A pamphlet on instruments in which Hooke hid, as a Latin anagram, the rule that the ideal shape of an arch is a hanging chain turned upside down.
Hooke published the principle as a string of jumbled letters to establish his priority without giving the secret away; the solution, printed after his death, reads ut pendet continuum flexile, sic stabit contiguum rigidum inversum: as hangs the flexible line, so but inverted will stand the rigid arch. A chain carries its load in pure tension; turned over, the same curve carries the load of an arch in pure compression. Christopher Wren used the idea for the dome of St Paul’s, Giovanni Poleni used it in 1748 to check the cracked dome of St Peter’s, and Antoni Gaudí designed the Colònia Güell church (1898–1914) with a hanging model of strings and small bags of shot, studied upside down in a mirror.
The book, part by part
I The hanging chain
The catenary as the funicular of a uniform chain; inversion into the ideal arch.
Rules: As hangs the chain, so stands the arch · The line of thrust must stay inside the arch