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plural mainsprings
(horology) The principal spring of a clockwork mechanism, which drives it by uncoiling. quotations
He was wise, for he had seen a long succession of mechanical toys arrive to boast and swagger, and by-and-by break their mainsprings and pass away.
1921 June, Margery Williams, “The Velveteen Rabbit: Or How Toys Become Real”, in Harper’s Bazar, volume LVI, number 6 (2504 overall), New York, N.Y.: International Magazine Company
(figuratively) The most important reason for, or element of, something. quotations examples
But in the England of Shakespeare’s history plays, the mainspring of the system has broken down.
2014 November 2, Paul Mason, “What Shakespeare taught me about Marxism”, in The Guardian