All About Engines

Raising Steam 73 for lagging, but these substances are of small dura- bility and tend to char. They may even take fire, and Mr. Edgar Allen, in his book on “ The Modern Locomotive,” describes how lagging of this kind once took fire when he was riding on the footplate. They were just entering a tunnel, and as it was impossible to stop, he and the driver and fireman were almost suffocated by the cloud of acrid smoke before they emerged. It was not an experience which one would care to repeat. Water-tube Boilers All the boilers so far considered belong to the fire- tube class—that is to say, the hot gases pass through the flues or tubes surrounded by water. In this arrangement the fire gets very close to some of the water, but most of the water remains a long way from the fire. Moreover, the circulation of the water over the hot surfaces is irregular, except in the Lanca- shire and some vertical boilers in which water tubes are used. The great advantage for rapid steaming of having a small quantity of water very near to the fire was recognised by inventors for many years, and a number of different forms have been devised, but of these only a few have survived. One of the earliest, and still a very useful form, is the Stirling, illustrated diagrammatically in Figs. 38 and 39- It consists of three upper and two lower drums connected by curved tubes. The grate was placed in front and the hot gases were caused to pass in the direction shown by the arrows by means of baffle