TAXK PRESSURE GAUGE 367

Note.—The tube starts oH with an initial internal air pressure
of 15 Ibs. per square inch, the equivalent of 33 feet depth of water,
bat x represents the actual depth which we are trying to find

Example.—What would be the length of the volume of air left in a
sounding tube after it has been lowered to 30 fathoms?

Volume is asked for in this esainple.
Xew volume old pressure
Old volume new pressure

x ins. _ 33 ft.
2fins. ^ (lSO-j-33) ft.
.'. z=33x24~213=3-71 inches

The scale of depths for K.B B tubes is calibrated for a barometer
reading of 29 0 inches. A reading higher than 29*0 indicates a higher
initial pressure of air within the tube which will be harder to compress
into smaller volume and will give a scale reading too low.
If barometer reads 29 5 add a fathom in every 40 fathoms.

30*0 „ „ 30 „

„ „ 30*5 ,, „ 20 ,)

TANK GAUGES.
The fact of the pressure of liquids increasing with the depth is
applied in the application of hydrostatic gauges to measuring the
height of the surface level of liquids inside tanks, the draught of a
ship, the depth of water in a river at any stage of the tide, or, indeed,
to the alteration of the surface level of any liquid above any desired
datum level. Such apparatus is designed to convert changing pressures
into scaled depths.
The Pneumercator Tank and Draught gauges manufactured by
Messrs. Kelvin, Bottomley & Baird, Glasgow, consist of five essential
parts shown in Figure 8 where (1) is the balance chamber, (2) an air
pump, (3) the mercury gauge calibrated in feet and inches of liquid
depth, (4) a control valve, (5) small solid drawn air piping led from
the chamber through devious paths to the gauge.
Figure 9 illustrates a tank with the air chamber inside of it and
iae air pump for expelling liquid from the chamber. It is necessary to
bring the level of the small residual amount of liquid that may collect