Mechanics.

*57

common Centre of Gravity of the Earthand Moon will be found about 1854 Milesfrom the Earth’s Surface; and it is thiscommon Centre of Gravity that describesthe Annual Orbit about the Sun, and notthe Earth itself, as is commonly said andthought.

In like manner there is a common Centreof Gravity of the Sun and all the Planetswhich circulate about him ; and were theyall placed in a Eight Line on one Side theSun, then would the common Centre ofGravity of the whole System be distantfrom the Sun’s Surface eight "Tenths ofhis Semidiameter: And it is about thiscommon Centre of Gravity, and not aboutthe Sun, that not only all the Planets, buteven the Sun itself, do constantly move(XXXVI).

The

ing of this Point over the Rope that People can walkor fly thereon ; and that Tumblers and Equilibrists per-form such Wonders. In fine, it is this single Principlethat regulates eve;y Kind of Motion, both Animal andMechanical, and to which we are naturally prone tohave a much stricter Regard than we commonly thinkof; as would be evident were we nicely to con-sider the Gait, Posture, and Configuration of the Bodytinder the various Modes of Acting, viz. of Walking ,

Sitting, Riding, Stooping, Carrying, Lifting, Drawing ,

Pushing, Reaching, Striking, Throwing, ctc. and theparticular Manner in which Artists six a Wind-Mill,

Crane, or any such like Machine.

(XXXVI) j. If two Bodies A and B be connected PI. VII.together, by a Wire or Chain AB, they will each of Fig. 13.

them