Sea Power was vital to the more powerful rival trading nations of the day and so several governments offered huge rewards to anyone who could find a method of accurately establishing longitude. Without benefit of radio communication an essential property of the device was that it had to be carried on board a ship and still have the required accuracy. A method found by Galileo was to observe the eclipses of the Jupiter moons at night - and then with a table of times when the eclipses occurred in relation to a particular meridian the mariner could measure the difference between this and local time on the ship from a star fix to find his longitude, allowing one degree of longitude to 4 minutes of time. The problem with such a method was that it was virtually impossible to observe the eclipses with a telescope from a ship at sea or from any other irregularly moving platform.
Nations offered substantial rewards to find a solution. The largest prize offered was £20,000 by the British Parliament in 1714. To win this, longitude had to be established to an accuracy of 30' of arc. Lesser sums were offered for accuracies of 40' of arc (£15,000) and 60' of arc (£10.000). A Board of Longitude was set up to examine proposals, and during its existence from 1714 to 1828 paid out the huge sum of £100,000. The problem was solved by a carpenter and joiner who had taught himself to repair clocks before joining with his brother to make a few precision clocks with wooden movements and wheels which were so brilliant that they kept time to within a few seconds per month. His name was John Harrison and he had no formal education. Fortunately he had a persistent nature which became important when he had to deal with the board and all the associated politics. The Royal Astronomer was Edmonds Halley who directed Harrison to another renowned clock and instrument maker called George Graham who helped finance the project.
JOHN HARRISON
H 1
120cm X 120cm
120cm X 120cm
H2
140cm X 60cm
140cm X 60cm
H3
60cm X 30cm
60cm X 30cm
An essential problem solved by Harrison was that
of rate correction. He had to make a timepiece that was predictable in its
consistency of going. If it gains a second a day all the time it will be 100
seconds fast after 100 days and an accurate correction can be applied.
However if it can vary from a second fast to a second slow it is impossible
to know whether it is slow, fast or on time. So after 100 days it could be
anywhere from 100 seconds slow to 100 seconds fast - not accurate enough to
claim any of the prizes.
Trials for the H4 were controversial because Harrison
would not divulge details of the machine and the results were so accurate
that board members thought it might be a fluke. Furthermore, Harrison was
paranoid about having his work stolen and the board's technical knowledge
was wanting. A second trial was conducted to Barbados under the scrutiny of
the board and 4 mathematicians and the results showed the watch to be 3 times
more accurate than necessary to win the top prize. The Royal Astronomer at
this time (1765), and also a board member, was Nevil Maskelyne who had pushed
for other methods to solve the longitude problem; consequently he and Harrison
clashed. Harrison was awarded only half the prize dependent on satisfactory
explanations of the principles involved with the balance to be paid after
other timekeepers had been made and tested. Harrison was by now 72 years old.
Until he died in 1776 Harrison continued to clash with Maskelyne and Harrison
only managed to get another £8,750.
Even though Harrison was the first to solve the longitude problem it is an irony of his inspiring qualities, that despite its feats of timekeeping, H4 had very little effect on the development of the true chronometer.
It is interesting to note that accurate timing signals are the basis for the very accurate modern day navigation with the Global Positioning System (GPS) - even though this system has nothing to do with local or meridian times.
To compute ranges directly, however, the user must have an atomic clock synchronized to the global positioning system. By taking a measurement from an additional satellite, the receiver avoids the need for an atomic clock. The result is that the receiver uses four satellites to compute latitude, longitude, altitude, and time.
GPS has three segments: space, control, and user. The space segment includes the satellites and the Delta rockets that launch the satellites from Cape Canaveral in Florida, United States. GPS satellites fly in circular orbits at 17,440 km (10,900 mi) altitude, each lasting 12 hours. The orbits are tilted to the equator by 55° to ensure coverage in polar regions. Powered by solar cells, the satellites continually orientate themselves to point the solar panels towards the Sun and the antennas towards the Earth. Each satellite contains four atomic clocks.
The control segment includes the master control station at Falcon Air Force Base, Colorado Springs, Colorado and monitor stations at Falcon AFB, Hawaii, Ascension Island in the Atlantic, Diego Garcia in the Indian Ocean, and Kwajalein Island in the South Pacific. The control segment uses measurements collected by the monitor stations to predict the behaviour of each satellite's orbit and clock. The prediction data is linked up to the satellites for transmission to users. The control segment also ensures that GPS satellite orbits remain within limits and that the satellite clocks do not drift too far from nominal behaviour.
User segment is a term originally associated with military receivers. Military GPS user equipment has been integrated into fighters, bombers, tankers, helicopters, ships, submarines, tanks, jeeps, and individual soldiers' equipment. In addition to basic navigation activities, military applications include target designation, close air support, "smart" munitions, and rendezvous. GPS is found on the space shuttle.
With millions of GPS receivers, the civil community has its own large and diverse user segment. Even before a full complement of satellites was in orbit, surveyors were using GPS to save days or weeks over standard survey methods. GPS is now used by aircraft and ships for en route navigation and airport or harbour approach. GPS tracking systems monitor delivery vans and emergency vehicles to provide optimum advice on routes. In a method called "precision farming" GPS is used to monitor and control the application of fertilizer and pesticides. It is also available as an in-car navigation aid and is used by hikers.
The practical use of GPS by mariners still requires requires a plot to be kept - either electronically or manually. Electronic charts built into some GPS units still need cross checking against against the proper current chart for the appropriate area. Where there is no built-in electronic chart standard navigational techniques need to be applied while treating the GPS as an accurate navaid.
For a comprehensive in-depth coverage of GPS visit
http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
To remove the air from the fuel system open the line at the filter and operate the priming pump until the fuel trickle from the line or bleeder screw is clear of bubbles. When the fuel is clear close off and repeat the procedure at other points in the system upstream of the injectors. If an injector line is loosened the engine will need to be turned over by hand.
Each diesel engine will have slightly different recommended methods for bleeding fuel lines. The manual should be read and the procedure practiced in calm water so that if it needs to be done in rough conditions the risk of confusion is reduced.
