Bolt Head Experiment consists of a glass globe of four or five inches in diameter, with neck about thirty inches long, having cemented on the neck a plate fitting the open top air pump receiver [bell jar]. To use, the plate is set on the open top receiver with the end of the neck immersed in a jar of water, which, to render the experiment more conspicuous, is usually colored red or blue; on exhausting the air from the receiver, the air in the globe is expanded and escapes from the neck, and is seen bubbling through the water. On returning the air to the receiver, it cannot enter the globe, but pressing on the water forces it up the neck into the globe, occupying the place of the air that escaped by its expansion, and showing the quantity [of air].
                      Price, $1.00