Miles. Jupiter 16,572 Saturn 18,678 Sun 15,552 Etna 17,763 Ajax 11,678 Firefly 15,608 ------ Making in all 95,851 ------ (96.) The advantages derivable from railroads are greatly abridged by the difficulty arising from those changes of level to which all roads are necessarily liable; but in the case of railroads, from causes peculiar to themselves, these changes of level occasion great inconvenience. To explain the nature of these difficulties, it will be necessary to consider the relative proportion which must subsist between the power of traction on a level and on an inclined plane. On a level railroad the force of traction necessary to propel any load, placed on wheel carriages of the construction now commonly used, may perhaps be estimated at 7-1/2 pounds,[28] for every ton gross in the load; that is to say, if a load of one ton gross were placed upon wheel carriages upon a level railroad, the traces of horses drawing it would be stretched with a force equivalent to 7-1/2 pounds. If the load amounted to two or three tons, the tension of the traces would be increased to 15 or 22-1/2 pounds, and so on. The necessity of this force of traction, arising from the want of perfect smoothness in the road, and from the friction of the wheels and axles of the carriages, must be the same whether the road be level or inclined; and consequently, in ascending an inclined plane, the same force of traction will be necessary in addition to that which arises from the tendency of the load to fall down the plane. This latter tendency is always in the proportion of the elevation of the plane to its length; that is to say, a plane which rises 1 foot in 100 will give a weight of 100 tons a tendency to fall down the plane amounting to 1 ton, and would therefore add 1 ton to the force of traction necessary for such a load on a level. [Footnote 28: The estimate commonly adopted by engineers at present is 9 pounds per ton. I have no doubt, however, that this is too high. I am now (November, 1835,) engaged in an extensive course of experiments on different railways, with a view to determine with precision this and other points connected with the full developement of their theory; and I have reason to believe, from the observations I have already made, that even 7-1/2 pounds per ton is above the average force of traction upon the level.]