History of Length measurement

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All knowledge that humans have known come from the past era that inherited and completed until to application domain. Are you Know that the basic from all of science is measurement of basic unit like length, mass, time and velocity ? then certainly amended become an extraordinary thing.
In the history of length measurement, in early, the human using unit unstandard like the length of ulna or length of the foot. Then this makes complicated caused by unreliability. Decided part of the body as unit length make the result of measurement relative, depend on who the humans are.

In the history, the first unit of length called cubic that is the size of a someone hand’s length. This Unit of cubic amended in Egypt. Different with Egypt in england the king edward II in years 1120 had announced a new unit of length that called yard which the yard is the length of king’s body form the nose tip until his hand tip.

 Up to 1791 where is the unit of meter first time heard and set by Francis Science Academi as 1/10.000.000 distance earth surface from North polar until equator trough Paris's meridians.

Sundial and water clock: from the 2nd millennium BC

The movement of the sun through the sky makes possible a simple estimate of time, from the length and position of a shadow cast by a vertical stick. (It also makes possible more elaborate calculations, as in the attempt of Erathosthenes to measure the world -. If marks are made where the sun's shadow falls, the time of day can be recorded in a consistent manner.

The result is the sundial. An Egyptian example survives from about 800 BC, but the principle is certainly familiar to astronomers very much earlier. However it is difficult to measure time precisely on a sundial, because the sun's path throug the sky changes with the seasons. Early attempts at precision in time-keeping rely on a different principle.
The water clock, known from a Greek word as the clepsydra, attempts to measure time by the amount of water which drips from a tank. This would be a reliable form of clock if the flow of water could be perfectly controlled. In practice it cannot. The clepsydra has an honourable history from perhaps 1400 BC in Egypt, through Greece and Rome and the Arab civlizations and China, and even up to the 16th century in Europe. But it is more of a toy than a timepiece.

The hourglass, using sand on the same principle, has an even longer career. It is a standard feature on 18th-century pulpits in Britain, ensuring a sermon of sufficient length. In a reduced form it can still be found timing an egg.

Hero's dioptra: 1st century AD

One of the surviving books of Hero of Alexandria, entitled On the Dioptra, describes a sophisticated technique which he has developed for the surveying of land. Plotting the relative position of features in a landscape, essential for any accurate map, is a more complex task than simply measuring distances.

It is necessary to discover accurate angles in both the horizontal and vertical planes. To make this possible a surveying instrument must somehow maintain both planes consistently in different places, so as to take readings of the deviation in each plane between one location and another.

This is what Hero achieves with the instrument mentioned in his title, the dioptra - meaning, approximately, the 'spyhole' through which the surveyor looks when pinpointing the target in order to read the angles.

Hero adapts, for this new and dificult task, an instrument long used by Greek astronomers (such as Hipparchus) for measuring the angle of stars in the sky. It is evident from his description that the dioptra differs from the modern theodolite in only two important respects. It lacks the added convenience of two inventions not available to Hero - the compass and the telescope.

The hour: 14th century

Until the arrival of clockwork, in the 14th century AD, an hour is a variable concept. It is a practical division of the day into 12 segments (12 being the most convenient number for dividing into fractions, since it is divisible by 2, 3 and 4). For the same reason 60, divisble by 2, 3, 4 and 5, has been a larger framework of measurement ever since Babylonian times.

The traditional concept of the hour, as one twelfth of the time between dawn and dusk, is useful in terms of everyday timekeeping. Approximate appointments are easily made, at times which are easily sensed. Noon is always the sixth hour. Half way through the afternoon is the ninth hour - famous to Christians as the time of the death of Jesus on the Cross.

The trouble with the traditional hour is that it differs in length from day to day. And a daytime hour is different from one in the night (also divided into twelve equal hours). A clock cannot reflect this variation, but it can offer something more useful. It can provide every day something which occurs naturally only twice a year, at the spring and autumn equinox, when the 12 hours of day and the 12 hours of night are the same length.

