The commentary doubles as a synopsis and has therefore been reproduced in full:

Nobody bothered much about smoke before the war, but when our food supplies depended on the convoys getting through, smoke on the horizon gave away the position of shipping to the enemy and was a potential danger.

At the Fuel Research Station, scientists at the Department of Scientific and Industrial Research concentrated on the problem of cutting out smoke from ships' furnaces. They developed a device to admit extra air at the top of the fire when necessary. Thus the smoke, which consists of incompletely burnt fuel particles, was driven onto the flames and burned up. Most smoke is made immediately after the furnace has been fired or raked. At such times, the heat is not sufficient for complete combustion. So these were the times when the stoker had to open his extra air jets for a while until, as he could judge from his indicator, hardly any smoke got through to the chimney.

In peacetime, smoke at sea is not very important, except that it indicates waste. On land, it's different, though. Smoke pollutes the air we breathe, shuts out the sunlight, and defiles our cities with a daily increasing deposit of soot and filth.

One of the worst offenders is the widely used Lancashire boiler. With certain types of fuel, whenever it's fired or raked, dense clouds of smoke billow out. So the same principle was applied to the firing door of the Lancashire boiler. In the new design, air is supplied continuously through the two large jets. An extra supply can be admitted by opening this flap, through these holes and round the main jets. As the door is opened the flap falls, and the right amount of air is admitted to ensure that all the volatile tarry matter is burnt. As soon as the volatile matter has been burnt, the flap can be closed.

Important as it is to cut down smoke pollution, it's even more important to reduce wasted fuel in present times. So besides keeping an eye on visible smoke during boiler trials, everything must be accurately measured. The fuel is weighed, and its chemical composition and its heating value are determined in the laboratory. The weight and the temperature of the water are taken. The boiler is fired for a test period of anything from 8 to 24 hours, and all the time a minute-by-minute record is taken of everything that happens. Steam temperature and pressure, the pull of the chimney, the air pressure over the fires, the temperature and composition of flue gases, and the density of the smoke. These all show that where there is smoke, heat is lost, not only in the form of unburnt particles but also invisible gases.

So a simple device not only saves the air from pollution, but is also an aid to economy in the most precious of our raw materials - coal.