Abstract: A new and improved mechanical jar apparatus for delivering downhole blows in a wellbore in either of opposite directions in response to the controlled application of a longitudinal force. A plurality of mechanically released collets are employed to releasably secure the concentric telescoping members of the jar apparatus in the ready condition until the longitudinal actuation force is applied to move the telescoping members to the longitudinally extended or compressed jarring positions. The plurality of collets may be arranged for controlling the magnitude and direction of the jarring force without inadvertently triggering a jarring force in the undesired direction.