Bit of a tricky Q, that. A gauged arch is one consisting of wedged stones, whose joints project to a single point. At first glance, it might seem to be that only a simple, semi-circular or Roman arch...
'hemi' means 'half' and 'sphere' means 'round ball', so, a hemisphere would be a shape like half a coconut, eggshell, or anything else that is shaped like half a round ball.
On a globe of earth, we...