5
Time
(a) THE CHRONOMETER
A CHRONOMETER is
neither more nor less than a very superior watch. Its motive power is a spring
whose varying force (as it uncoils) is accurately compensated for by the form
of the drum upon which the chain is wrapped. What is called the
"escapement" (that is, the mechanism which prevents the watch from
running down all at once) is somewhat different to that of an ordinary English
lever watch. The seconds' hand only moves two steps for each second, and the
balance-wheel receives an impetus only when swinging in one direction. The
hair-spring is of the cylindrical form, and the balance-wheel itself is very
carefully "compensated" for temperature. The result of all the
successive improvements in the construction of Chronometers, and of the
extraordinary care taken in their manufacture, is that a good Chronometer will
continue to go at a constant rate, even when subjected successively to two
extreme temperatures (say 50° and 90°), and very nearly at the same rate for
intermediate temperatures.
A Chronometer should be kept in a padded box in a part
of the ship where it will be as free as possible from vibration, and should
never be moved until taken ashore, and should be wound up regularly. The use of
a Chronometer on board ship is mainly to keep Greenwich time from port to
port; hence the navigator requires to know on leaving port the error of his
Chronometer as compared with Greenwich Mean Time, and also the extent to which
it is gaining or losing per day. This information is usually supplied by an
Optician or Chronometer "rater," who has exceptional facilities for
doing this important work. But the navigator takes every opportunity of
checking the daily rate from time to time, for the rate may change from the
moment the Chronometer is taken on board.
(b) SHIP TIME
For the purpose of
discipline on shipboard and to divide the watch fairly, the crew is mustered in
two divisions: the Starboard (right side, looking toward the head), and the
Port (left). The day commences at noon, and is thus divided: Afternoon Watch,
noon to 4 p.m.; First Dog Watch, 4 p.m. to 6 p.m.; Second Dog Watch, 6 p.m. to
8 p.m.; First Watch, 8 p.m. to midnight; Middle Watch, 12 a.m. to 4 a.m.;
Morning Watch, 4 a.m. to 8 a.m.; Forenoon Watch, 8 a.m. to noon. This makes
seven Watches, which enables the crew to keep them alternately, as the Watch
which comes on duty at noon one day has the afternoon next day, and the men who
have only four hours' rest one night have eight hours the next. This is the
reason for having Dog Watches, which are made by dividing the hours between 4
p.m. and 8 p.m. into two Watches.
Ship Time
|
No. of Bells
|
Time of Day
|
|
No. of Bells
|
Time of Day
|
Mid-day
|
1 bell
2 bells
3 "
4 "
5 "
6 "
7 "
8 "
|
12.30
1.00
1.30
2.00
2.30
3.00
3.30
4.00
|
Afternoon
|
1 bell
2 bells
3 "
4 "
5 "
6 "
7 "
8 "
|
12.30
1.00
1.30
2.00
2.30
3.00
3.30
4.00
|
Morning
|
1 bell
2 bells
3 "
4 "
5 "
6 "
7 "
8 "
|
4.30
5.00
5.30
6.00
6.30
7.00
7.30
8.00
|
1st Dog
2nd
Dog
|
1 bell
2 bells
3 "
4 "
1 bell
2 bells
3 "
8 "
|
4.30
5.00
5.30
6.00
6.30
7.00
7.30
8.00
|
Forenoon
|
1 bell
2 bells
3 "
4 "
5 "
6 "
7 "
8 "
|
8.30
9.00
9.30
10.00
10.30
11.00
11.30
Noon.
|
First
Watch
|
1 bell
2 bells
3 "
4 "
5 "
6 "
7 "
8 "
|
8.30
9.00
9.30
10.00
10.30
11.00
11.30
M'dn't
|
(c) LIST OF TIME SIGNALS IN GREAT BRITAIN
Place
|
Signal adopted
|
Greenwich
Sheerness
Deal
Dover
Portsmouth
Southampton
Devonport
Falmouth
Swansea
Liverpool
Dundee
Edinburgh (Leith)
North Shields
Cork
Queenstown
Dublin
|
Black Ball (Royal Observatory)
Black Ball (Garrison Fort)
Black Ball (Telegraph Tower)
Gun (near Drop Battery)*
Black Ball (Dockyard Semaphore Tower)
Ball (South Castle)
Black Ball, also Gun on Mount Wise *
Black Ball (Pendennis Castle)
Gun on Old Eastern Pier*
Gun (Morpeth Dock Pier)*
Gun*
Ball (Calton Hill), also Gun (Edinburgh Castle)*
Gun (near Albert Edward Dock)*
Gun (Victoria Quay)*
Gun (near Military Hospital)*
Ball (Port and Docks Board Building)
|
* The signals at the above places are made at 1 p.m.
