_Inclined Plane._—The advantage gained by the inclined plane, is as great as its length exceeds its perpendicular height. The force wherewith a rolling body descends upon an inclined plane, is to the force of its absolute gravity, as the height of the plane is to its length. _Wedge._—This may be considered as two equally inclined planes, joined together at their bases. When the wood does _not_ cleave at any distance before the wedge, there will be an equilibrium between the power impelling the wedge, and the resistance of the wood acting against its two sides; when the power is to the resistance, as half the thickness of the wedge at the back, is to the length of either of its sides; because the resistance then acts perpendicular to the sides of the wedge: but when the resistance on both sides acts parallel to the back, the power that balances the resistance on both sides will be, as the length of the whole back of the wedge is to double its perpendicular height. When the wood cleaves at any distance before the wedge, (as it generally does) the power impelling the wedge will be to the resistance of the wood, as half the length of the back is to the length of either of the sides of the cleft, estimated from the top, or acting part of the wedge. _Screw._—Here the advantage gained is as much as the circumference of a circle described by the handle of the winch, exceeds the interval or distance between the spirals of the screw. There are few compound engines, but what, on account of the friction of parts against one another, will require a third part more power to work them when loaded, than what is required to constitute a balance between the power and the weight. FERGUSON’S _Nat. Philosophy_. _MILE._—Comparison of the different miles, in geometric paces, each of which is equal to 5 feet French royal, 5.6719 feet Rhinland, or 6.1012 English feet. The mile of Sweden = 5761 geometric paces. ” ” ” Switzerland 4512 ” ” ” Denmark 4071 Common, of Germany 4000 ” ” Holland 3158 League of France 2400 ” ” Spain 2286 ” ” Scotland 1500 Mile of Italy 1000 ” ” England 868 Werste of Russia 575 _MINE._—The excavation formed by the blowing up of a mine is found by experiment to be nearly a paraboloid. It was formerly supposed that the diameter of the entonnoir, or excavation, was always equal to only double the line of least resistance; but experiments have proved, that the diameter of the excavation may be increased to six times the line of least resistance; and that the diameter of the globe of compression may be increased to eight times that line; this is called the _maximum_ of a mine, or the greatest effect that can be produced by a globe of compression. In any mine intended to produce an effect within this extent, the effects will be nearly as the charges.