He was getting old and was having trouble seeing both up-close and at a distance. Getting tired of switching between two types of glasses, he devised a way to have both types of lenses fit into the frame. The distance lens was placed at the top and the the up-close lens was placed at the bottom. (About.com)
A list of Benjamin Franklin's inventions reveals a man of many talents and interests. It was the scientist in Ben that brought out the inventor. His natural curiosity about things and the way they work made him try to find ways to make them work better.
The Franklin stove is a metal-lined fireplace named after its inventor, Benjamin Franklin. It was invented in 1741. It had a hollow baffle near the rear (to transfer more heat from the fire to a room's air) and relied on an "inverted siphon" to draw the fire's hot fumes around the baffle. It was intended to produce more heat and less smoke than an ordinary open fireplace. It is also known as a "circulating stove" or the "Pennsylvania fireplace."