cocks in the feed-pipes, which allow the
pumps to act; and the height of water in the boiler is commonly shown by
a glass gauge-tube, and by three gauge-cocks at the side, which should be
opened from time to time, (especially when stopping,) as they afford a
more correct indication of the quantity of water and steam than the
gauge-tube.

One pump, if constantly at work, would, in most Engines, supply as much,
or rather more water than is required by the Engine as equivalent to the
steam consumed; so that by turning on or off either or both pumps, the
Engine-man has the power of regulating the height of the water in the
boiler at discretion.

It may be laid down as an invariable rule, that water alone should always
blow off from the bottom cock (which is from 1 inch to 1.5 inch above the
top of the fire-box), in order that there may be enough water over the
fire-box and tubes to prevent their burning; and few Engines will carry
their water much above the top cock without _priming_, so that the height
of the water may be made to range between these two points, according as
more or less steam is required.

The water is higher when the Engine is running than when stopping: a good
working height for it in most Engines is when _water_ blows off from the
middle cock while running, and _water and steam_ when stopping: an
Engine-man is sometimes obliged to run the water rather lower, if he has
heavy work; but it is always better to keep the level of the water as
high as possible.

It is observed that when any variation takes place in the pressure of the
steam, a corresponding change occurs in the level of the water,--that
when the pressure of the steam rises or falls, the height of the water
rises or falls simultaneously. Partly for this reason, and partly to
allow the more rapid generation of steam, the feed-pumps are not
generally allowed to act when the Engine starts: a knowledge of this fact
also shows the necessity of the water being above the ordinary level,
before a decrease is allowed in the pressure of the steam.

When the Engine is highest on an inclined plane, rather a greater height
of water must be kept over the fire-box than on a level, in order that
the chimney ends of the tubes may be well covered.

The most favourable time for allowing the feed-pumps to act, is when the
steam is blowing off with force from the safety-valve, and the fire
strong; and the least favourable time is when the steam and fire are