Final answer:
The figure of speech used in the phrase 'as if I were a tortoise' is a simile, which compares two different things using 'like' or 'as'.
Step-by-step explanation:
The phrase 'as if I were a tortoise' in The Dilemma of a Ghost represents a figure of speech known as a simile. A simile is a figure of speech that makes a comparison between two unlike entities and often includes the words 'like' or 'as'. In this case, the word 'as' initiates the comparison, suggesting similarity in some respect between the speaker and a tortoise, which could be in terms of speed, protection, behavior, or another characteristic.