_G._ Eccentric. _K. K._ Eccentric-Strap. _L._ Eccentric-Rod.] QUESTION 6. _How is the slide-valve moved so as to admit and exhaust the steam at the right time?_ _Answer._ This is done by what is called an _eccentric_, which is a circular disc, _G_ (fig. 6), the axis of which is not in the centre. The outside of the eccentric is embraced by a metal ring, _K K_, made in two halves, called an _eccentric-strap_. The eccentric is attached to the shaft by screws or keys, and revolves with it and inside of the eccentric-strap. To the latter is also attached a rod, _L_, called an _eccentric-rod_. It is obvious from fig. 6 that if the eccentric revolves inside of the strap it will impart a reciprocating motion to the rod _L_. The eccentric, _G_, strap, _K_, and rod, _L_, are represented in fig. 2. Before describing their operation, or rather their connection with the valve _V_, it is necessary to understand that usually the valve-seat is placed on top of the cylinder, in which position it is difficult to connect the eccentric-rod with the valve. For convenience, therefore, what is called a _rocker_, _r r_, is placed between the cylinder and the main shaft of the engine. This rocker has two arms attached to a shaft, _s_, and the two arms have a vibratory motion about it, as indicated by the dotted lines. The eccentric-rod _L_ is attached by a pin to the lower arm of the rocker, and the valve is connected to the upper arm by the rod _M_, called the _valve-stem_, or _valve-rod_. It is obvious that as the shaft _S_ and eccentric _G_ revolve, a reciprocating or vibratory motion will be given to the rocker, which will be communicated to the valve by the valve-stem; and it is only necessary to fix the eccentric in the proper position on the shaft, in relation to the crank and piston, to give the valve the required motion for admitting and exhausting the steam to and from the cylinder at the right time. PART II. THE FORCES OF AIR AND STEAM. QUESTION 7. _What is meant by the pressure of the air?_ _Answer._ It is the pressure exerted by the weight of the air on every point with which it is in contact. The globe of the earth is surrounded by a layer of air about 50 miles thick, and, like every other substance, the air possesses weight, and hence presses upon every object with which it is in contact. QUESTION 8. _How can it be shown that the air possesses weight?_ _Answer._ By weighing a flask when it is filled with air, and again when the air is exhausted from it. In the latter condition the weight of the flask will be found to be sensibly less than it was when full of air, showing that the air which the flask contained when it was first weighed increased its weight. QUESTION 9. _Why do we not feel this pressure on our bodies?_