Disposed approximately midway of the uprights C, C¹ and within casing M, secured at its ends to said uprights, is rotatably mounted a power transmission shaft M¹, keyed to which is a spur wheel M². Also mounted on the shaft M¹ is a spur wheel M³, meshing with which at its upper and lower sides are two spur wheels M^{4}, M^{5}, loosely mounted upon short supporting shafts M^{6}, M^{7}, journaled in uprights M^{8}, M^{8} secured to the casing M. To each of the spur wheels M^{4}, M^{5} is secured the outer end of a coil spring M^{9}, M^{10}, respectively, the inner ends of said springs being secured to the respective shafts M^{6}, M^{7}, the arrangement being such that when the springs are placed under tension by the rotation of the shafts M^{6}, M^{7}, the force of the springs rotates the spur wheels M^{4}, M^{5}, thereby rotating the spur wheel M³, shaft M¹ and the spur wheel M². Also mounted upon each of the respective short shafts M^{6}, M^{7}, and keyed thereto, is a ratchet wheel M^{11}, M^{12}, and adjacent thereto and loosely mounted upon each of said shafts M^{6}, M^{7} is a pawl carrier plate M^{13}, M^{14}, each carrying a pawl indicated at M^{15}, M^{16}, which is adapted to engage the teeth of the ratchet wheels M^{11}, M^{12}, being held in engagement therewith by springs, one of which is shown at M^{17}, secured to said pawl carrier M^{13}. Suitably mounted upon the casing M, and adapted to engage the teeth of the ratchet wheels M^{11}, M^{12}, is a detent M^{19}, to prevent reverse movement of said ratchet wheels. The pawl carrier plate M^{13} is provided with a pin M^{21}, and secured thereby loosely to said carrier is one end of a connecting rod M^{21ª}, the other end of said connecting rod being connected to one end of a longitudinally flexible coupling M^{22}, the other end of said coupling being secured by means of the connecting rod M^{23} to the power transmission lever L. The function of the flexible coupling M^{22} will be hereinafter referred to. The pawl carrier M^{13} also carries, at its lower end, a pin N, and loosely mounted thereon is one end of a connecting rod N¹, the other end of said rod being connected to a pin N² secured to the pawl carrier M^{14}, whereby, when motion is imparted to pawl carrier M^{13} and, through the pawl M^{15} to the ratchet wheel M^{11}, motion is imparted to the pawl carrier M^{14}, and through its pawl M^{16} to the ratchet wheel M^{12}. From the pin N² is suspended a weight N³ to return the pawl carriers to their lowermost positions when they complete their upward travel. The flexible coupling M^{22} comprises a tubular casing N^{4}, which is provided at one end with an opening N^{5}, through which projects a rod N^{6} having a head N^{7}, which is adapted to bear against a spiral spring N^{8} mounted within said casing, the other end of said rod N^{6} being connected to the rod M^{23}.