Mechanics.

I 5S

Whence, since the Earth and Moonare to each other as about 40 to 1, and

the

the Distance of the Centre of Gravity from the VertexB in the Part D B E ; and so | of B G is the Point inthe Axis of the whole Parabola ABC, from the Ver-tex B.

15. The Phænomenon of the double Cone is easily p;„.accounted for, from what we have said of the Centre

of Gravity. For let A B D be the common Base ofthe two Cones, its Centre C will be the Centre ofGravity of the Whole ; therefore if D F be the Leg ofa Ruler elevated to an Angle F D G, whose Sine F G isless than the Semidiameter of the Cone C D, it is plainthe Centre of Gravity C at the Position-of the Cone inD is more distant from the Centre of the Earth than inits Position between the Legs of the Ruler at F ; andtherefore it will descend (as on an inclined Plane C F E)from C to F, where it will stop, as being supported onthe Ends of the Ruler.

16. Let A BED represent a Section of a Cylinder pjg^of Wood biafs’d on one Side with a cylindric Piece ofLead as B, this will bring the Centre of Gravity out

of the Centre of Magnitude C to some Point G be-tween C and B ; let F H be an inclined Plane, whoseBase is F L. It is evident the Cylinder laid upon thePlane will no where rest but there, where a perpendi-cular to the Horizon F L passes through the Centre ofGrav ty G, and that Point of the Plane E in whichthe C Under touches it; and this in all Angles of Incli-nation of the Plane less than that whose Sine is equalto C G (the Radius being C D) will be in two Situ-ations ABED and abed , because when the Cylindermoves, the Centre of Gravity describing a Circle roundthe Centre of Magnitude C, this Circle will meet thePerpendicular in two Points G and g, in each of whichthe Centre of Gravity being supported the Cylinderwill rest. Therefore the Cylinder moves from E to eby the Descent of the Centre of Gravity from G to g,in the Arch of the Cycloid G hg.

17. If