I haven't touched my pages on map projections in a while, after a recent burst of activity.
But now I've made a very small change.
To my page on the Mercator projection,
http://www.quadibloc.com/maps/mcy0101.htm
and to my page on special slide rule scales
http://www.quadibloc.com/math/sr02.htm
I have added links to a new page on my site,
http://www.quadibloc.com/math/hypint.htm
which says a few words about hyperbolic trig functions and the Gudermannian. The function giving the vertical coordinate for the Mercator projection in its conventional aspect happens to be the inverse Gudermannian, hence the link.