236 THE MATHEMATICAL PRINCIPLES [BOOK 1. contents u.ider A X AZ, and B X BZ. Join AB, and let it be cut in G, so that AG may be to BG as the particle B to the particle A : and G will be the common centre of gravity of the particles A and B. The force A X AZ will (by Cor. 2, of the Laws) be resolved into the forces A X GZ and A X AG ; and the force B X BZ into the forces B X GZ and B X BG. Now the forces A X AG and B X BG, because A is proportional to B, and BG to AG, are equal, and therefore having contrary directions de stroy one another. There remain then the forces A X GZ and B X GZ. These tend from Z towards the centre G, and compose the force A + B X GZ ; that is, the same force as if the attractive particles A and B were placed in their common centre of gravity G, composing there a little globe. By the same reasoning, if there be added a third particle G, and the force of it be compounded with the force A -f B X GZ tending to the cen tre G, the force thence arising will tend to the common centre of gravity of that globe in G and of the particle C ; that is, to the common centre oi gravity of the three particles A, B, C ; and will be the same as if that globe and the particle C were placed in that common centre composing a greater globe there ; and so we may go on in injinitum. Therefore the whole force of all the particles of any body whatever RSTV is the same as if that body, without removing its centre of gravity, were to put on the form of a globe. Q,.E.D. COR. Hence the motion of the attracted body Z will be the same as if the attracting body RSTV were sphaerical ; and therefore if that attract ing body be either at rest, or proceed uniformly in a right line, the body attracted will move in an ellipsis having its centre in the centre of gravity of the attracting body. PROPOSITION LXXXIX. THEOREM XLVI. If there be several bodies consisting of equal particles whose jorces are as the distances of the places from each, the force compounded of all the forces by which any corpuscle is attracted will tend to the common centre of gravity of the attracting bodies ; and will be the same as if those attracting bodies, preserving their common centre of gravity, should unite there, and be formed into a globe. This is demonstrated after the same manner as the foregoing Proposi tion. COR. Therefore the motion of the attracted body will be the same as if the attracting bodies, preserving their common centre of gravity, should unite there, and be formed into a globe. And, therefore, if the common centre of gravity of the attracting bodies be either at rest, or proceed uni formly in a right line, the attracted body will move in an ellipsis having Us centre in the common centre of gravity of the attracting bodies.