NavList:

Frank you wrote:  Here's a graphic representation of the difference between a pure cylindrical projection and the actual Mercator projection. I think I've drawn this correctly but no guarantees... Please note, this sort of representation will be immediately instructive to some people and meaningless to plenty of others. If it doesn't strike a chord, don't waste your time on it. 

I can see what you’ve done Frank.  You’ve fed the mathematical formula for a Mercator projection into a curve sketching programme and overlaid it onto the geometrical projection of a sphere onto a cylinder.  I hadn’t seen this when I sent my post, because posts seem to be published in batches.  If our two attempts equate, the distance between your 50 and 70 degree yellow intersections with the cylinder should be roughly four times the distance between 0 and 10 degrees, which my ruler tells me it is, whereas the distance between your 50 and 70 degree white intersections with the cylinder should be more like eight times this value, which my ruler also confirms.  I've to saved your diagram, because you never know when a slide like that might come in handy.  DaveP