12inch x 18inch Poster There are two forms of diving bell in use at the turn of century, the ordinary bell and the air lock or caisson bell. The air lock bell is constructed of steel plates and is rectangular in form. The interior is fitted with a telephone apparatus and electric lights. Owing to its lower cost and greated simplicity the ordinary bell is usually employed in harbour construction and under water work; generally. - There is a border around the image. Image size is correct.