Gravity Catapult

I have long been enamored of catapults. Invented around 400 B.C., they were used until nearly A.D. 1300. So for 1,700 years, catapults were arguably the largest, most expensive, and most powerful machines on the planet. Today, people build catapults for fun: Scout troops, historical reenactors, fathers and daughters, beer-stoked... Read more »

The Atlatl

Before the bow and arrow there was the atlatl*, or spear-thrower, an ancient weapon that could throw a spear or dart with enough force to penetrate a mammoth’s hide. It was used in North America for about 10,000 years, and used by native Australians and Aleuts as recently as 50... Read more »

Electromagnetic Aluminum Levitator

About 13 years ago I learned of some military research into a satellite- and missile-defense device that would propel projectiles using Lenz’s law, which governs the direction of electrical current induced by a changing magnetic field. I decided to make a little gadget based on the same principle for my... Read more »

Basic Stilts

By Molly Graber and Chris Merrick Have you ever dreamed of being really tall? Walking on stilts is a fun and adventurous sport. But where are you going to get a decent-priced pair of stilts to try for your first time? With these instructions, you can add as much height... Read more »