JF Ptak Science Books Post 1637 (From the series on the History of Blank, Empty and Missing Things.)
Such is the case with this great drawing by Theo. G. Scott in the Architectural Record (July 1910) of Christopher Wren's "Geometrical Staircase" in the southern tower of St. Paul's Cathedral. The stairs of course are not missing, they're just not visible from the standard I'm-looking-at-stairs viewing scenario, as we can see in another classic view, this one found Theodore Cook's The Curves of Life (1914):
[The double staircase as described in Cook's book as the "Tamworth Church Tower", which is actually St. Editha, in Staffordshire. The church is Medieval, and building dates from 1369, though it is the fourth iteration of a structure there, the first of which (begun in the 9th century) was destroyed by the Danes soon after its completion. The staircase is remarkable because it allows one person to ascend and another to descend without either ever having to see the other.]
Engineers and architects had confronted the idea of spiral stairs for many centuries before Wren--perhaps as many as 40 generations, perhaps more. The "invention" of the spiral staircase, or its discovery in the places in which it sort of exists in nature (like in the shells terebra dimidiata and voluta scalaris), seems like a very interesting thing, though I am not sure where it stands in the history of importance in architectural development. Perhaps Pisa wouldn't have nearly so many narrow towers without the spiral staircase, and I know for a fact that it is far easier to deflect marauders from ascending a castle's spiral staircase than any other (the defenders getting at least an order of magnitude (and maybe more) bump from that design than any other staircase. I doubt in comes anywhere close to the indoor fireplace/chimney, or downspout, and a lot of other stuff. But it is definitely much prettier to look at in perspective and plan than just about anything else in architecture.