It has doubtless been observed by most of our readers, that the very dense black smoke which issues from the chimneys of steam engines and other furnaces, is not constant; that it commences at the time of putting on fresh fuel, and continues for a few minutes afterwards. At this time the air finds its way through the fuel with less opposition, and the evolution of dense smoke ceases until the next charge of coals. To supply the requisite quantity of air to burn this black smoke, the late Mr. Murray (the celebrated steam-engine manufacturer of Leeds), devised a very ingenious machine. It is described in a letter addressed to the Editor of the London Journal of Arts, dated February 15, 1821, wherein he observes, - "The most effectual method yet known for consuming smoke, is by the admission of a large quantity of air to the hottest part of the fire, at the time the smoke is bursting from the recent charging of coal. The necessary quantity of air to be admitted ought not to be less than may pass through an aperture of four square inches for each horse power that the boiler or fire is equal to; this will consume the smoke in from three to five minutes, according to the quantity and quality of coal put on at each time, - the times of charging being not more than five times in an hour, nor less than three.

The air rushing into the flue is the moving force for giving motion to my new regulating machine, which continues in motion during the consumption of smoke, but no longer." The machine may be thus described, without the drawings given in the work before alluded to. It consists of a light fan-wheel, from which proceeds a capacious tube, communicating with the fire-place, and containing a turning valve, which opens or closes the passage. When the fire-door is opened to take in fresh fuel, it discharges (by means of a slip catch connected to the door) a pall, which sets at liberty a suspended weight; the descent of this, turns a ratchet wheel, which places the turn-valve edgeways against the current, and leaves a free communication between the atmosphere and the upper side of the fire. In this state of rest the machine remains until the fire-door is shut, when the current of air enters the machine, turning rapidly the fan-wheel, which having a pinion on its axis of only one tooth, gives a slight motion to a light spur-wheel of many teeth; this wheel, through the medium of a catch-rod, and other simple mechanism, gradually closes the turn-valve.

The smoke having been consumed, the fire continues burning until a fresh supply of fuel is necessary, when the fire-door is opened, which puts the machine in a state for measuring off the required quantity of air to be admitted to consume the smoke of each charge; the operation is then repeated.