A jar for use in a downhole toolstring

A jar (10), for use in a downhole toolstring comprising: a hollow housing (11); a jar mandrel (13); a latch sub (14); one or more latch keys (16); a cam surface (17); a chamber (23); a compression spring; and an adjuster (24). The hollow housing (11) supports, moveably retained therein, the jar mandrel (13). The jar mandrel (13) and the latch sub (14) are releasably securable together by means of the one or more latch keys (16); and each latch key (16) is moveable between a latching position, in which the latching sub (14) and the jar mandrel (13) are connected together, and a release position permitting separation thereof. An adjuster portion (510 is threadedly connected to the jar mandrel (13) such that rotation of the adjuster mandrel (47) relative to the jar mandrel (13) alters the length of the chamber (23) and hence the degree of compression of the compression spring (24).