There are a couple of lines that are almost the same when it comes to alliteration and they should all be equally correct. "Slowly, silently, now the moon", "Her beams beneath the silvery thatch", "One by one the casements catch", and "With paws of silver sleeps the dog". Alliteration is when consecutive words in lines begin with the same group of letters and in the cases mentioned here it occurs regularly like "slowly silenty", or "casements catch".