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Let us consider the task of comprehending how a fire extinguisher works from the following description that appears on page 147 of The Way Things Work:

An extinguisher puts out a fire by excluding oxygen so that combustion (see p.154) can no longer continue. The extinguisher must smother the whole fire as quickly as possible, and therefore produces a powerful spray of water, foam, or powder. Some extinguishers produce a jet of carbon dioxide, a heavy gas that prevents burning. A fire extinguisher works in much the same way as a spray can. The extinguishing substance, such as water, is put under high pressure inside the extinguisher, and the pressure forces the substance out of the nozzle.

This text is accompanied with a cutaway diagram of a fire extinguisher revealing its structure, and some brief descriptions of the individual components such as the gas cartridge and the release valve. Figure 1 illustrates this diagram. Note that the annotations on the diagram are more specific to the structure of the extinguisher shown in the diagram than to any part of the text itself. The text describes the behavior of the fire extinguisher, making explicit reference to the descriptions of combustion and spray cans. The reference to the concept of combustion is a forward reference; presumably the reader has not yet read it but may do so for further elaboration and specification. But the spray can is described on the previous page of the book (p. 146), just opposite to the description of the fire extinguisher.

Figure 1: The Fire Extinguisher.

Kavi Mahesh
Mon Sep 30 15:11:01 MDT 1996