Writing a Melody under the Cantus Firmus?

Hey Peeps,

I've brought a copy of "The Study of Counterpoint" and I'm slightly stuck on like, the third/fourth page...

I fully understand everything so far up to what is Page 31 of my copy, here Joseph has been given the task of writing the counterpoint underneath a Cantus Firmus and I've got completely lost as to how he's doing it. It doesn't help that I can't actually read some of the numbers aswell because the print blotches.

This is the example given:

What I'm lost how you decide what the interval is... Let's take the fourth bar for example because 1 and 2 are Joseph's mess ups (bless him) and I can't actually read the Interval he's written in number 3 because it's either a 3, 5, 6, or 8.

So, in the lower stave of bar 4, there's an F and then there's a D in the higher stave. So basing the interval from the Counterpoint Stave then we have a Major Sixth. But then, if you base the interval from the Cantus Firmus we have a Minor Third.

This is where I get confused. What way should I be relating it to? It kinda depends on whether the interval is classed as Perfect or Imperfect. I understand that you're meant to go on the mode of the Cantus Firmus, does that mean I should just be treating the Cantus Firmus as the one that I start counting the interval from? Then I just get confused because he's written "10" instead of simplifying to a Third and stuff like that.

Basically I just want to confirm I'm looking at this in the right way? Any help would be awesome!

Cheers!

Daryl :)

You always measure the interval from the lowest note; in this case, the counterpoint staff, because of the little eight under the clef which tells you that the notes sounds an octave below how they look on the page; so, the one you can't read in bar three, is the distance between A below middle C and the next E above that, which is a fifth. The 10's are written instead of 3's because there's not just a third between the two parts; there's an octave and a third.