the points of intercourse are connected only by a single line; but, in cases of great intercourse, two lines are sometimes provided for carriages moving in opposite directions, in which the delay produced by carriages meeting in opposite directions is avoided.
The power of traction required on a well-constructed level railway is generally estimated at the 240th part of the load drawn. The smallness of this proportion gives rise to a consequence of great practical importance when inclined planes occur; as must always be the case at points where the level of the country road changes. In addition to the ordinary resistance of the rails, the power of traction in ascending must overcome the tendency of the load to descend by its gravity. This tendency, as is well known, bears a proportion to the load equivalent to the elevation of the plane. If the plane rise 1 foot in 100, the tendency of a load of 100 tons to descend will be 1&1nbsp;ton. Upon this principle, if the plane rise 1 foot in 240, the power of traction, compared with that which is necessary upon a level, will be double. An ascent of 2 feet in 240, or 1 in 120, will require a three-fold power of traction; an ascent of 3 feet in 240, or 1 in 80, will require a four-fold power of traction, and so on. Hence it is obvious how enormously the drawing power must be increased even by the slightest incurvation. An ascent of 1 in 240, or 17 feet in 1400 yards, which requires the power of traction to double its energy, is scarcely perceptible to the eye; and the rise of 1 in 96 at Rainhill, on the Manchester line, which is barely perceivable, requires the power of traction to increase its intensity in nearly a four-fold proportion. It follows, therefore, that whatever agent may be employed as a propelling power on a railroad having incurvations upon
