Up, Down and Sideways

David talks about ""the geometry of not-falling."" It is rooted in the right angle. Fighting gravity depends on building things at right angles. But how do you ensure a proper right angle? David moves up in tools, from the spirit level to the theodolite. Interwoven in this program is the Pythagorean Theorem and the first glimpse of what would become the Chunnel linking London to Paris — ""a straight line that starts by going down and ends by going up.""