This section is from the book "The Tinman's Manual And Builder's And Mechanic's Handbook", by Isaac Ridler Butt. Also available from Amazon: The Tinman's Manual And Builder's And Mechanic's Handbook.

Mr. Tredgold gives the following Table, which will show how the power of the steam as it issues from the boiler, is distributed.

Let the pressure on the boiler be | 10.000 | |

Force required to produce motion of the steam in the cylinder will be | 0.069 | |

Loss by cooling in the cylinder and pipes | 0.160 | |

Loss by friction of the piston and waste | 2.000 | |

Force required to expel the steam into the atmosphere | 0.0G9 | |

Force expended in opening the valves, and friction of the various parts | 0.622 | |

Loss by the steam being cut off before the end of the stroke | 1.000 | |

Amount of deductions - | 3.920 | |

Effective pressure | 6.080 |

Let the pressure on the boiler be | 10.000 | |

Force required to produce motion of the steam in the cylinder | 0.070 | |

Loss by cooling in the cylinder and pipes | 0 160 | |

Loss by friction of the piston and waste | 1.250 | |

Force required to expel the steam through the passages | 0.070 | |

Force required to open and close the valves, raise the injection water, and overcome the friction of the axes. | ||

0.630 | ||

Loss by the steam being cut off before the end of the stroke | 1.000 | |

Power required to work the air pump | 0.500 | |

Amount of deductions - | 3.680 | |

Effective pressure. | 6.320 |

If we now suppose a cylinder whose diameter is 21 inches, the area of this cylinder and consequently the area of the piston in square inches, will be,

242 X .7854 = 452.39

Let us also make the supposition that steam is admitted into the cylinder of such power as exerts an effective pressure on the piston of 12 lbs. to the square inch; therefore, 452.39 X 12 = 5128.68 lbs., the whole force with which the piston is pressed. If we now suppose that the length of the stroke is five feet, and the engine makes 41 single or 22 double strokes in a minute, then the piston will move through a space of 22 X 5 X 2 = 220 feel in a minute; the power of the engine being equivalent to a weight of 5428 lbs. raised through 220 feet in a minute.

This is the most certain measure of the power of a steam engine. It is usual, however, to estimate the effect as equivalent to the power of so many horses. This method, however simple and natural it may appear, is vet, from differences of opinion as to the power of a horse, not very accurate; and its employment in calculation can only be accounted for on the ground, that when steam engines were first employed to drive machinery, they were substituted instead of horses; and it became thus necessary to estimate what size of a Steam engine would give a power equal to so many horses.

There arc various opinions as to the power of a horse. According to Smeaton, a horse will raise 22,916 lbs. one foot high in a minute, Desaguliers makesthe number 27,500; and Watt makes ii larger still, that is 33,000.

There is reason to believe that even this number is loo small, and that we may add at least 11,000 to it, which gives 41,000 lbs. raised one foot high per minute.-Grier.

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