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Page:Popular Science Monthly Volume 24.djvu/547

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FIFTY YEARS OF MECHANICAL ENGINEERING.

laws relating to the strength of material, but were well adapted for the convenience of the firemen, in that the flues were of such size that a man could pass through them to remove accumulated soot.

The result was, that the boilers were incapable of withstanding an internal pressure of more than four or five pounds to the square inch. The low pressure made a large cylinder necessary to secure the required power, and the size of the cylinder restricted the speed, which rarely exceeded 250 feet a minute. The boilers were commonly fed by a tank situated high enough to enable the water to overcome the pressure of the steam. The low pressure and slow piston-speed necessitated very large cylinders relatively to the power obtained. The consumption of fuel was about ten pounds to the one horse-power per hour.

The governing was done by slowly-revolving pendulum-arms scarcely securing centrifugal force enough to raise the balls and actuate the butterfly-valve in the steam supply-pipe, thus making a very poor and inefficient governor. The low speed made a very heavy fly-wheel necessary to secure uniformity of motion, also costly trains of gear-wheels to secure the rotative speed required for factory-work.

In 1882 the boilers are cylindrical, frequently internally fired, and, thanks to Sir William Fairbairn's circumferential bands, the flue, subjected to external pressure, is so strengthened that the danger of collapse is removed even with our present high pressures. The tendency of the day seems to incline toward the water-tube sectional type of boiler and a rational system of inspection and test. The pressures in use to-day vary from 80 to 150 pounds. The piston-speed is nearer 500 feet per minute, often 800 and 1,000. An engine of 1832 capable of exerting 25 one horse-power to-day would indicate about 250 working under fair conditions. The same expenditure of fuel to-day would give nearly four times the power.

The decrease in size of the cylinder due to the higher pressures has made higher rotative speeds possible; hence, the engine requires a much lighter fly-wheel, and the governing is made more effective. The most efficient engines of to-day are found in our city pumping-stations. Here the conditions are favorable for securing the highest economy, a duty of 100,000,000 foot-pounds being frequently secured. The engine of to-day for mill-use is, comparatively speaking, a portable engine requiring nothing but a foundation to bolt it to. The engine of fifty years ago was not self-contained or self-supporting, but required to be built from the ground up, and the support of walls and timbers.

To-day the practice is to make large engines condensing and often compound, expanding the steam in some instances ten volumes. The higher pressures and rotative speeds of to-day have made the use of high expansions possible in comparatively small engines, and economies are secured which, but a few years ago, would have been wonderful for large engines. The governing is done by quick-running governors which either throttle the supply-pipe or alter the point of cut-off,