I have done quite a bit of recreational writing on the history of military technology, mainly on the Wikipedia (e.g. Torpedo Data Computer). I am particularly interested in the problems associated with fire control calculations. I remember as a boy being fascinated when reading about Galileo and his compass, which was used for early fire control calculations with cannons. So my interest goes way back.
I am particularly interested in the mechanical analog computers used for fire control calculations. Youtube has some excellent videos on this topic. One is a World War II training video discussing how mechanical multipliers, adders, subtractors, reciprocal generators, and integrators work. It is ~40 minutes long, but it is also available broken up into seven parts for those of you with adult-onset ADD (like me). The second video is the best discussion of how a differential works that I have ever seen. While this video is focused on car differentials, the same hardware can be used as an adder or subtractor.
First of Seven Parts (Same as Above But in Parts)
Great Video on Differentials