Greenwich mean time, with the exception of Dover, where the gun is fired at
noon Greenwich mean time.
At most of these
places no signal is made on Sundays or general holidays.
NOTE (Liverpool). — Chronometers tested gratis at
Bidston Observatory.
[The above is from Lloyd's Calendar, by kind permission.]
(d) DIFFERENCE IN TIME
Time in different parts of the world corresponding to London time at 12 o'clock (noon).
Locality
|
Time of Day
|
Aden (Arabia)
Amsterdam
Athens
Auckland (New Zealand)
Berlin
Bombay
Bremen
Brussels
Buenos Ayres
Cairo
Calcutta
Constantinople
Genoa
Gibraltar
Hong Kong
Honolulu (Hawaii)
Lima (Peru)
Lisbon
Madrid
Manila (Philippine Islands)
Marseilles
Melbourne (Victoria)
Mexico City
Milan
Moscow
Munich (Germany)
New York
Odessa (Russia)
Paris
Pekin
Rio de Janeiro (Brazil)
Rome
St. Petersburg
San Francisco
San Juan (Porto Rico)
Shanghai
Singapore (Malay Peninsula)
Stockholm (Sweden)
Sydney (New South Wales)
Teheran (Persia)
Vienna
Venice
Warsaw (Russia)
Yokohama (Japan)
Zanzibar ( East Africa)
|
3.00 p.m.
12.20 p.m.
1.35 p.m.
11.39 p.m.
12.54 p.m.
4.51 p.m.
12.33 p.m.
12.17 p.m.
8.07 a.m.
2.06 p.m.
5.53 p.m.
1.56 p.m.
12.36 p.m.
11.39 a.m.
7.37 p.m.
1.29 a.m.
6.53 a.m.
11.24 a.m.
11.45 a.m.
8.04 p.m.
12.21 p.m.
9.40 p.m.
5.24 a.m.
12.37 p.m.
2.30 p.m.
12.46 p.m.
7.00 a.m.
2.03 p.m.
12.09 p.m.
7.46 p.m.
9.07 a.m.
12.50 p.m.
2.01 p.m.
4.00 a.m.
8.00 a.m.
8.06 p.m.
6.54 p.m.
1.12 p.m.
10.05 p.m.
3.27 p.m.
1.06 p.m.
12.48 p.m.
1.24 p.m.
9.19 p.m.
2.37 p.m.
|
(e) REDUCTION OF
LONGITUDE INTO TIME
Rule 1. — Divide the number of degrees,
minutes and seconds by 15, and the quotient will be the time. If longitude is
west of Greenwich, the result will be the time at Greenwich when it is noon at
the place. Example: Longitude 74° 48' 15" W. What is the time ? Divide by
15 — 4 h., 59 m., 15 s., slower than Greenwich.
Rule 2. — To find difference in time
between two places divide the difference in longitude by 15. Example: Paris,
longitude 2° 20' E.; Philadelphia, longitude 75° 10' W. Difference in longitude
77° 30' divided by 15 — 5 h., 10 m., difference in time.
Rule 3. — To find difference in
longitude (e.g. distance sailed) when difference in time is known, multiply the
difference in time by 15.
A ship at 1 knot per hour goes about
1.69 feet per second.
A ship at 10 knots per hour goes
about 16.89 feet per second.
A ship at 15 knots per hour goes
about 25.33 feet per second.
A ship at 16 knots per hour goes
about 27.02 feet per second.
A ship at 17 knots per hour goes
about 28.71 feet per second.
A ship at 18 knots per hour goes
about 30.40 feet per second.
A ship at 19 knots per hour goes
about 32.09 feet per second.
A ship at 20 knots per hour goes
about 33.78 feet per second.
A ship at 21 knots per hour goes
about 35.47 feet per second.
(g) COMPARATIVE
VELOCITIES
Velocities
|
Metres per Second
|
Wave 30 metres high and 300
metres in breadth
|
6.81
|
Ship 9 knots an hour
|
4.63
|
Ship 17 knots an hour
|
8.75
|
Ordinary wind
|
5 to 6
|
Carrier pigeon
|
18.
|
Ocean wave during tempest
|
21.85
|
Storm-wind
|
25 to 30
|
Cannon-ball
|
632
|
Electricity in submarine wire
|
4,000,000
|
Light
|
300,400,000
|
A
metre is equal to 39.37 English inches.
|