## Introduction: Mounting an Anvil With a Puddle

It is distressing when things around us seem uncertain. Here's a little reminder that you're always surrounded by something absolutely true.

## Supplies

You'll need:

An enormous planet, warm enough for liquid water

Wire

String

A nail

A stick

Some more sticks

A brick

A chainsaw (optional)

An anvil (optional)

## Step 1: You Are Unsure If You Are Sure That the Painting Is Crooked

You've done it. You've hung the picture and you were certain it was straight and then you left the room and it was crooked when you came back. Or you thought your kitchen floor was straight until you dropped that Gobstopper- then off it goes to live under the stove forever. I'm not getting it.

But the surface of still water is level- that is, it's a plane perpendicular to a straight line drawn from where you are standing down to the center of the Earth. It's enough to make you feel like you're a pretty big deal.

And so you are. You are perpetually on the top of the world! And now you'll know how to use that water surface to your advantage.

## Step 2: Find a Puddle

If you can't find a puddle today you can make one. I'll let you figure out how to do that on your own. Here's the rest of the plan:

1. Put a brick in the puddle and squish it down to the water level.

2. Bend a piece of wire 45 degrees and fasten each leg to a stick.

3. Hang a nail from the peak of the bent wire. This nail is now a plumb bob and will always point to the center of the Earth.

4. Set the stick on the brick and mark where the nail is pointing.

5. Retrieve the brick. Bricks are useful and don't belong in the mud.

6. Give the brick a pat. He did his part.

Hereafter, when the nail is pointing at this mark on the stick, you can trust that the stick is perpendicular to the radius of the Earth.

## Step 3: Transferring the Puddle

With a little help from stick #2 and stick #3 and stick #4 I can transfer that puddle surface plane to the irregular, cylindrical surface of a log.

With each stick a greater length than the diameter of the log I nail the center of each to the bark, leveling as I go. When the sticks' ends all line up I know that they have made a circuit of the log that is parallel to the surface of the puddle, which is as true as anything you ever saw, down to the margin of molecules. The margin of the chainsaw is less precise.

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