Hate when your stupid calculator runs out of batteries?!
well go oldschool with the classic slide rule.
Do complex calculations in a flash! go slide rules!
This instructable will only teach you how to multiply, and do functions with pi.
The rest you will have to figure out on your own!

Step 1: Get a Slide Rule.

Get one from ebay here
Amazon does not carry them
David Crate's rules here
Slide rule guy here
And for you canadians eh.
This instructable is based on stick slide rules, and not circular ones. However, the concept is the same.

Step 2: What to Do!

1. slide the middle part of the slide until the first number you want to multiply by is on the "c" scale, and is lined up with the second number on the "d" scale (bottom slide).
2. slide the clear part with the line on it until it reaches your second number ON THE "c" SCALE!
3. The number that the line lines up with on the "d" scale is your final result.

Step 3: Example.

For this example I will multiply 2 by 4
See pictures!

Step 4: Pi!

For this example I will multiply pi by 7, as a simple indication of what you can do.

Step 5: Trouble Shooting

If the slide doesn't compute correctly by sliding it to the right, slide it to the left.
More complex options probably have more trouble.

Step 6: More Complex Things.

"Phil B says:

There are a lot of links to other resources at this Wikipedia article , including some you gave in your Instructable. A couple provide templates to download for making your own slide rules.

Here is how slide rules work for multiplying and dividing. Each number has a logarithm. To multiply two numbers, add their logarithms and find the anti-logarithm of the sum. Division involves subtraction of one logarithm from the other. A slide rule represents logarithms as a distances along a linear scale. Add the distances to multiply. Subtract the distances to divide. The numbers on the scale are the anti-logarithms of the logarithms represented by distances on the scale, so there is no need to consult a table of anti-logarithms.

All of the great engineering marvels until the 1970s were done on slide rules. Because a slide rule is accurate to only a couple of decimal places, those marvels are all a little over-built.
"

Here are some links if you want to know more:
Bing
Google
YouTube
Other 'Ibles