If, now, we have the resistance which is due to the _ascent or gravity alone_, we must add to this the resistance on a straight and level track, at the speed at which the train runs, in order to determine the total resistance on the grade. On a level road at a speed of 5 miles per hour it would be 6.1 lbs. per ton, so that on a grade of forty feet to a mile at that speed the resistance would be 6.1 + 15.1 = 21.2 lbs., per ton, and at 10 miles it would be 6.6 + 15.1 = 21.7 lbs., and at 30 miles per hour on the grade the resistance would be 11.2 + 15.1 = 26.3 lbs. per ton. To get the total resistance on a grade for any speed, we ADD THE RESISTANCE FOR THAT SPEED ON A STRAIGHT AND LEVEL LINE TO THE RESISTANCE DUE TO THE ASCENT ALONE. The resistances for various rates of speed and grades has been calculated, and is given in the table in the appendix. The top horizontal row of figures of that table gives the rates of speed. The left-hand vertical row gives the rise of grade in feet per mile. The resistance for any given grade and speed is given where the vertical row of figures under the rate of speed and the horizontal row opposite the rise of the grade intersect each other. Thus, for a grade of 30 feet per mile and a speed of 45 miles per hour, we follow the vertical column under the 45 downward, and the horizontal column opposite 30 to the right, and where the two intersect the resistance, 29.1 lbs. is given. QUESTION 409. _What effect do curves have on the resistance of trains?_ _Answer._ They increase the resistance, but in what proportion or to what degree is not known accurately. European authorities say that the resistance is increased, over what it would be in a straight and level line, about 1 per cent. for every degree of the curve occupied by a train. It is probable, however, that the resistance of American cars, which nearly all have double trucks, is not so great on curves as that of European cars, which nearly all have long and rigid wheel-bases, and whose wheels therefore can not adjust themselves so easily to the curvature of the track as they can when the American system of double trucks is used. QUESTION 410. _What is meant by a degree of a curve?_ _Answer._ In order to measure circles, they are all supposed to be divided into 360 equal parts, which are called degrees. One degree of a curve is therefore ¹⁄₃₆₀ of a complete circle; but if the curve has a long radius, one degree of such a curved track will be longer than one degree of a curve with a short radius, but each will have the same amount of “bend” or curvature. It is this latter which increases the resistance of trains, and the greater the number of degrees of a curve occupied by a train of cars, the greater will be the “bend” of the track, and therefore the greater the resistance. QUESTION 411. _What other causes affect the resistance of trains?_