WRITING AN ANALOGY
An analogy is an extended comparison between two things usually thought of as unlike. Analogies illustrate and explain by moving from the familiar to the unfamiliar, comparing several points, each of which has a counterpoint. For example, here is an analogy in which an engineering student explains something relatively unknown (loading a tanker) by using her knowledge of something known (filling pop bottles):
A tank truck usually holds between 4,000 and 6,000 gallons of gasoline. Depending on the tanker, three to six individual compartments hold 600 to 900 gallons of gasoline apiece. The tank that contains the compartments is elliptically shaped to distribute the pressure equally and to allow a more complete flow of air when the gasoline is delivered.
Until recently the only way to load a tanker was to climb up on top, where the openings to the compartments are located. You can easily picture this by visualizing six pop bottles lined up in single file on a table. A man wants to fill up bottle three, so he takes the cap off. He then inserts a small hose into the neck of the bottle and turns on a faucet which is connected to the hose.
A gasoline tanker is loaded in a similar way, but on a much larger scale. A man climbs on top of the tanker and opens a particular compartment by removing the cap. He then takes a hose with a four-foot metal pipe down into the "bottle" (the compartment hole), which measures four inches in diameter. A pump is then turned on, allowing the gasoline to flow into the compartment.
Know your audience
In the (admittedly unlikely) event her readers had no prior knowledge of pop bottles, however, this analogy might not be particularly informative. The writer chose this analogy based on the likely knowledge of her audience. When you construct an analogy, be certain that the familiar or known side of the analogy is really familiar and known to your reader. It is useless to explain a mineral's crystal-lattice structure by reference to analytic geometry if your reader knows nothing about analytic geometry.
All of us know many things that we can use to help a reader understand an idea better. Here a geology major shows how the oil seismograph works by comparing it to shouting at a cliff wall:
The oil seismograph is a small portable electronic instrument that detects and measures artificial earthquakes. The purpose of the instrument is to find geological structures that may contain oil. The oil seismograph instrument is not mysterious because it can be compared to shouting at a cliff wall.
Imagine yourself standing near the base of a large cliff. If you shout at the cliff face, you will get an echo because the sound waves bounce back from the "interface" where air meets rock. The sound waves travel at 1,100 feet per second. You can find out how far you are standing from the cliff by measuring the time it takes for your shout to travel from you to the cliff and back again, and then by solving a simple formula for distance.
The function of the oil seismograph is to find out how far down in the earth the horizontal layers of rock are. To discover this distance, the oil seismologist digs a deep hole (usually 100-200 feet). At the bottom of the hole, he explodes a heavy charge of dynamite. Ground waves travel from the explosion down to the layers of rock. At each major interface between the layers, the waves bounce back to the surface. The explosion is similar to shouting at the cliff. Just as sound travels through the air at a certain speed, ground waves travel through the earth, although much faster. Ground waves bounce from rock interfaces as sound waves bounce from a cliff face. And the seismologist can determine distance just as you can determine the distance between you and the cliff.
Know your limits
It is said that all analogies limp, that is, they are useful for illustration only as far as they remain reasonable. Therefore, do not try to stretch an analogy too far. Like the fabled camel who first put his nose in the man's tent, then his head and finally his whole body, pushing the man out of the tent, analogies can get out of control unless you know when to stop. Cut out or explain any points that cannot be logically compared.
For example, it might be a fair analogy to say that some professional athletes are treated like kings, that they receive special homage from the public and exemption from some rules, that they are more an expense and a pampered group than an asset to the community. But, except for comic effect, it would be overstatement to compare the equestrian charge of a king at an enemy with a football lineman's charge from the line of scrimmage. Likewise, it would be ridiculous to claim that modern athletes believe themselves divinely ordained to lead their country, or that professional athlete-ship is handed down from father to son by divine right. Just because certain similarities between athletes and kings exist, it doesn't follow that every kingly attribute manifests in modern-day athletes. Do not overconnect the subjects being compared.
Good analogies are vivid and logical, and while they cannot prove an argument, they can offer a picture that is very persuasive.