METEOROLOGICAL INSTRUMENTS 141
Example.—A vessel's consumption of coal is 30 tons per day at 12
knots, required her consumption at a reduced speed of 10 knots.
new 0 _ new speed3 _ G 103
oldC ~~ old speed3 ' 30 ^ 12s
„ 30 X 10 X 10 X 10
°= 12X12X12 -"***•
Example —In bad weather a vessel makes 10 knots for 4 days of 24
hours each on 25 tons of coal per day and finds she has 1000 miles to go
and only 80 tons of coal left. Find the reduced speed to enable her tp
reach port under the same weather conditions.
Write down the equation, fill in the quantities given in the question
and solve for speed.
new G new speed2 X new distance
old G old speed2 X old distance
80 tons _ speed2 X 1P00 miles . _ ^ _
100 tons "10 X 10 X 960 miles " * Speed ~ * X ^ ' 10 ~ 7T
Speed = \/77=8-8 knots. Reduce to 8-5 knots until the weather
conditions for steaming improve.
Example.—The average speed is 12 knots on 40 tons of coal per day.
After 10 days' steaming there is 350 tons of coal left and 3000 miles to go.
Required the reduced speed to reach port.
Ans.—11 knots.
THE MARINE BAROMETER—CONSTRUCTION AND
PRINCIPLE.
The barometer consists of a glass tube about 33 inches in length,
closed at one end and filled with mercury. The tube is then inverted
and its lower end immersed in a cistern containing mercury. The
column of mercury remains stationary in the tube at a height corres-
ponding to the pressure exerted by the atmosphere on the surface of the
'mercury in the cistern. The column lengthens when the pressure is
increased and shortens when the pressure is diminished, thus the weight
of the column exactly balances the pressure of the atmosphere. The
height of the top of the column above the surface of the mercury in
the cistern is measured by means of scales graduated in inches or milli-
bars or both. The space between the mercury and the top of the tube
10 called the Torricellian vacuum.
4n analogy exists between the action of a pump and the action of a