A while ago I got on a bit of a slide rule kick. I think they are the shit. Seriously. They are awesome.

Yesterday I got a chance to use one for fun. En was on Facebook with someone who’s pregnant and about to pop out a kid. The question that was posed is “will the tub for a water birth be a problem with my house?”

So, the tub is 70″ x 40″ x 30″. And it’s filled with water.

Oh, back to the slide rule… it’s a good one. Fuck, it’s an astonishing one. It’s the one by which others are measured.

The Faber-Castell 2/83N.

And it’s new. Well, never been used. Sitting in a warehouse for 4o years. I’m the first person to use it in anger. It’s the Ferrari that’s sat in the garage for all those years.


So, onto the math. Some of it’s easy, but I’ll do it with the rule. Let’s start with 30 x 40. Obviously 1200 – 3×4=12, 10×10=100.

In this case I’m using the fancy pants W scale of the 2/83N. It’s the equivalent of a 20″ rule normally… but it’s a bit trickier to use. I set the end mark to 3 on the W1 scale and the cursor on the 4 on the w2′ scale:

I’m not going to go into a long explanation of which scale to read… it’s kind of annoying. Some of it is common sense though and knowing what number to expect. But anyway, I’m at 1.2. Slide rules don’t tell you where to put the decimal. I know that I have a 10×10 so I have at least a x100 on something. I also know that 3×4 can’t be 1.2, so it must be 12. Together that gives you 1200 — the right answer.

Next, do the same with 1200×70:

This give 8.4. Applying the same principal, that gives us 84000 in^3.

Now, let’s kick it old school:

We just multiply the cubic inches by 0.00433 to get gallons.

That gives us 3.636 — the trick now is to get the right decimal place.

84000 x 0.00433 = 8.4E4 * 4.33E-3 = 8.4E1 * 4.33 = 84 * 4.33. From here we know it’ll be in the hundreds. Therefore: 363.6.

Now, after a bit of a mis-step — a pint is not a pound the world around. Almost though. It’s actually 8.35 pounds per gallon of water.

Now the answer is 3.035… well, 3035 pounds. Which is about right. 3037 is the correct answer — but this is plenty close to let you know if your house is going to fall down.

Of course I’m only touching the surface of what this beast is capable of! :-)

A bit more work and it turns out that this is a static load of just over 100 pounds per square foot which should be within the design specs of most structures.

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