_Answer_. Yes; probably all the air that is admitted which is not necessary for combustion, or, in other words, the oxygen of which does not combine with the fuel, instead of increasing diminishes the amount of water converted into steam. It does this in two ways; first, by reducing the temperature of the gases in contact with the heating surfaces, and second, by increasing the volume or quantity of the gases which must pass through the tubes. Heat is transmitted through the heating surface of a boiler in proportion to the _difference_ of the temperature of the products of combustion on one side and the water on the other.[94] Thus, if the temperature of the water on one side is 250 degrees, and the hot gases on the other is 500, there will be only half as much heat transmitted to the water in a given time as there would be if the gases had a temperature of 750 degrees. If the volume of gases is doubled by the admission of too much air, then obviously in order to pass through the tubes they must move at double the velocity, so that not only is their temperature diminished, but the time they are in contact with the heating surface is diminished in like proportion. This is shown by the effect of opening the furnace door, or of allowing the fire to burn away so that portions of the grate are left uncovered. The volume of cold air which will in either of these cases enter the fire-box will be so great that the pressure of the steam in the boiler will begin to fall at once. [94] This law is perhaps not absolutely correct, but is near enough for our present illustration. QUESTION 387. _What determines the amount of air which must be admitted to the fire-box of a locomotive to effect perfect combustion?_ _Answer_. This depends chiefly upon the _rate of combustion_, that is, the number of pounds of coal consumed per hour on each square foot of grate surface. Of course if 100 pounds is burnt it will require twice the supply of air that would be needed if only 50 pounds were burnt. QUESTION 388. _How should the air be admitted so as to burn the coal perfectly?_