Abstract: Apparatus for altering the length of a down-hole tool assemblly is described. The apparatus comprises a main housing (1), a slide (13) movable within the housing between a retracted position and an extended position, and latching means (6,755,33,41) to releasably secure the slide (13) to the housing (1) when the slide (13) is in either the extended position or the retracted position. The latching means (6,7,33,41,55) is activated and de-activated by fluid pressure within the apparatus where the apparatus forms part of a drill string, the fluid is typically drilling mud which is pumped through the drill string and the apparatus and the pressure is changed by changing the rate of mud flow through the drill string.