In general as thin fires are found necessary, the intervals of firing should be made more frequent and the quantity of coal fired at each interval smaller. As thin fires become necessary due to the character of the coal, the tendency to clinker will increase if the thickness be increased over that found to give the best results. There are two general methods of hand firing: 1st, the spreading method; and 2nd, the coking method. [Illustration: Babcock & Wilcox Chain Grate Stoker] In the spreading method but little fuel is fired at one time, and is spread evenly over the fuel bed from front to rear. Where there is more than one firing door the doors should be fired alternately. The advantage of alternate firing is the whole surface of the fire is not blanketed with green coal, and steam is generated more uniformly than if all doors were fired at one time. Again, a better combustion results due to the burning of more of the volatile matter directly after firing than where all doors are fired at one time. In the coking method, fresh coal is fired at considerable depth at the front of the grate and after it is partially coked it is pushed back into the furnace. The object of such a method is the preserving of a bed of carbon at the rear of the grate, in passing over which the volatile gases driven off from the green coal will be burned. This method is particularly adaptable to a grate in which the gases are made to pass horizontally over the fire. Modern practice for hand firing leans more and more toward the spread firing method. Again the tendency is to work bituminous coal fires less than formerly. A certain amount of slicing and raking may be necessary with either method of firing, but in general, the less the fire is worked the better the results. Lignites--As the content of volatile matter and moisture in lignite is higher than in bituminous coal, the difficulties encountered in burning them are greater. A large combustion space is required and the best results are obtained where a furnace of the reverberatory type is used, giving the gases a long travel before meeting the tube surfaces. A fuel bed from 4 to 6 inches in depth can be maintained, and the coal should be fired in small quantities by the alternate method. Above certain rates of combustion clinker forms rapidly, and a steam jet in the ashpit for softening this clinker is often desirable. A considerable draft should be available, but it should be carefully regulated by the boiler damper to suit the condition of the fire. Smokelessness with hand firing with this class of fuel is a practical impossibility. It has a strong tendency to foul the heating surfaces rapidly and these surfaces should be cleaned frequently. Shaking grates, intelligently handled, aid in cleaning the fires, but their manipulation must be carefully watched to prevent good coal being lost in the ashpit.