depress the primary piston 25 within hydraulic cylinder 27 as noted above this movement of the primary piston 25 within the hydraulic cylinder 27 will produce a larger movement of the secondary piston 26 which correspondingly moves the trigger arm 29 downwards relative to the counter wheel 21 downward movement of the trigger arm 29 engages pawl 34 with one of the series of teeth 22 on the counter wheel 21 which causes the wheel 21 to rotate through an incremental amount to advance the dose count release of the pressurised dispensing container 3 allows the ferrule 11 to move upwardly under the internal spring biased of the metering valve 12 this disengages the ferrule 11 from the upper elongate extension 31 of the primary piston 25 which allows the hydraulic mechanism 24 to reset by virtue of the biasing effect of the spring 32 resetting occurs because the biasing force of spring 32 acts upwardly on the secondary piston 26 to move the secondary piston 26 upwardly relative to the hydraulic cylinder barrel 27 and this upward force is transmitted to the primary piston 25 through the hydraulic working fluid 28 upward movement of the trigger arm 29 causes the pawl 34 to move upwardly and ride over the neighbouring tooth 22 on the counter wheel 21 counter rotation of the counter wheel is prevented by standard means such as a ratchet not shown various modifications may be made to the inventive apparatus the primary piston 25 may be fixed or coupled to the ferrule 11 rather than merely being engaged by the ferrule 11 for example the upper elongate extension 31 may be welded to the ferrule 11 in such a case resetting of the hydraulic mechanism 24.