[Illustration: Fig. 35. U-tube Draft Gauge]

[Illustration: Fig. 36. Barrus Draft Gauge]

An instrument which has given excellent results is one introduced by Mr.
G. H. Barrus, which multiplies the ordinary indications as many times as
desired. This is illustrated in Fig. 36, and consists of a U-tube made
of one-half inch glass, surmounted by two larger tubes, or chambers,
each having a diameter of 2-1/2 inches. Two different liquids which will
not mix, and which are of different color, are used, usually alcohol
colored red and a certain grade of lubricating oil. The movement of the
line of demarcation is proportional to the difference in the areas of
the chambers and the U-tube connecting them. The instrument is
calibrated by comparison with the ordinary U-tube gauge.

In the Ellison form of gauge the lower portion of the ordinary U-tube
has been replaced by a tube slightly inclined to the horizontal, as
shown in Fig. 37. By this arrangement any vertical motion in the
right-hand upright tube causes a very much greater travel of the liquid
in the inclined tube, thus permitting extremely small variation in the
intensity of the draft to be read with facility.

[Illustration: Fig. 37. Ellison Draft Gauge]

The gauge is first leveled by means of the small level attached to it,
both legs being open to the atmosphere. The liquid is then adjusted
until its meniscus rests at the zero point on the left. The right-hand
leg is then connected to the source of draft by means of a piece of
rubber tubing. Under these circumstances, a rise of level of one inch in
the right-hand vertical tube causes the meniscus in the inclined tube to
pass from the point 0 to 1.0. The scale is divided into tenths of an
inch, and the sub-divisions are hundredths of an inch.

The makers furnish a non-drying oil for the liquid, usually a 300
degrees test refined petroleum.

A very convenient form of the ordinary U-tube gauge is known as the
Peabody gauge, and it is shown in Fig. 38. This is a small modified
U-tube with a sliding scale between the two legs of the U and with
connections such that either a draft suction or a draft pressure may be
taken. The tops of the sliding pieces extending across the tubes are
placed at the bottom of the meniscus and accurate readings in hundredths
of an inch are obtained by a vernier.

[Illustration: Fig. 38. Peabody Draft Gauge]

EFFICIENCY AND CAPACITY OF BOILERS

Two of the most important operatin