In two experiments, we investigated the ability of participants to report the lengths of rods wielded in air or water, Homogeneous aluminum rods were employed in Experiment 1. The inertia of the rods was manipulated in Experiment 2 through the use of attached masses. Although the torques required in order to wield rods in water are substantially greater than those required to wield rods in air, the perceived lengths of rods wielded in the two media were very similar. Perceived length was found to be a function primarily of inertia in both media. The experiments also revealed a small influence of resistance due to the denser medium of water. The results demonstrate the ability of perceivers to extract a physical invariant from a complex array of forces. The discussion is focused on the role of invariants in dynamic touch.The present article is directed at the spatial abilities of dynamic touch. This is the kind of touching that takes place when an object such as a hockey stick, a hammer, a cup, or a pencil is grasped and carried, turned, swung, positioned, and so forth. It is the kind of touch that is dominant in everyday manipulatory activity: The hand and fingers are in contact with only a part of the manipulated object, and the object, both in stasis and in motion, affects the tensile states of the muscles, tendons, and ligaments of the hand and arm. The latter tissue deformations (induced by the forces and torques engendered by holding and manipulating) characterize dynamic touch, more so than deformations of the skin and changes in joint angles (Gibson, 1966). Traditionally, cutaneous touch (e.g., an object resting on the skin) and haptic touch (e.g., the hands enveloping an object and sweeping thoroughly and freely over its surfaces) have been the types of touch, or kinds of haptic subsystems (Gibson, 1966), that have received the most attention. In the service of manipulatory activity, and in the service of locomotionThe authors thank James Liburdy of the Clemson University Mechanical Engineering Department for his assistance with fluid dynamics, Ben Stephens and Eugene Galluscio for their assistance during the early stages of this project, and Geoffrey Bingham and Gregory Burton for comments made on an earlier version of the manuscript. Preliminary versions of these results were presented at