_Answer._ By connecting the end of the piston-rod _R_ (fig. 2) by another rod, _E_; called a _connecting-rod_, with a crank, _P_, which is attached to a revolving shaft, _S_. It is apparent that if the piston _B_ is moved in the direction shown by the dart _R_, a rotary motion will be given to the crank in the direction of the dart _N_. When, however, the crank reaches the position shown by the dotted lines in fig. 5, it is plain that a force applied to move the piston in either direction will no longer produce a rotary movement of the crank and shaft. The same thing will occur when the crank is in the opposite position, shown by the full lines. These two positions are called the _dead-points_ of the crank. QUESTION 4. _How is the crank of an ordinary steam engine carried past the dead-points?_ _Answer._ Stationary engines are usually provided with a large and heavy wheel, called a _fly-wheel_ (_F F_, fig. 2) which is attached to the shaft _S_. This wheel receives a sufficient amount of momentum from the crank, while the latter is moving from one dead-point to the other, to carry it past those points. QUESTION 5. _How is the steam admitted to and exhausted from the cylinder?_ _Answer._ It is admitted through two channels, _c_, _d_, fig. 2, called _steam-ways_, cast in the cylinder. These ways terminate in a smooth flat surface, _f f_, called the _valve-seat_. Their openings in the valve-seat are called _steam-ports_. Between them is another port or cavity, _g_, called the _exhaust-port_, which communicates with the open air. The form of these ports is long and narrow, as shown in fig. 4, which represents a plan of them. Over these ports a valve, _V_, called a _slide-valve_, usually made of cast iron, with a cavity, _H_, on its under side, is fitted so that by moving it backwards or forwards it will alternately cover and uncover the two steam-ports. The valve and valve-seat are inclosed in a sort of box, _I I_, fig. 2, made of cast iron, called a _steam-chest_, into which steam is admitted from the boiler by a pipe, _J_. When the valve is in the position represented in fig. 2, the front steam-port is uncovered and the steam is admitted to the front end of the cylinder, and thus forces the piston towards the back end. If, when the piston reaches the back end, the valve be moved into the position shown in fig. 3, the back steam-port will be uncovered and steam will be admitted to that end of the cylinder. At the same time it will be observed that the aperture of the front steam-port _c_ and that of the exhaust-port are both covered by the cavity in the slide-valve, so that the steam which was admitted to the front end of the cylinder can escape through the steam-port _c_ into the exhaust-port, and thus into the open air. In this way, by moving the valve alternately back and forth, steam is simultaneously admitted first to one end and exhausted from the other, and _vice versa_. [Illustration: _Fig. 6._