In the 14th century, coinciding with the first practical the meaning of an hour gradually changes. It becomes a specific amount of time, one twenty-fourth of a full solar cycle from dawn to dawn. And the day is now thought of as 24 hours, though it still features on clock faces as two twelves.
Minutes and seconds: 14th - 16th century
Even the first clocks can measure periods less than an hour, but soon striking the quarter-hours seems insufficient. With the arrival of dials for the faces of clocks, in the 14th century, something like a minute is required. The Middle Ages, by a tortuous route from Babylon, inherit a scale of scientific measurement based on 60. In medieval Latin the unit of one sixtieth is pars minuta prima ('first very small part'), and a sixtieth of that is pars minute secunda ('second very small part'). Thus, on a principle 3000 years old, minutes and seconds find their way into time.
Minutes are mentioned from the 14th century, but clocks are not precise enough for anyone to bother about seconds until two centuries later.

Barometer and atmospheric pressure: 1643-1646

Like many significant discoveries, the principle of the barometer is observed by accident. Evangelista Torricelli, assistant to Galileo at the end of his life, is interested in why it is more difficult to pump water from a well in which the water lies far below ground level. He suspects that the reason may be the weight of the extra column of air above the water, and he devises a way of testing this theory.

He fills a glass tube with mercury. Submerging it in a bath of mercury, and raising the sealed end to a vertical position, he finds that the mercury slips a little way down the tube. He reasons that the weight of air on the mercury in the bath is supporting the weight of the column of mercury in the tube.
If this is true, then the space in the glass tube above the mercury column must be a vacuum. This plunges him into instant controversy with traditionalists, wedded to the ancient theory - going as far back as Aristotle - that 'nature abhors a vacuum'. But it also encourages von Guericke, in the next decade, to develop the vacuum pump.

The concept of variable atmospheric pressure occurs to Torricelli when he notices, in 1643, that the height of his column of mercury sometimes varies slightly from its normal level, which is 760 mm above the mercury level in the bath. Observation suggests that these variations relate closely to changes in the weather. The barometer is born.

With the concept thus established that air has weight, Torricelli is able to predict that there must be less atmospheric pressure at higher altitudes. It is not hard to imagine an experiment which would test this, but the fame for proving the point in 1646 attaches to Blaise Pascal - though it is not even he who carries out the research.

Having a weak constitution, Pascal persuades his more robust brother-in-law to carry a barometer to different levels of the 4000-foot Puy de Dôme, near Clermont, and to take readings. The brother-in-law descends from the mountain with the welcome news that the readings were indeed different. Atmospheric pressure varies with altitude.
Gabriel Daniel Fahrenheit, a German glass-blower and instrument-maker working in Holland, is interested in improving the design of thermometer which has been in use for half a century. Known as the Florentine thermometer, because developed in the 1650s in Florence's Accademia del Cimento, this pioneering instrument depends on the expansion and contraction of alcohol within a glass tube

Alcohol expands rapidly with a rise in temperature, but not at an entirely regular speed of expansion. This makes accurate readings difficult, as also does the sheer technical problem of blowing glass tubes with very narrow and entirely consistent bores.
By 1714 Fahrenheit has made great progress on the technical front, creating two separate alcohol thermometers which agree precisely in their reading of temperature. In that year he hears of the researches of a French physicist, Guillaume Amontons, into the thermal properties of mercury.

Mercury expands less than alcohol (about seven times less for the same rise in temperature), but it does so in a more regular manner. Fahrenheit sees the advantage of this regularity, and he has the glass-making skills to accomodate the smaller rate of expansion. He constructs the first mercury thermometer, of a kind which subsequently becomes standard.
here remains the problem of how to calibrate the thermometer to show degrees of temperature. The only practical method is to choose two temperatures which can be independently established, mark them on the thermometer and divide the intervening length of tube into a number of equal degrees.

In 1701 Newton has proposed the freezing point of water for the bottom of the scale and the temperature of the human body for the top end. Fahrenheit, accustomed to Holland's cold winters, wants to include temperatures below the freezing point of water. He therefore accepts blood temperature for the top of his scale but adopts the freezing point of salt water for the lower extreme.

Measurement is conventionally done in multiples of 2, 3 and 4, so Fahrenheit splits his scale into 12 sections, each of them divided into 8 equal parts. This gives him a total of 96 degrees, zero being the freezing point of brine and 96° (in his somewhat inaccurate reading) the average temperature of human blood. With his thermometer calibrated on these two points, Fahrenheit can take a reading for the freezing point (32°) and boiling point (212°) of water.

A more logical Swede, Anders Celsius, proposes in 1742 an early example of decimilization. His centigrade scale takes the freezing and boiling temperatures of water as 0° and 100°. In English-speaking countries this less complicated system takes more than two centuries to prevail.

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