=Automatic Air Vents.=--These vents depend for their action on the expansion of a part of the valve due to the temperature of the steam. The valve remains closed when hot and opens when cold. The difference in temperature between the steam and the expelled air from the radiator is the controlling factor. In the automatic vent shown in Fig. 11, the part _A_ is screwed into the radiator loop. The discharge _C_ is open to the air or connected with a drip pipe, which returns the water to the basement. The cylinder _D_, which closes the passage _B_, is made of a material of a high coefficient of expansion. The piece _D_, when cool, is contracted sufficiently to leave the passage _B_ open to the air. When the steam is turned on, the expelled air from the radiator escapes through _B_ and _C_, but when the steam reaches _D_ the heat quickly expands the piece and closes the vent. Most automatic vents require adjusting when put in place and occasionally need readjustment. The cap _O_, of Fig. 11, may be removed with a wrench and a screw-driver used to adjust the piece _D_, so as to shut off the steam when the radiator is filled with steam. The expanding piece is simply screwed down until the steam ceases to escape. Fig. 12 is another style of automatic vent, constructed on the same principle as that of Fig. 11, but probably more positive in action. In this vent the part _A_ attaches to the radiator. The expanding portion _B_ is made in the form of a hollow cylinder, through which the air and steam escape to the atmosphere. It is longer than the corresponding piece in the other vent and is more sensitive because of its greater length and exposed surface. As the piece _B_ elongates from expansion, the upper end makes a joint with the conical piece _D_. The shape of this latter piece gives better opportunity for a tight joint than in the other form of vent and in practice gives better service.