shown up with the calender's horse at his house in Ware. Gilpin explains to the calender the whereabouts of his hat and wig by saying that "My hat and wig will soon be here, / They are upon the road."
This is a play on words using two figures of speech called an idiom and a personification. Literally, "upon the road" means lying upon the road, which is exactly where the hat and wig are. However the idiomatic meaning of "upon the road," as applied to humans, means that people are traveling at that precise time. If you go from home to Burger King, you are "on the road," meaning traveling and not squashed or dropped somewhere along the way. In addition, by placing his wig and hat as travelers on the road, Gilpin has very wittily employed personification, which is to give inanimate objects, like hats and wigs, human powers, feelings, thoughts or attributes. In this case, Gilpin has imbued his wig and hat with the human power of locomotion.
Some other figures of speech in Cowper's poem are as follows:
Idiom: through thick and thin.
Onomatopoeia: Smack went the whip.
Onomatopoeia: The stones did rattle underneath.
Metaphor: the cloak did fly.
Simile: Like streamer long and gay.
Metaphor Combined with Simile: the cloak did fly / Like streamer long and gay.
Simile: So like an arrow swift he flew.