1. A return mechanism for a chronograph or a stopwatch, comprising a hammer-fulcrum, a hammer mounted rotatably on said hammer-fulcrum and having an integral hammer spring and two integral acting surfaces one of which has flexibility with respect to the remainder of said hammer, two essentially heartshaped cams, each mounted rotatably on its own cam-fulcrum and disposed to coact with one of said acting surfaces when said return mechanism is actuated, said acting surfaces being at essentially equal distances from said hammer-fulcrum, and said cam-fulcrums being at essentially equal distances from said hammer-fulcrum, whereby the forces exerted by said acting surfaces against said cams during actuation will be essentially equal, and a pin so disposed as to press against said hammer spring and urge said hammer away from said cams, the shape of said hammer-spring being such that the force exerted by said spring is near maximum during the first portion of said actuation, decreases during a further portion of said actuation and rises during the last portion of said actuation.