Friend of mine showed me one of these today.
I'd heard of them, but never actually seen one before. He still had it from high school, which should tell you everything you need to know about the generation from which my friend hails. :) This thing is a slide rule, for the benefit of anyone who doesn't recognize it. The precursor to the calculator.
Let me tell you, it is a pain in the BUTT to operate. Line your first number up with this. Line your second number up with that. Slide the clear thing over once the first two alignments are made and then look at where the marker on it is located, but don't forget to adjust your answer for the right number of zeros, and be sure to pay careful attention to the value of all the little line divisions. Ye gods. I tried a couple of problems on it and gave up. I'm sure it's not as bad when you've used it for a while, as with most things, but the calculator is just so much easier! I looked it up: the first calculator was invented in 1972. I was born in 1973, right at the dawn of the electronic era...I can only imagine the joy with which people chucked out their slide rules once the newfangled calculators became available.
And yet...this one was used by the crew of Apollo 13. Read this article! Wow.
“I am, and ever will be, a white-socks, pocket-protector, nerdy engineer, born under the second law of thermodynamics, steeped in steam tables, in love with free-body diagrams, transformed by Laplace and propelled by compressible flow.” -- Neil Armstrong