r each,
square foot of fire grate, which, with the proportion of heating surface
already mentioned, leaves 5 square feet of heating surface to evaporate a
cubic foot of water in the hour. This is only about half the amount of
surface usual in land and marine boilers per cubic foot evaporated, and its
small amount is due altogether to the high temperature of the furnace,
which, by the rapidity of transmission it causes, is tantamount to an
additional amount of heating surface.

232. _Q._--You have stated that the steam and vacuum gauges are generally
glass tubes, up which mercury is forced by the steam or sucked by the
vacuum?

_A._--Vacuum gauges are very often of this construction, but steam
gauges more frequently consist of a small iron tube, bent like the letter
U, and into which mercury is poured. The one end of this tube communicates
with the boiler, and the other end with the atmosphere; and when the
pressure of the steam rises in the boiler, the mercury is forced down in
the leg communicating with the boiler and rises in the other leg, and the
difference of level in the legs denotes the pressure of the steam. In this
gauge a rise of the mercury one inch in the one leg involves a difference
of the level between the two legs of two inches, and an inch of rise is,
therefore, equivalent to two inches of mercury, or a pound of pressure. A
small float of wood is placed in the open leg to show the rise or fall of
the mercury, and this leg is surmounted by a brass scale, graduated in
inches, to the marks of which the float points.

233. _Q._--What other kinds of steam and vacuum gauges are there?

_A._--There are many other kinds; but probably Bourdon's gauges are now in
more extended use than, any other, and their operation has been found to be
satisfactory in practice. The principle of their action may be explained to
be, that a thin elliptical metal tube, if bent into a ring, will seek to
coil or uncoil itself if subjected to external or internal pressure, and to
an extent proportional to the pressure applied. The end of the tube is
sharpened into an index, and moves to an extent corresponding to the
pressure applied to the tube; but in the more recent forms of this
apparatus, a dial and a hand, like those of a clock, are employed, and the
hand is moved round by a toothed sector connected to the tube, and which
sector acts on a pinion attached to the hand. Mr. Shank, of Paisley, has
lately introduced a form of stea