To represent a Man moving a Burthen, consider that the Motion may be made in different Lines; viz. either from below upwards, with a simple Motion, as in heaving a Weight from the Ground; or from behind forwards, as in drawing a Weight after him; or simply forwards, as in shoving any thing before him: Or, Lastly, From above downwards, as in pulling at a Rope which plays in a Pully. Here it must be remarked that the Weight of a Man's Body draws so much the more, as the Centre of his Gravity is further distant from the Centre of the Axis which sustains him: To this, you must consider the Effort wherewith the Reins, and Legs, when bent, strive to recover their Straightness; and that a Man neither Ascends, Descends, nor Walks in any other Line whatever, without drawing up the Heel of the hind Foot.