When the temperature of the apartment has risen so as to expand the thermostatic element _A_, the pressure on _K_ and _L_ is relieved and the spring _N_ closes the port _G_. The air admitted through the restriction screw _S_, since it cannot escape through the port _G_, accumulates in the passage _Y_ and pipe _D_, filling the diaphragm and moving the valve into the position to decrease the supply of heat. When the temperature of the apartment has decreased so as to produce pressure on the connecting rod _K_, through the contraction of the thermostatic element _A_, the port _G_ will be opened by the valve lever _O_, allowing the air in the pipe _D_, together with that which flows through the restriction _S_, to escape through the passage _W_ to the atmosphere, allowing no air to accumulate in the pipe _D_, and thus permitting the spring at the diaphragm to actuate the damper or valve for more heat. The amount of air released through the port _G_ by the valve lever _O_ varies the pressure accumulated in the pipe _D_ and produces the graduated or intermediate action desired. [Illustration: FIG. 169.--Cross-section of pneumatic radiator valve showing its internal construction.] [Illustration: FIG. 170.--Pneumatic motor valve for automatic control of dampers, etc.] A further application of air pressure in temperature regulation is that of the type of motor shown in Fig. 170. This device is intended to open and close dampers such as are used in the automatic regulation of temperature where heated air is used to warm the buildings. The operation of the motor is the same as that which controls the steam valve. The pressure exerted by the diaphragm is applied at _A_ and the attachment to the damper is made at _B_. The motors indicated at _V_ and _N_ in Fig. 174 and _D_ in Fig. 175 are examples of its application. =Mechanical Ventilation.=--Draft ventilation produced by open windows, flues and chimneys is influenced by extremes of temperature and by the force and changing direction of the wind; it is, therefore, but imperfectly controlled. The superiority of mechanical ventilation is generally recognized because the amount of entering air may be regulated to suit any circumstance and its temperature and humidity varied to conform to any desired atmospheric conditions. Mechanical ventilating plants are seldom employed in any but the more pretentious dwellings, but their use has extended to a degree that they are occasionally installed in apartment buildings and their further application is likely to grow. Neither the cost of installation nor the expense of operation is prohibitive in dwellings of the better types. Mechanical ventilation is quite generally employed in school buildings, auditoriums, hospitals, public buildings and others where means will permit, and there is a universal recognition of the effects of the agreeably conditioned air.