Artists have always found it difficult to make a living off their art. Even a master like Leonardo was forced to sell out in order to support himself, so he adapted his drawing skills to the more lucrative fields of architecture, military engineering, canal building and weapons design. Although a peacenik at heart, Leonardo landed a job working for the Duke of Milan by calling himself a military engineer and outlining some of his sinister ideas for weapons and fortifications. Like many art school types in search of a salary, he only briefly mentioned to the Duke that he could paint as well.
Lucky for Leonardo, he was actually really talented as an engineer. Good illustrators were a dime a dozen in Renaissance Italy, but Leonardo had the brains and the diligence to break new ground, usually leaving his contemporaries in the dust. Like many crackpot geniuses, Leonardo wanted to create "new machines" for a "new world."
Throughout his life he had brilliant and far-out ideas, ranging from the practical to the prophetic. As military engineer and architect to the notorious Cesare Borgia (son of the Pope!), Leonardo proposed creating a dry route across the Gulf of Istanbul, connecting the Golden Horn and the Bosporus with a bridge. Alas, like most great ideas, the bridge plan was squelched by those killjoy engineers, who flipped when they found out how big it was supposed to be. Leonardo watchers got the last laugh, though, because modern engineers have determined that the bridge would have been completely sound. Furthermore, they show its construction would have been entirely feasible, proving yet again that Leonardo was the smartest man ever.
Nearly a century before Galileo, Leonardo butted heads with the challenge of measuring time. For him, the most interesting part was the use of mechanical gears, and he studied them with relish (see "Levers and Gears"). Based on the gear, he came up with loads of different thingamajigs, including the bicycle, a helicopter, an "auto-mobile", and some gruesome weapons of course.
The biggest mechanical bee in his bonnet, however, was water. Recall that nobody had harnessed electricity yet, so water was at that point the ultimate source for power. Leonardo studied all forms of water -- liquid, steam, and ice -- and he had all sorts of swell ideas of what to do with it. He cooked up plans for a device to measure humidity, a steam-powered cannon, many different waterwheels, and oodles of useful industrial machines powered by flowing water. He also devised some highly ambitious plans to revitalize Milan with canals, which he intended to implement with some equally ambitious construction machines. In fact, once he started on the subject of water te couldn't really stop, forever envisioning things like floating snowshoes to walk on water, breathing devices (including a diving hood) and webbed gloves to explore underwater, a life preserver to remain afloat, devices to attack and sink ships from underwater, and an "unsinkable" double-hulled ship and dredges for clearing harbors and channels.