The Two Falling Globes diagram

They date this to late 1664 when Newton was living at Trinity College, probably in Great Court (ie before he returned to Woolsthorpe due to the plague.) The diagram shown here is based on Newton's and differs from Mc&T's version in several respects (but is quite similar to Westfall's in Force in Newton's Physics p401.)

First, Newton's mechanism at f has b suspended from a string attached to a block at f, which rests on a small lip on the wall and the lever knocked away by a. Mc&T's diagram has a small overhang rather than a lip, above f and not part of the mechanism. Also b is suspended from a second lever attached to the wall rather than the block as in Newton's diagram. Finally the top of the wall in Newton's diagram is less regular (less square) than my own with only some up and down sections good right angles (it is a free hand sketch after all.)

It seems obvious that these are meant to be the top of the crenellated walls of Great Court, with the solid merlons and gaps (crenels) familiar from medieval castles. (Mc&T's diagram has the crenels replaced by upturned semicircles.) The photograph shows part of the Library Range, although all the ranges of rooms in Great Court share this feature. This identification is also relevant to the lip omitted by Mc&T and replaced by an overhang: just such a lip can be seen at two thirds of the height of the wall in the photograph (and corresponding lips are present on the other ranges of the Court.)

Even though the Two Falling Globes can be viewed as a typical thought experiment, I believe it can also be viewed as a practical experiment designed to be feasible with the resources Newton had available to him - even if from the start he never intended to carry it out. This is underlined by the inclusion of a sketch of the place where he was living.