38.5 + 4.8 = 43.3 inches,
as the inside diameter of boiler, whence the following locomotive to meet the requirement as stated.
| Weight upon driving wheels, | 16,938 lbs., |
| Cylinders, | 12 × 12 inches, |
| Wheels, | 5 feet, |
| Tubes, | 135—11 feet × 2 inches, |
| Grate, | 13.16 square feet, |
| Barrel, (inside diameter,) | 43.3 inches, |
and under the most favorable circumstances, the chimney may be 40 inches high, 12.7 inches in diameter; the blast orifice 5.8 inches in diameter; and the capacity of smoke box 39½ cubic feet.
363. We may vary the tractive power of an engine by using the steam at a greater or less degree of expansion, but the adhesion remains the same. If an engine was built able to work a road partly level, and partly on steep grades, varying the power simply by varying the expansion, it would be unnecessarily heavy for the easy parts of the road. The expansive principle may be advantageously employed in adjusting the power to the difference of resistance on any one division of a road, and also to the varying load which each day’s traffic will present.
Suppose we would move a load of two hundred tons over the road below; and suppose, also, that we require the cylinder pressures set opposite the several divisions.
| 10 miles, level, | 60 lbs., |
| 10 miles, 10 feet per mile, | 80 lbs., |
| 10 miles, 20 feet per mile, | 100 lbs., |
| 10 miles, 30 feet per mile, | 120 lbs. |
The boiler pressure being 150 lbs., and the pressure at entering the cylinder 145 lbs.,
An admission of 71 per cent. gives a mean pressure of 120 lbs.,
An admission of 55 per cent. gives a mean pressure of 100 lbs.,