each of the probes 18 20 are preferably fluidly coupled to a cylinder or chamber 26 the chamber 26 preferably has disposed therein a piston 28 by displacing the piston 28 within the chamber 26 formation fluids are pulled through one or more of the probes 18 20 into the chamber 26 alternatively each probe could have its own piston and chamber except for acceleration and deceleration times of the piston 28 it is preferred that the movement of the piston and thus the increasing volume within its respective chamber provides a constant fluid flow rate from the formation referring still to fig2 preferably each of the probes 18 20 has an associated pressure transducer being pressure transducer 30 for the source probe 18 and transducer 32 for the vertical probe 20 a sequence for determining the permeability of the formation will now be described referring simultaneously to fig2 a and 3 preferably the wireline formation tool 10 is placed within the borehole at a particular depth level of interest the probes 18 20 are extended to be in fluid communication with the formation and likewise stabilizer 22 is extended to aid in providing sufficient force for sealing the probes 18 20 against the formation initially valve 34 is open and thus both the source probe 18 and the vertical probe 20 are in fluid communication with the chamber 26 and piston 28 as shown in fig3 the pressure transducers 30 32 initially sense the hydrostatic pressure within the borehole which in the exemplary drawing is shown by line 50 and is between 6 050.