1. A ventilating device for ventilating through a ridge, preferably through a top ridge of a roof, wherein said ventilating device comprises a ridge member intended for extending longitudinally along and beneath the ridge, said ridge member having a rigid part consisting of two opposed panels and a number of cross pieces extending between the opposing panels, and said ridge member having passages extending from a lower part of the ridge member to an upper part of the ridge member, said upper part being an uppermost part of the ridge member. 2. A ventilating device according to claim 1, wherein said cross pieces extend from the lower part to the upper part of the ridge member transverse to a longitudinal direction of the ridge member and where ventilating passages are formed in spaces between the two opposing panels and between the cross pieces, said ventilating passages allowing air to pass from the lower part of the ridge member along the ventilating passages and to the uppermost part of the ridge member. 3. A ventilating device according to claim 1, wherein said cross pieces extend between the lower part to the upper part of the ridge member parallel to a longitudinal direction of the ridge member, and where ventilating passages are formed as holes penetrating the cross pieces, said ventilating passages allowing air to pass from the lower part of the ridge member along the ventilating passages and to the uppermost part of the ridge member. 4. A ventilating device according to any of the preceding claims where the cross pieces are attached to inner surfaces of the two opposing side panels and where the cross pieces establish a mutual distance between the opposing side panels. 5. A ventilating device according to claim 4 wherein the ridge member is made of plastic, preferably is made of sheets of plastic, and where the two opposing panels and the cross pieces after manufacture are mutually joined by welding or gluing. 6. A ventilating device according to claim 4 wherein the ridge member is made of plastic, preferably is made by extrusion, alternatively by injection moulding, and where the two opposing panels and the cross pieces after manufacture form a corporate unit. 7. A ventilating device according to claim 4 wherein the ridge member is made of metal, preferably is made of sheet metal, and where the two opposing panels and the cross pieces after manufacture are mutually joined by welding, gluing or soldering. 8. A ventilating device according to any of the preceding claims wherein the ventilating device furthermore comprises flexible parts extending from lower edges of the ridge member downwards and being capable of bending outwards away from each other, said flexible parts being made in continuation of the opposing side panels of the ridge member and where the flexible parts have a flexibility along the outwards extension of the flexible parts enabling the flexible parts at least to flex along an underroof having a different slope at each side of the ridge member and preferably also to flex along non-plane underroofs. 9. A ventilating device according to claim 8 wherein the ventilating device furthermore comprises flexible parts extending from lower edges of the ridge member downwards and being capable of bending outwards away from each other, said flexible parts being made in continuation of the opposing side panels of the ridge member and where the flexible parts have a thickness being smaller than a thickness of the opposing panels so that the flexible parts are less rigid and more flexible than the opposing side panels. 10. A ventilating device according to claim 8 wherein the ventilating device furthermore comprises flexible parts extending from lower edges of the ridge member downwards and being capable of bending outwards away from each other, said flexible parts being provided with ribs in order to weaken the strength of the flexible parts so that the flexible parts are less rigid and more flexible than the opposing side panels. 11. A ventilating device according to any of the preceding claims wherein the flexible parts have lower edges provided with a flap extending downwards and being provided for establishing a substantially water tight abutment between the lower edge of the flexible parts and an underlying surface, such as an underroof, which the flexible parts are intended for abutting. 12. A ventilating device according to any of the preceding claims where mounting means are provided for co-operating with the ridge member, said mounting means being intended for mounting of a ridge coping such as top ridge tiles to the ridge member. 13. A ventilating device according to claim 12 where the mounting means consists of plugs being intended for cooperating with and being secured to the ventilating passages established in the ridge member, and where the plugs are provided with means such as screws for mounting of the ridge coping to the mounting plugs and thereby securing the ridge coping to the plug. 14. A ventilating device according to claim 12 where the mounting means consists of number of mounting clips being intended for mounting in a groove provided in the top part of the ridge member and being provided with means for securing the clips to the groove, and where the mounting clips are provided with means for securing of a ridge coping to the mounting clips, thereby securing the ridge coping to the ventilating device. 15. A ventilating device according to claim 12 where the mounting means consists of number of mounting plates being intended for mounting in a groove provided in the top part of the ridge member and being intended for being secured to the groove, and where the mounting plates are intended for co-operating with fastening means such as screws for securing of a ridge coping to the mounting clips, thereby securing the ridge coping to the ventilating device. 16. Ventilating device according to claim 14, where the mounting clips comprise a loop for securing a rear end of one roof tile and a front end of another roof tile to the mounting clip, and where the mounting clip also comprise legs with guiding sections intended for engaging with a dove-tail groove in the top of the ridge member. 17. A ventilating device according to claim 16, where the legs also comprise means for securing the legs to the dove-tail groove, said means preferably being at least ends, more preferably being at least pointed ends, engaging with lateral sides of the groove, most preferably being ends and a toggle joint like bend between a leg and a transverse legging. 18. A ventilating device according to any of the preceding claims where a connecting piece is provided, said connecting piece comprising two oppositely opposed ends having a cross section corresponding to a cross section of a canal formed between longitudinally extending cross pieces of the ridge member, said connecting piece preferably being provided with holes intended for extending in extension of holes provided in the cross pieces. 19. A ventilating device according to any of the preceding claims where a connecting piece is provided, said connecting piece comprising two oppositely opposed ends having a cross section corresponding to a cross section of a canal formed between longitudinally extending cross pieces of the ridge member, said connecting piece preferably being provided with wedge like bodies intended for engaging with holes provided in the cross pieces. 20. A ventilating device according to any of claims 8-10 where a band is stretched between rafter of a roof construction that the ventilating device is to be mounted to, where part of a non-rigid underroof is supported by the band, and where the flexible parts of the ventilating device abut the underroof at least where the band support the underroof. 21. A ventilating device according to any of the preceding claims where the ridge member is manufactured by extrusion and preferably is made of plastic.