Why do some s's look like f's in old text?

I was looking at

Micrographia: or, Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries thereupon By

Robert Hooke

 on Google Books and noticed that some of the s's looked like f's, but not all of them.  So, I wanted to see why that was and what the rules are for this strange looking letter.

http://babelstone.blogspot.com/2006/06/rules-for-long-s.html