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Men of Invention and Industry-Part13_Samuel Smiles(1812-1904)_English Literature(文学名著)-第2页

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sextants. These were for the most part ponderous fixed



instruments of little or no use for the purposes of navigation.

But Tycho Brahe's sextant proved the forerunner of the modern



instrument. The general structure is the same; but the vast

improvement of the modern sextant is due, firstly, to the use of



the reflecting mirror, and, secondly, to the use of the telescope

for accurate sighting. These improvements were due to many



scientific men--to William Gascoigne, who first used the

telescope, about 1640; to Robert Hooke, who, in 1660, proposed to



apply it to the quadrant; to Sir Isaac Newton, who designed a

reflecting quadrant;[8] and to John Hadley, who introduced it.



The modern sextant is merely a modification of Newton's or

Badley's quadrant, and its present construction seems to be



perfect.

It therefore became possible accurately" target="_blank" title="ad.准确地；精密地">accurately to determine the position



of a ship at sea as regarded its latitude. But it was quite

different as regarded the longitude that is, the distance of any



place from a given meridian, eastward or westward. In the case

of longitude there is no fixed spot to which reference can be



made. The rotation of the earth makes the existence of such a

spot impossible. The question of longitude is purely a question



of TIME. The circuit of the globe, east and west, is simply

represented by twenty-four hours. Each place has its own time.



It is very easy to determine the local time at any spot by

observations made at that spot. But, as time is always changing,



the knowledge of the local time gives no idea of the actual

position; and still less of a moving object--say, of a ship at



sea. But if, in any locality, we know the local time, and also

the local time of some other locality at that moment--say, of the



Observatory at Greenwich we can, by comparing the two local

times, determine the difference of local times, or, what is the



same thing, the difference of longitude between the two places.

It was necessary therefore for the navigator to be in possession



of a first-rate watch or chronometer, to enable him to determine

accurately" target="_blank" title="ad.准确地；精密地">accurately the position of his ship at sea, as respected the



longitude.

Before the middle of the eighteenth century good watches were



comparatively unknown. The navigatormainly relied, for his

approximate longitude, upon his Dead Reckoning, without any



observation of the heavenly bodies. He depended upon the

accuracy of the course which he had steered by the compass, and



the mensuration of the ship's velocity by an instrument called

the Log, as well as by combining and rectifying all the



allowances for drift, lee-way, and so on, according to the trim

of the ship; but all of these were liable to much uncertainty,



especially when the sea was in a boisterous condition. There was

another and independent course which might have been



adopted--that is, by observation of the moon, which is constantly

moving amongst the stars from west to east. But until the middle



of the eighteenth century good lunar tables were as much unknown

as good watches.



Hence a method of ascertaining the longitude, with the same

degree of accuracy which is attainable in respect of latitude,



had for ages been the grand desideratum for men "who go down to

the sea in ships." Mr. Macpherson, in his important work



entitled 'The Annals of Commerce,' observes, "Since the year

1714, when Parliament offered a reward of 20,000L. for the best



method of ascertaining the longitude at sea, many schemes have

been devised, but all to little or no purpose, as going generally



upon wrong principles, till that heaven-taught artist Mr. John

Harrison arose;" and by him, as Mr. Macpherson goes on to say,



the difficulty was conquered, having devoted to it "the assiduous

studies of a long life."



The preamble of the Act of Parliament in question runs as

follows: "Whereas it is well known by all that are acquainted



with the art of navigation that nothing is so much wanted and

desired at sea as the discovery of the longitude, for the safety






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