3. To so arrange heating-surfaces that, without checking draught, the available heat shall be most completely taken up and utilized.
4. To make the form of boiler such that it shall be constructed without mechanical difficulty or excessive expense.
5. To give it such form that it shall be durable, under the action of the hot gases and of the corroding elements of the atmosphere.
6. To make every part accessible for cleaning and repairs.
7. To make every part as nearly as possible uniform in strength, and in liability to loss of strength by wear and tear, so that the boiler when old shall not be rendered useless by local defects.
8. To adopt a reasonably high “factor of safety” in proportioning parts.
9. To provide efficient safety-valves, steam-gauges, and other appurtenances.
10. To secure intelligent and very careful management.
In securing complete combustion, the first of these desiderata, an ample supply of air and its thorough intermixture with the combustible elements of the fuel are essential; for the second—high temperature of furnace—it is necessary that the air-supply shall not be in excess of that absolutely needed to give complete combustion. The efficiency of a furnace in making heat available is measured by
| E = | T - T′ | ; |
| T - t |