Bifocals are eyeglasses with an upper and lower half, the upper for distance, and the lower for reading. Bifocals are commonly prescribed to people with presbyopia, a condition that Franklin suffered. Franklin wrote, in August 1784 to his friend George Whatley, that he was "happy in the invention of double spectacles, which serving for distant objects as well as near ones, make my eyes as useful to me as ever they were."
Once Franklin had an understanding of the behavior of electricity, he set about to protect houses from the destructive forces of lightning. A lightning rod, simply, is a rod attached to the top of a building, connected to the ground through a wire. The electric charge from lightning strikes the rod and the charge is conducted harmlessly into the ground. This protects houses from burning down and people from electrocution.
In colonial America, homes were warmed by a fireplace. The Franklin stove, invented in 1742, is a metal-lined fireplace that stands in the middle of a room. It has rear baffles for improved airflow. It provides more heat and less smoke than an open fireplace and uses less wood. This cast-iron furnace would radiate heat from the middle of the room in all directions, and the iron walls even absorbed heat, providing warmth to the room long after the fire went out.
A popular entertainment in England in the early 18th century was playing music on upright wine goblets, with tones made by rubbing one's fingers around the lip of glasses filled with different quantities of fluid. In 1761, Franklin created a mechanized version, and called it the Armonica (after the Italian word for harmony.) Franklin worked with London glassblower Charles James to build his Armonica, and it had its world premiere in early 1762, played by Marianne Davies.