Slide rulers. Take a inch ruler and mark at two inches. Add another two inches to that and you will be at 4 inches. You have just done an addition with a regular ruler. Slide rulers take advantage o the fact that adding the log of two numbers results in a multiplication. To make life simple, it's just two scales side by side with log spacing. Other scales are added so you can do trig functions. Most functions that can be calculated on a scientific calculator can be accomplished on a slide ruler however if you need to add or subtract, you have to get the paper and pencil out again.
The other option was to carry a book with log and trig calculations in it to replace the slide ruler. The book was about the size of a good sized dictionary and while it gave better precision, most of the time a slide ruler was sufficient.
I still have my metal slide ruler from college and every so often, I need to clean it off and apply vaseline to the sliding edges to lubricate it. That also means you have to be careful how you handle it or you get grease all over your hand.