A correspondent, who has practically investigated this subject extensively, says: Slack requires the prate-bars to be very open. I have used bars with openings of 1-1/2 inches. The only secret in using slack with any kind of a furnace is to have the grate-bars open enough so that the fire can be kept open from the under side of the grates with the poker. Some coal, of course, will go through at first; but coarse coal and wood can be used to start with, and what falls through the grate must be raked out and put in again. The coal will soon cake so that it will not waste. Make the furnace wider than usual, in building it, with doors in the side of the front, similar to furnaces for burning sawdust. For some varieties of coal, it will be found beneficial to wet the coal before throwing it into the furnace; this helps to run the coal together. Then put in the coal at the side doors, and let it alone till it cakes; then with the poker roll it into the centre of the fire. It will be in large lumps and will not waste, and there will always be a good fire in the centre. .Never smother it with fresh coal. A system of introducing comminuted fuel with the air required for its combustion, by means of a fan-blower, has been introduced by a Boston firm.