boring 62 extending through the piston 60 parallel to the longitudinal axis of the piston rod 18 this boring 62 provides a further fluid flow passage interconnecting the portions of the cylindrical chamber 20 at opposite sides of the piston 16 and allows a restricted flow of the fluid past the piston 16 in this embodiment of the present invention the annular seal 32 is replaced by the annular seal 38 by this means the piston 16 is enabled to move gradually toward the housing wall 22 consequently when this modified piston 60 is substituted for the piston 16 in the holdown 10 the latter acts as a dampener 0038 fig7 illustrates a further modified piston indicated by reference numeral 64 which is identical to the piston 60 of fig6 except that the piston 64 is additionally provided with a flow control screw 66 in the form of a grub screw which can be screwed into a correspondingly threaded opening 68 in the piston 64 when thus inserted into the piston 64 the flow control screw 66 can be adjusted so as to obstruct to a greater or lesser extent the cross sectional area of the boring 62 and thus so as to adjustably restrict the flow of fluid through this further flow passage and thereby to control the damping effect of the piston 64 0039 fig8 shows a view in perspective of the holdown 10 of fig1 in an assembled condition 0040 fig9 shows a view corresponding to that of fig8 but illustrating a modification of the holdowns of fig6 and 7 indicated generally by reference numeral 10 a in which the piston 16 is provided i e the piston lacks the further flow passage formed by the boring 62 in the.