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 becomes no considerable addition; while the power of traction on a level railroad is so small, that the increase produced by the smallest inclination is severely felt.

That a railroad should be effective, it is therefore necessary that a propelling power should be used capable of great variation in its intensity, or that additional powers of traction should be provided at every inclination, or, finally, that, in the original construction of the road, a level be maintained as near as possible, and in no case should the inclination exceed 14 feet in a mile. Valleys must, therefore, be traversed by embankments or aqueducts, and hills intersected by artificial chasms of open cutting. To penetrate them by tunnels, except in very rare cases and short distances, is inexpedient; for the travelling steam-engine generally used on railroads cannot be used in a tunnel, owing to the air being rendered unfit for breathing by the effect of the fire. Besides the resistance that the air in the tunnel will give to the carriage passing through it, even were tunnels practicable, the great original expense of construction forms a strong objection.

A turnpike road, on the other hand, is usually carried in a winding course, through an undulating country, avoiding hills of great acclivity; and though the length will be thereby increased, yet the total expenditure of the power of traction will be diminished. The power of traction necessary on common roads in different states of repair, or differently constructed, is subject to great variation. Experiments