A boiler with horizontal sections is shown in fig.
It has the horizontal racking motion mentioned FIG.
In the last case the mother-cell divides by a number of horizontal walls.
The gradient or grade of a line is the rate at which it rises or falls, above or below the horizontal, and is expressed by stating either the horizontal distance in which the change of level amounts to r ft., or the amount of change that would occur in some selected distance, such as roo ft., r000 ft.
When the four cranks are placed with two pairs at 180°, the pairs being at 90°, the forces are balanced without the introduction of a hammer-blow, but there remain large unbalanced couples, which if balanced by means of revolving weights in the wheels again reintroduce the hammerblow, and if left unbalanced tend to make the engine oscillate in a horizontal plane at high speed.