Probably you think you can tell all about a chimney, but you may be able to learn something interesting from this selection.

You should all begin reading at the same time. Your teacher will give the signal when to close your books. She will then ask you to write answers to the questions at the end.

A hollow tree was the first chimney of our unlettered forefathers. Accidentally set on fire, this tree illustrated the principle upon which all chimneys have been constructed. It showed that warm air, being lighter than cold air, tends to rise. When this warm air is confined within an enclosure open at the top and bottom, a strong upward current fills the space. As the warm air rises, the cold air rushes in through the opening at the bottom of the shaft, and in this way a draft is created which supplies the fire at the foot of the chimney with the oxygen it needs to support combustion.

Simple chimneys are constructed of logs and mortar, or of stones and mortar, such as those built for log cabins; of brick, also of cement and of iron pipes made for the purpose. Since a long column of hot air produces a stronger current than a short one, the tallest chimneys, other things being equal, produce the strongest draft. Tall chimneys are larger at the base than at the top. This is to make the structure stable and to increase the draft by contracting the flue at the top. At the bottom the chimney is usually connected with the fire by a flue. A fireplace, however, is practically an enlarged part of the chimney.

The size and height of a chimney depend upon the size of the furnace. For larger furnaces there is greater danger of making the flue too small than too large. The stacks or chimneys of the largest steamships like the Mauritania and the Imperator are so large that two railway trains could run through them abreast, and they are about 175 feet in length. The difference between a chimney and a smokestack is in name only; chimneys constructed of tubing made from iron plates are usually called stacks.

Questions

1. How does a difference between warm and cold air make chimneys useful?

2. Why is a tall chimney better than a short one?

3. How large are the chimneys or stacks on the largest steamships?