Air-lock.

Air-lock. When hollow cylinders (Caissons, q.v.) of iron are used for founding the piers of bridges under water, it is now the custom to use condensed air in them, the pressure of which does not generally exceed two atmospheres beyond the ordinary atmospheric pressure. This iron shell is open at the bottom, but air-tight and water-tight at all other points; thus water is prevented from rising in it. As part of the arrangement, it is necessary to have at the top of the caisson or elsewhere a chamber, called an air-lock, to serve for the exit and entrance of men and materials. This comparatively small chamber has an outer and an inner door. The outer door is shut after a man enters, and just then the air around him is at its ordinary pressure. But before the inner door is opened, the air in this chamber is compressed like that in the caisson, and he can then descend to his work.