A simple model bell (Fig. 159) is easily made out of a glass tumbler which has had a tap fitted in a hole drilled through the bottom. We turn off the tap and plunge the glass into a vessel of water. The water rises a certain way up the interior, until the air within has been compressed to a pressure equal to that of the water at the level of the surface inside. The further the tumbler is lowered, the higher does the water rise inside it. Evidently men could not work in a diving-bell which is invaded thus by water. It is imperative to keep the water at bay. This we can do by attaching a tube to the tap (Fig. 160) and blowing into the tumbler till the air-pressure exceeds that of the water, which is shown by bubbles rising to the surface. The diving-bell therefore has attached to it a hose through which air is forced by pumps from the atmosphere above, at a pressure sufficient to keep the water out of the bell. This pumping of air also maintains a fresh supply of oxygen for the workers. [Illustration: FIG. 160.] Inside the bell is tackle for grappling any object that has to be moved, such as a heavy stone block. The diving-bell is used mostly for laying submarine masonry. "The bell, slung either from a crane on the masonry already built above sea-level, or from a specially fitted barge, comes into action. The block is lowered by its own crane on to the bottom. The bell descends upon it, and the crew seize it with tackle suspended inside the bell. Instructions are sent up as to the direction in which the bell should be moved with its burden, and as soon as the exact spot has been reached the signal for lowering is given, and the stone settles on to the cement laid ready for it."[34] For many purposes it is necessary that the worker should have more freedom of action than is possible when he is cooped up inside an iron box. Hence the invention of the DIVING-DRESS,