A page with a good explanation can be found at:
http://www.aviationshop.com.au/avfac...ydro/index.asp
You'll need to click the 'click here for free access' bit and get a user id and password but it's well worth it.
There are plenty of other interesting topics by the same guy at:
http://www.aviationshop.com.au/avfacts/editorial.htm