Similes are comparisons that use the word "like" or "as."
a) The author describes the scene of hundreds of British redcoats crowding the narrow Tremont Street--"like a river of blood."
b) Johnny surprises Mr. Tweedie, the journeyman silversmith whom he dislikes: "The timid creature jumped like a shot rabbit..."
c) "country cheeses--big as a baby's head."
Metaphors are comparisons that do not use the words "like" or "as."
a) "The narrow course of Tremont Street was filled to the brim and overflowing with the waiting scarlet-coated men."
The author is comparing the crowded street to a cup that is overflowing with liquid.
b) "The kitten was no longer gnawing inside him, trying to get out. In fact it was no longer a kitten [but] a Newfoundland dog."
The author is describing the feeling in Johnny's stomach after a big meal.
c) "'That squeak-pig [Mr. Tweakie] is trying to horn in on breakfast."
Johnny is comparing Mr. Tweakie to a squeaking pig.