FIFTY YEARS OF MECHANICAL ENGINEERING. 531 



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 scarce- 

 ly 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, sub- 

 jected to external pressure, is so strengthened that the danger of col- 

 lapse is removed even with our present high pressui'es. 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 work- 

 ing 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 govern- 

 ors which either throttle the supply-pipe or alter the point of cut-off, 



