Performing a scansion on a poem is a somewhat subjective process. In a scansion, you mark each syllable of each line with a dot or a dash--a dot to show an unaccented syllable and a dash to show an accented syllable. Then you group the dots and dashes into poetic feet. However, the way you say a line can at times influence whether a syllable is accented or not, and not all lines of poetry break evenly into poetic feet. Your goal in performing a scansion is to identify the predominant rhythm or rhythms used. In this poem by Edgar Allan Poe, the poet uses a variety of rhythms, giving the poem an unstable, almost erratic feeling at times.
The first two lines can be scanned as follows:
../ ../ ../ ./
../ ./ ./
Once you have arrived at these markings, you can translate them into poetic feet. A foot that has two dots followed by a dash is called an anapest. A foot that has one dot followed by one dash is an iamb. Thus you would identify the first line as three anapestic feet followed by one iambic foot. The second line could be one anapestic foot followed by two iambic feet.
An alternate way of marking the second line would be:
/. /. /. /
This is how you'd mark it if you stressed the word "in." Such a marking would give you three trochaic feet plus a catalexis--an incomplete foot that is missing a syllable. To make a decision about which version of labeling line two is preferable, you should take into account the entire poem, listening for the predominant rhythms. Since the rest of the poem makes heavy use of the anapestic rhythm, it seems better to retain the anapest as the first foot of line two and consider the remaining two feet as iambs.
Deciding how to mark the poetic feet in a poem is somewhat subjective, but when making a decision in the case of two or more possibilities, you should choose the one that results in the greatest consistency with the rest of the poem.