An arch stands up by pushing outwards. Ev stone i it le against i neighbours, s the lo travels side along t curve ins of pres straight down, and the walls at either end must be heavy enough to absorb the thrust. Builders discovered this long before anyone could calculate it. A Roman surveyor knew from experience how thick a supporting pier had to be, though he could never have explained why.

Type only the missing letters. Each box is one letter, and the number of boxes matches the letters that are missing.