When the water is very impure, what is called a _mud-drum_, _M_, fig. 41, is used. Much of the mud and dirt is deposited in this receptacle, from which it can easily be removed by taking off the cast-iron cover on the bottom of the drum. The cover is also provided with a blow-off cock, which is shown in the figure referred to. QUESTION 151. _What other attachments are there to the boiler of a locomotive?_ _Answer._ There are two cocks, _a_, _a_, fig. 71, called _heater-cocks_, which are connected with pipes to the feed-pipes _D D_, to admit steam to the latter to prevent the water in them from freezing. There is also another cock, _b_, called a _blower-cock_, which is connected to the smoke-stack by a pipe _b_, _b_. Steam is conducted through this pipe and escapes up the chimney in a jet, thus producing a draft when the engine is not working. This arrangement is called a _blower_ and is used to blow the fire when the engine is standing still. The action of the jet is similar to that of the exhaust steam which escapes up the chimney, excepting that the steam from the jet escapes in a continuous stream instead of distinct “puffs,” as it does when it is liberated alternately from one end of the cylinders and then from the other. _T′_ is a handle which is connected by a rod, _T′ T_, with the feed-cock (not shown in the engraving) in the pipe _D_. This cock can be opened or closed by the handle, and the supply of water fed into the boiler by the pump can thus be regulated. _J_ is a handle on the other side of the engine, for regulating the working of the pump on that side. _e_, _e_ are handles, also connected by rods with the pet-cocks on the pumps. These cocks can thus be opened or closed, and it can then be known whether the pumps are working. _A_ is the furnace door, which is fastened by a latch. The latter has a chain, _Q_, attached to it by which it can be conveniently opened or closed. The door also has a circular register with six holes to admit air into the furnace. These holes can be opened or closed by the revolving circular disc shown in the engraving. [Illustration: Fig. 82. Scale ³⁄₄ in. = 1 foot.] [Illustration: Fig. 83. Scale ³⁄₄ in. = 1 foot.] [Illustration: Fig. 84. Scale ³⁄₄ in. = 1 foot.] QUESTION 152. _How are the grates constructed?_