Sentence:  A force pump comprising a well casing having a bottom packing ring, a cylinder held in said ring and having a substantially closed upper and lower end, inlet ports in said cylinder below the ring and outlet ports in said cylinder above the ring, a pull rod extending into the cylinder, a lower piston in the cylinder fixed to the lower end of the rod, an upper piston slidably mounted on the rod, means in said cylinder for complementary engagement of means carried by the upper piston for retaining said upper piston above the inlet ports upon <WSD>seatment</WSD> of the lower piston on the lower end of the cylinder whereby fluid entering the cylinder is received between the pistons, said lower piston moving responsive to an upward pressure thrust relative to the upper piston upon engagement of the upper piston with the upper end of the cylinder to force the fluid retained between the pistons through the outlet ports into the casing. 
Meaning of the word marked with <WSD> tags:  The act or manner in which something has been seated; placement.