How does Poe use imagery to sustain tension in "The Black Cat"?

You might want to consider this question by thinking of
how the black cat itself is described throughout this excellent tale. Remember, of
course, that the story is told from the first person point of view, and so it is the
murderous and slightly insane narrator himself who reports what he sees and what
happens, and it is clear that his relationship with the black cat is what leads to his
act of murder and his ever-more tenuous grip on reality and
sanity.

Thus it is that tension is sustained throughout the
tale by descriptions of the black cat as follows:

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I started, hourly, from dreams of unutterable
fear, to find the hot breath of the thing upon my face, and its vast weight--an
incarnate Night-Mare that I had no power to shake off--incumbent eternally upon my
heart!

References made to the
black cat which describe it as a "thing" of demonic proportions, an "incarnate
Night-Mare," serve to heighten our interest and the tension of the tale as we wait to
see what will happen and how the curious relationship between the narrator and the cat
will resolve itself.