on the end of a rod 10 which in turn slides within the tool 4 when a base 8 is provided the rod 10 passes through it and is thus guided in its movement the rod 10 may be manoeuvred by any known means and is preferably the longitudinally extending piston rod of a hydraulic jack 11 arranged above the drilling tool 4 of course the jack 11 may be housed within the sleeve 5 but it is preferable for the jack 11 itself to form part of the sleeve 5 for this purpose the cylinder 12 of the jack 11 is provided with flanges 13 at both ends so as to be mounted to provide an extension of the sleeve 5 this jack 11 is a double action jack with a simple action or a telescopic action as required when the jack 11 is mounted coaxially with the drilling tool 4 as has just been explained its cylinder 12 serves to effect rotational driving of this tool for this reason a rotary joint 14 is interposed between the jack 11 and the speed reducer 3 so as to ensure an unobstructed supply of fluid under pressure it is not essential for the valve action member 9 to be rotated the present invention is also applicable to embodiments where this member 9 is only movable in translation however it is more advantageous as in the embodiment shown in fig1 for the member 9 to rotate at the same time as the drilling tool 4 in this embodiment rotation of the member 9 is caused by rotation of the jack 11 however it is preferable for the rod 10 which passes through the base 8 to have a cross sectional profile which is circular e g to have an hexagonal profile.