Mechanical stop and start system for a function, and watch comprising such a system

Mechanical system for stopping and starting a function of a timepiece, comprising a mass, movably mounted on a frame between a first and a second extreme position, defined respectively by a first and a second stop, said mass being displaced, under the effect of gravity, thereby loading of a spring between the first and second extreme positions, and thereby relaxing said spring between the second and first extreme positions,

• said mass being kinematically or directly connected to a control member for said function,
• the mass and a force of the spring being determined as a function of the force required to command said function and displace the control member, and of predetermined spatial orientations of the frame, in which the mass is bi-stably displaced from one to the other of its extreme positions, according to a resultant of the forces exerted by the spring and by gravity, the displacement of the mass from the first extreme position to the second extreme position being able to start said function via the control member, and the displacement of the mass from the second extreme position to the first extreme position being able to stop said function via the control member.