Scan the poetic meter in the following line by placing the accent mark ( ' ) over stressed syllables and the breve (˘) over unstressed syllables. on a high hill far away stood an old rugged cross

  '   | ˘ |   '   |  ˘  | '  |  ˘ | '  |   '     |  ˘ |  '  |  '  |  ˘  |    '
On  a  high hill far away stood an old rugged cross

When you're marking for scansion, the first place you want to look are two syllable words. Look at the word "away" and pronounce it two ways. First by emphasizing the first syllable and then flipping it. You should notice that one of the ways (the second) feels more natural. This should be the same with "rugged". Try to clap out the rhythm for each syllable to get a sense of which words feel most stressed or important. 

The author's purpose is to be persuasive