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

Sleep

Why is it sometimes you feel like you slept for hours when it's only a couple hours and then sometimes it feels like you haven't slept at all when you slept a full 8 hours?
 
I can sleep 7-8 hours one night and still feel tired, then the next night I can sleep the same amount but feel amazingly rested.

I don't get it.