There's been a lot of discussion in the criminal defense bloggosphere about the role of the criminal defense lawyer. Basically, prosecutors are supposedly charged with seeking justice. The question naturally arises then, are criminal defense lawyers also supposed to seek justice? And if so, what is their duty to their client?

As near as I can tell, it started with a comment left by John Kindley at Defending People, which he expanded into a post on his blog (followed by several more) which quotes Vincent Bugliosi, which lead to a rebuttle by Mark Bennett and comments by Scott Greenfield, Jeff Gamso, Norm Pattis, Gideon, and probably a whole bunch of other bloggers (feel free to drop a link in the comments if I missed you).

I figure I might as well take a shot at it and explain what I think criminal defense lawyers are supposed to be doing. Naturally, I see a science angle. And it has something to do with toilets. I'm going to digress a bit before I come to my point.

You may have heard the science factoid that toilets in the southern hemisphere flush clockwise and toilets in the northern hemisphere flush counter-clockwise. The supposed reason for this is the Coriolis effect, which is caused by the Earth's rotation.

Basically, everything on the surface of the Earth revolves eastward in a circle around the planet's axis. However, objects closer to the equator have to follow a longer path than objects that are closer to the poles. Since everything has the same amount of time to complete a rotation, the objects taking the longer path -- those closer to the equator -- must be moving faster.

Now imagine an object that starts near the equator and begins moving northward. At the equator, both the object and the land beneath it are moving eastward with the Earth's rotation. As it moves north, however, it passes over land that is closer to the pole and therefore moving eastward slower than at the object's starting point. The object will still have it's original eastward velocity, so it will tend to get ahead of the land below and begin to drift eastward. Similarly, an object moving southward will be starting with a slower speed and therefore fall behind, drifting to the west.

In the southern hemisphere, the exact same thing happens except that the directions are reversed because the rotational speed at the surface declines as you get closer to the south pole.

When you flush a toilet, all the water in the bowl flows toward the center. The water in the northern half of the bowl moves south, and therefore Coriolis forces push it to the west. The water in the southern half of the bowl moves north and gets a shove to the east. Thus, the story goes, the water is pushed in a counter-clockwise direction, and that's the way toilets drain in the northern hemisphere. And vice versa in the southern hemisphere.

At least that's the theory. But does it really happen? Do toilets really flush in different directions in different hemispheres?

No, as you can easily determine for your self by watching a bunch of different toilets. The reason is simple: Toilets are too small.

The Coriolis effect works quite well with very large things because the speed difference is much greater over large distances.  The low pressure zone in the center of a hurricane pulls air in from all directions for hundreds of miles, and the northern and southern edges are far enough apart that the Coriolis effect induces a gigantic spinning cyclone with air speeds that can easily reach over a hundred miles an hour. Even better, hurricanes spin clockwise in the south, and counter-clockwise in the north, exactly as we'd expect from the Coriolis effect.

On the other hand, the surface of the water in a toilet is less than a foot across. When you do the math, it turns out that as the toilet follows the Earth's daily revolution, the maximum possible speed difference between opposite edges is only about three inches per hour. That's so small that it's overcome by water movement caused by vibration of the floor or air currents stirred up by somebody moving around in the same room as the toilet. Even worse, the currents set up in the toilet when it fills (or when certain other materials are added to the bowl water, if you know what I mean) can persist for hours and have enough inertia to resist Coriolis forces. Finally, imperfections in the shape of the bowl can cause the water to flow in a preferred direction that has nothing to do with Coriolis forces. So, in theory, the Coriolis effect is there, but it's too small to detect in a real-world toilet.

Scientists have tested this under laboratory conditions, however, and it really does work: When the run the test in the southern hemisphere, the water picked up a gentle clockwise swirl every time. Repeating the experiment in the northern hemisphere, the water swirled counter-clockwise.

The experimental setup is interesting (meaning I'm finally getting to the point of this post). The scientists wanted to eliminate all sources of water motion that might hide the Coriolis effect. They began by setting up their apparatus on a base that isolated it from vibration and in a room with no vents or heat sources to cause air motion. Instead of an off-the-shelf toilet bowl, they carefully constructed a funnel about six feet across and as near to perfectly round and smooth as possible.

The scientists knew that after filling the bowl with water, they'd have let it sit for several hours to allow the residual currents to die down. But just to make sure the residual currents didn't contaminate the experiment, they did one more thing: They filled the bowl in the opposite of the expected direction of spin, so that the water was initially swirling the wrong way. If any of that swirl remained when the drain was opened, it would not cause a false positive result.

A criminal trial is a bit like that experiment. It's a highly artificial environment created to get an answer to an important question. It's isolated from the real world, and an attempt is made to remove distracting influences which might lead to an erroneous result. As for criminal defense lawyers, they're that swirl in the wrong direction.

You see, by starting with the water swirling in the wrong direction, the scientists added an influence that would have produced the the exact opposite result from the hypothesis they were testing. This opposing swirl would have to be overcome by the forces hypothesized to be at work in order to get the expected result.

That's the role that criminal defense attorneys play in our justice system: They oppose the government's theory that their client is guilty. This opposition is necessary so that if the government achieves the result it's hoping for -- a guilty verdict -- we can be confident that it was arrived at for the right reasons.

If my explanation of the role of criminal defense attorneys doesn't seem very original, that's because it's not. It's basically a variation on "putting the government's case to the test" or "ensuring procedural due process." I'm sorry if you were expecting something more enlightening or profound. I've no reason to seek a novel explanation when the existing explanations are so good. But I do hope you found the science of toilets and hurricanes a bit entertaining.

I'm not saying that this is the only role a defense attorney should play in the system. There are other ways to serve justice in the larger sense, and there are other ways to serve their clients, and doing either of those things need not compromise their role as the opposition.

Nor am I saying that this is what motivates criminal defense lawyers on a personal level. They may serve out of compassion for the oppressed, abhorrence of the urge to punish, thirst for social justice, or desire to be a badass. But whatever their reason and whatever else they do to serve clients or justice, they also serve the purpose of justice simply by standing in defiance.

Do note, however, that the need to swirl the water in the wrong direction has nothing to do with whether or not the Coriolis effect is an accurate theory. Similarly, the criminal defense lawyer's role does not change with the defendant's guilt or lack thereof.

(Note for science geeks: Wikipedia has a more accurate and rigorous treatment of the Coriolis effect, including a description of the conditions under which Coriolis forces are significant in a physical system. My description of the experiment is from my memory of a book or article I read a long time ago. If you're like me, you're probably wondering how the water would have drained from the funnel if they did the experiment at the equator. Fortunately for both of us, the scientists wondered the same thing. As expected, the water drained quietly away with no significant rotation in either direction.)