Over the pressure-valve _G_ is a chamber, _J_, called an air chamber. When water is forced into this chamber, it is obvious that as soon as it rises above the mouth of the pipe _E_, the air above the surface, _c d_, of the water will be confined in this chamber. This confined air, being elastic, will be compressed and expanded by the pressure of the water, so that it forms a sort of cushion, which relieves the pump and the pipes from the sudden shocks to which they are subject, owing to the rapid motion of the pump-plunger. Another air-chamber, _K K_, is sometimes placed below the suction-valve _F_. The object of this is to supply a cushion to relieve the suction-pipe from the shock which is caused by the sudden arrest of the motion of the water in the pipe when the valve _F_ is closed. When the pump-plunger is drawn out, the water flows through the valve _F_ to fill the vacuum in the pump-barrel, _A A_, and consequently all the water in the suction-pipe is put in motion. As soon as the plunger returns, the valve _F_ is closed and the motion of the water is suddenly arrested, thus producing more or less of a shock in the pipe _D_. When the water in the air chamber _K K_ rises above the line _a b_, it is evident that the air above that line will be confined in the space surrounding the pipe _L_. This air then forms a cushion in the same way as that in the upper air chamber _J_ does, which has already been explained. The advantages of the lower air chamber are, it is thought, more imaginary than real. QUESTION 125. _How can the pump be taken apart and the valves examined?_ _Answer._ By removing the bolts _e_, _e_, the upper air chamber can be taken off, and by taking out the bolts _f_, _f_, the lower one can be taken down, and the valves and cages removed. The check-valve _H_ can be taken out by removing the bolts _l_, _l_, which hold up the valve-seat _h_ and the valve and cage. QUESTION 126. _How can it be known whether the pump is forcing water into the boiler?_ _Answer._ To show this a cock, called a _pet-cock_, is attached to the upper air chamber in the position shown by the dotted circle _m_.[33] By opening this cock, if the pump is working, a strong jet of water will be discharged from it during the backward stroke of the pump-plunger. If the pump is not forcing water into the boiler, or is working imperfectly, the stream discharged from the pet-cock will be weak, and the backward and forward strokes of the plunger will thus not be very definitely indicated by the discharge from the pet-cock. [33] The pet-cock is sometimes attached to the feed-pipe. Another small cock is often attached to the lower air chamber, or to the feed-pipe, to allow the water to escape from the pump in cold weather, when the engine is not working, so as to prevent it from freezing. QUESTION 127. _Why is it necessary to be able to regulate the quantity of water which is forced into the boiler by the pumps?_