Means to enable an elongate boom (1) to maintain a constant attitude with respect to the ground, e.g. the boom of an agricultural spray apparatus comprising a carrier (12) on which the boom (1) is pivotally mounted for movement about a substantially horizontal axis (P1), a levelling member (3) pivotally mounted on the boom for movement about a substantially horizontal axis (P2) and linkage means (6) pivotally connected between the levelling member (3) and the carrier (12), the pivotal connections being disposed to cause the boom (1) to rotate at a desired angular velocity in response to rotation of the carrier (12).
