fig1 a and 1 b show a schematic view of a feed 1 of a rock drilling apparatus . a rock drill 2 moves lengthwise of the feed by means of feed elements , such as feed cylinders 3 . to the rock drill 2 is connected a drill rod or pipe 4 , at the other end of which there is a similar drill rod or pipe or a drill bit for the collaring step . this kind of extension rod drilling technique is commonly known and obvious to a person skilled in the art , and so it will not be described in greater detail in this connection . at the rear end of the feed can be seen the tips of rear supports 5 . the rear supports 5 are arranged at the ends of the piston rods of support cylinders 6 in the longitudinal direction of the feed , and they are made to support the feed by supplying pressure fluid to the cylinder 6 , the pressure fluid pushing the piston rod out toward the rear end of the feed . the figures also show a front support 7 , which is mounted on the piston rod of its support cylinder 8 and is pushed outward through the front end of the feed when pressure fluid is supplied to the support cylinder 8 . correspondingly , the front and rear supports can be positioned as shown in the figure , i . e . retracted by supplying pressure fluid to the support cylinders 6 and 8 , to the other side of the pistons thereof . instead of two rear supports , it is naturally possible to use one support 5 and one support cylinder 6 in a similar manner as described in connection with the front support . fig1 a and 1 b show two rear supports , but it is also possible to have two front supports , if so desired . fig1 b also shows a schematic view of a first and a second pressure detector 9 and 10 , which detect the pressure in support cylinder 6 on the one hand and the pressure of the pressure fluid in support cylinder 8 on the other hand . the pressure detectors 9 and 10 are arranged to control a control unit 11 that controls the supply of the pressure fluid to the support cylinders 6 and 8 . the pressure value in both support cylinder 6 and support cylinder 8 of the invention is set as necessary for proper implementation of the bracing . the pressure prevailing in the cylinders 6 and 8 is measured by the pressure detectors 9 and 10 , and the supply of the pressure fluid to the cylinders is conducted on the basis of the detected pressure so that the pressure in the support cylinders 6 and 8 remains substantially as originally set . depending on the technique used , this can be performed in various ways without deviating from the basic idea of the invention . for example , using previously known electronics , a desired pressure value can be pre - set for both cylinders in the memory of the control unit 11 , after which the control unit compares the pressure value given by the detectors 9 and 10 with the values stored in the memory and , if necessary , supplies pressure fluid to the support cylinders 6 and 8 of either the front or the rear or both supports so as to keep the pressure value as originally set . in this embodiment , the set pressure value can be easily changed in the middle of the drilling operation if it is considered necessary . when new pressure values have been set , the control unit 11 automatically corrects the pressure values of the pressure in the support cylinders 6 and 8 to correspond to the new set values . in another embodiment of the invention , the pressure detectors 9 and 10 can simply be pressure detectors in which a pressure value can be set in such a way that if the current pressure value exceeds the set value and / or is lower than the set value , the detectors 9 and 10 supply a certain kind of signal . in this embodiment , fixed pressure values are set in the feed cylinders 6 and 8 by adjusting the pressure detectors 9 and 10 , for example , to detect when the pressure of the pressure fluid in the support cylinders 6 and 8 , respectively , is at least the same as the predefined pressure value . the control unit 11 controls the supply of the pressure fluid to the support cylinders 6 and 8 when the feed is being braced : the supply of the pressure fluid to cylinder 6 and cylinders 8 is stopped when the pressure detectors 9 and 10 detect that the pressure in the cylinder has reached the predefined set value . when the drilling is started , the feed is subjected to drill forces , which may make the support surface yield . the pressure in the cylinders and particularly in the support cylinders 6 of the rear supports may then drop , whereby pressure detector 9 makes the control unit 11 control the supply of the pressure fluid to support cylinders 6 in such a way that the pressure in the cylinders remains essentially the same as the predefined set value . pressure fluid is supplied to the system by a pressure fluid pump 12 in a manner fully known per se , and the supply of the pressure fluid to the cylinders 6 and 8 is controlled by valves 13 and 14 controlled by the control unit . fig2 in turn , is a schematic view illustrating the functional circuitry of the hydraulic support cylinders 6 and 8 of the front and rear supports . as shown in the figure , pressure detectors 9 and 10 are connected to the pressure fluid conduits of support cylinders 6 and 8 , respectively . when the pressure in support cylinder 6 , for example , drops below the set value of the first pressure detector 9 , the detector supplies a signal to the control unit , which switches control valve 13 to a position in which pressure fluid can flow from the pressure fluid pump 12 through conduit 15 to cylinder 6 . when the pressure in cylinders 6 reaches the set value of pressure detector 9 , the pressure detector detects the situation and the control unit switches control valve 14 to a position in which the supply of pressure fluid to cylinders 6 will stop . this is repeated every time the bracing of cylinder 6 changes , for example , because the ground yields , so as to keep the pressure in cylinder 6 essentially the same as the set pressure value throughout the drilling operation . correspondingly , in a pressure fluid conduit 16 of support cylinder 8 of the front support 7 there is another pressure detector 10 , which detects a pressure drop in a similar manner as described above and supplies a signal to the control unit concerning the supply of the pressure fluid by controlling control valve 14 so as to keep the pressure in support cylinder 8 essentially constant , at least at the pre - set pressure value . fig2 also shows a pressure fluid container 17 from which the pressure liquid pump obtains pressure fluid and to which the pressure fluid returns from the cylinders . in addition , the figure shows pressure - controlled check valves 18 connected to the pressure fluid conduits of the support cylinders 6 and 8 , the check valves hindering the flow of the pressure fluid from the cylinders otherwise , but allowing it when pressure fluid is supplied to either one of the conduits through valve 13 or 14 . the structure and operation of the valves is commonly known per se and obvious to a person skilled in the art , and so we do not consider it necessary to describe them in greater detail herein . it is also possible to implement the invention in such a way that the pressure value of the pressure prevailing in the support cylinders 6 and / or 8 can be set by the user , although a certain minimum pressure value has been set in the memory of the control unit 11 or in the pressure detectors 9 and 10 , the supporting pressure and the corresponding supporting force never being lower than this minimum pressure value . this can be implemented in various ways in accordance with the above embodiments . the invention is described in the above description and the drawings by way of an example , and it is not to be construed as being limited by them . the essential feature is that the pressure prevailing in the support cylinders of the front and rear supports of a rock drilling apparatus is monitored and that any change of pressure that makes the pressure in the cylinder drop below a pre - set pressure value makes the control unit control the supply of the pressure fluid to the cylinders so as to essentially maintain the set value of the pressure in the cylinders . the invention can also be applied in solutions where , for example , a separate front support is not used , but where there are one or more rear supports . this is particularly useful when there are two rear supports spaced apart in the transverse direction of the feed . if the support surface yields , for example , at one of the supports , the pressure will drop in both support cylinders , and when more pressure fluid is supplied , the pressure will rise evenly in both cylinders or will remain essentially even , although one of the supports is pushed further out , until there is a sufficiently firm surface behind both supports .