And just to follow up with the final post in this trilogy on mixed metaphors (after this and this), here is the exegesis of what goes on when you commit this crime.
1) The writer
From the writer’s point of view, the reason for mixing metaphors is usually fear and laziness, a toxic cocktail.
The fear is that whatever he is writing is not interesting enough in and of itself. It is not a murder mystery, but a friggin’ merger of two banks, for instance. Or a wobble in the stockmarket. Or something else that Truman Capote never chose to write about. So, out of insecurity, he and his editor dramatize. They do that by aggrandizing the thing in question with words from more primal situations. Mergers become either “takeover battles” or “marriages.” Divestitures become “divorces”. Usually, war, love, sex, floods, fires, mountains and geology (“erosion,” “tectonic shifts,” “rifts”) find their way into the passage. And so on.
The laziness consists of not even noticing. They stuff these templates of primal experience into their paragraphs and don’t bother to think about what the words actually mean.
2) The reader
When the reader sees the outcome, he has one of two reactions. If sophisticated, he will notice the mixed metaphors and lose respect for the writer, usually in excess of what is justified, and probably stop reading the article, or at least taking it seriously.
If less sophisticated or hurried, he may not notice the mixed metaphor per se. But something more insidious now occurs. The different metaphors (floods, fires, quakes, wars) will cancel one another out in his mind. Instead of evoking strong and specific images, which is what metaphors are supposed to do, they produce a verbal goo. Its effect is tedium. The text loses energy and the reader gets tired and bored. The subject that the writer feared was not sufficiently interesting is now even less interesting. It’s excruciating.
3) The solution
In 99% of all cases, the solution is for the writer to address first the laziness (because that’s easier) and then the fear. To stop being lazy, just get in the habit of loving words and seeing their original meanings. To stop being afraid, get in the habit of forgetting about your audience entirely. You must find your subject interesting (otherwise, why not choose something else to write about?), and so it simply is interesting.
At this point, you’re ready to take all metaphors out of your text and see what’s left. Usually, it will be much better. The big secret about metaphors is that you don’t usually need them at all! Other details, from direct observation, take over.
Then, if you really feel something is missing, choose something evocative–but just one single image for the entire article–and stick with that. It doesn’t need to be “literary”. I heard a Congressman complain about the “bailout” package last week by saying that it was “a giant cow patty with a little marshmellow in the middle of it”. He didn’t even need to spell out that he was not going to swallow the marshmellow (by voting Yes), given what it was served in. Now that’s an effective metaphor, don’t you think?
Hannibal and Me: life lessons from history 
What do you mean by “original” meanings of words in light of semantic drift? How far back are we to go to determine an original meaning? The word “silly,” for instance, used to mean “blessed” (related to the German selig) before it passed through various permutations in meaning until settling on “foolish,” at least for the time being.
Are you talking about a “common sense” sort of quasi-original meaning rather than an paleo-etymological one, by which standard “silly” indeed means “foolish”?
Many words didn’t simply change meaning but underwent an extension in lexis, i.e., over time they assumed a range of new meanings while retaining their original one (other than “silly,” which simply lost its original meaning and assumed a different one). So for instance, “storm” originally may have denoted an atmospheric disturbance manifested in strong winds, but by now it also means a disturbance/upheaval of any kind.
So the question is, if I use the term “storm” to refer to an upheaval in the stock-market, is it still a “metaphor” or is it already a word that quasi-originally means a disturbance of any type?
If we keep using certain metaphors, don’t they at some point stop being metaphors by assuming abstract “original” meanings in addition to their “original” original ones?
Very good point.
Let’s take your two examples: silly and storm.
In the case of silly, the “blessed” meaning has vanished. So the word has changed meaning, full stop. It is no longer a metaphor or pun.
Storm, however, has not changed meaning. It sill means an atmospheric disturbance manifested in strong winds. If you apply it to the markets or politics or a relationship, it is therefore a metaphor. Its strength is that it CAN make us think of an atmospheric storm. So you would not want to make, for instance, the “storm in your marriage evaporate”. Nor should the storm ebb or flow, or caress, or hit, or do anything else that atmospheric storms just don’t do.
the key is to use empathy. Every word evokes some sensual reaction. Which one? You have to be aware of what synapse in the reader’s brain will fire. Storm: My brain remembers being tossed about, sand in my eyes, hair flapping….
The other thing to say, of course, is that storm is used rather a lot. There may be a BETTER metaphor than storm. But maybe not…
Be aware of what synapse in the reader’s brain will fire … like go up in flames?
I know what you’re saying: to keep in mind whatever the reader may associate with a particular word. It’s just that sometimes certain poetic expressions simply scan better than others that may be more accurate in terms of original meaning, or vice versa. Certain vowels may sound better in combination with those in surrounding words.
For similar reasons, I suppose, we love those blessed … um, I mean, silly … clauses like “Let me add” and “I’d like to say,” simply because our internal sense of rhythm calls for a few extra syllables that add nothing of substance to the discussion.
Technically, the synapse would have sparked, not fired, I suppose.
But on the matter of “Led me add”, “I’d like to say”, and so forth: I am categorically against all of those fillers (they are not metaphors). What they do is buy us time while speaking. But in writing they should be cut.
There are chemical synapses and electrical synapses. I guess only the latter fire or spark. Chemical synapses release neurotransmitters. So some synapses fire, others spit. Given that, I suppose, both types are involved in all cognitive processes, perhaps we should say that synapses spitfire. Or spitspark.
So, how would you rate these, Andreas?
“… If you can fill the unforgiving minute
With sixty seconds’ worth of distance run…”
and
“This royal throne of kings, this sceptered isle,
This earth of majesty, this seat of Mars,
This other Eden, demi-paradise,
This fortress built by Nature for herself
Against infection and the hand of war,
This happy breed of men, this little world,
This precious stone set in a silver sea…
This blessed plot, this earth, this realm, this England,
This nurse, this teeming womb of royal kings…”
The first is a great unmixed Kipling metaphor.
The second is Shakespeare in a metaphor spasm. Yes, he’s mixing them up. But perhaps he wants to recreate Richard II’s excitement. In direct speech we often list the many ways we love something. That said, if this passage appeared not in a direct speech in a play but in a non-fiction essay, I might look askance at it.
Over to you.
Does this mean we are in a position to establish a comprehensive definition of a mixed metaphor?
You have hereby volunteered to provide it.
Let’s have it.
I regret I cannot do so because I do not know why the Kipling extract is an unmixed metaphor. Has he filled an unforgiving minute with a distance or deftly avoided doing so by use of the word “worth”? Or perhaps the minute and the sixty seconds are a measures of angle (rather small ones).
Since you have satisfied yourself in this regard, I must defer to you for a definition.
[Implied smiley.]
Well, a minute is sixty seconds. so far so good.
And you can run for sixty seconds, so that is coherent. (Ie, we have motion during a time frame. Fine, no?)
What you’re flagging is the “unforgiving”. Can a minute forgive? Not really, I admit. But it doesn’t bother me here, because what is unforgiving is the ticking clock. It stops at 60, whether you have a good excuse or not. So it’s unrelenting, unforgiving.
I don’t know. It’s dangerous to get too left-brain about it, but I still think the metaphor works, don’t you?