Fig. 13 is a cross-section of the Allen vent. This is an example of a vent which depends for its action on a float. Whenever sufficient water accumulates in the body of the vent to raise the float, it closes the vent by means of its buoyancy. The body of the vent shown in Fig. 13 is composed of two concentric cylinders. The float _E_ occupies the inner cylinder, while surrounding it is the outer cylinder _D_. The outer cylinder is entirely closed except a little hole at _G_. The float is made of light metal and fits loosely in the inner cylinder. The steam from the radiator condenses in the vent until the inner cylinder is filled with water, up to the opening _A_. The float by its buoyancy keeps the opening in _B_ stopped, and no steam can escape. The air of the outer cylinder _D_ is expanded by the heat of the steam and most of the air escapes through the hole _G_. When the radiator cools, the rarefied air in _D_ contracts and draws the water from the inner cylinder into the space _D_; this allows the float to fall and unstop the opening in _B_. When the steam again reaches the vent, the heat expands the air in _D_ and forces the water into the inner cylinder; the float is again raised and stops the opening in _B_. Many other air vents are in common use but most of them operate on one or the other of the principles described. Fig. 11 is a relatively inexpensive vent, while Fig. 12 is higher-priced. [Illustration: FIG. 14.--Steam radiator valve. FIG. 15.--Sectional view of a steam radiator valve.]