nciple of a
weighing machine, in which a small spring pressure at the index will
balance a very great pressure where the thrust is employed; and in each
case the variations of pressure are recorded by a pencil on a sheet of
paper, carried forward by suitable mechanism, whereby the mean thrust is
easily ascertained. The tractive force of paddle wheel steamers is
ascertained by a dynamometer fixed on shore, to which the floating vessel
is attached by a rope. Sometimes the power of an engine is ascertained by a
friction break dynamometer applied to the shaft.

345. _Q._--What will determine the amount of thrust shown by the
dynamometer?

_A._--In locomotives and in paddle steamers it will be determined by the
force turning the wheels, and by the smallness of the diameter of the
wheels; for with small wheels the thrust will be greater than with large
wheels. In screw vessels the thrust will be determined by the force turning
round the screw, and by the smallness of the screw's pitch; for with any
given force of torsion a fine pitch of screw will give a greater thrust
than a coarse pitch of screw, just as is the case when a screw works in a
solid nut.

246. _Q._--Will you explain the use of the glass gauges affixed to the
boiler?

_A._--The glass gauges are tubes affixed to the fronts of boilers, by the
aid of which the height of the water within the boilers is readily
ascertainable, for the water will stand at the same height in the tube as
in the boiler, with which there is a communication maintained both at the
top and bottom of the tube by suitable stopcocks. The cocks connecting the
glass tube with the boiler should always be so constructed that the tube
may be blown through with the steam, to clear it of any internal concretion
that may impair its transparency; and the construction of the sockets in
which the tube is inserted should be such, that, even when there is steam
in the boiler, a broken tube may be replaced with facility.

247. _Q._--What then are the gauge cocks?

_A._--The gauge cocks are cocks penetrating the boiler at different
heights, and which, when opened, tell whether it is water or steam that
exists at the level at which they are respectively inserted. It is unsafe
to trust to the glass gauges altogether as a means of ascertaining the
water level, as sometimes they become choked, and it is necessary,
therefore, to have gauge cocks in addition; but if the boiler be short of
steam, and a partial