The earliest hourglasses used marble dust for the sand. Also, the hourglass required expertise in glass-blowing. The likeliest location for these two features of the hourglass to be brought together is Italy, particularly Venice, where glass-blowing was a highly developed art and marble was readily available. Not only must the hourglass have become common, but its construction was clearly something that could be contributed to by a regular household.
Anonymous December 15, at PM. Anonymous August 14, at AM. The invention of eyeglasses has increased productivity over the ages. In the past, active, productive members of society had to stop working, writing, reading and using their hands for skillful tasks at a relatively young age.
With eyeglasses, these members were able to continue their work. In addition to appearing more intelligent, people who wear full-rim glasses are seen as more distinctive, according to a study published in the Swiss Journal of Psychology. They are gifted with more eye contact and overall attention. Some of the most common causes of low vision include age-related macular degeneration, diabetes and glaucoma. Low vision may also result from cancer of the eye, albinism, brain injury or inherited disorders of the eye including retinitis pigmentosa.
How many people wear glasses? A projected million American adults wear glasses, according to The Vision Council. As a general rule, yes: glasses make you look smarter but less attractive. In face perception, besides physiognomic changes, accessories like eyeglasses can influence facial appearance.
According to a stereotype, people who wear glasses are more intelligent, but less attractive. There are many different medical reasons people wear glasses. The earliest writings referring to sand glasses are from when Thomas de Stetsham, a clerk on a ship called La George in the service of King Edward III of England, ordered 16 hourglasses.
In , following the death of King Charles V of France, an inventory of his possessions included a "large sea clock … in a large wooden brass-bound case. John Harrison. John Harrison and his brother James were introduced to clock repair by their father, Henry.
At the time, clock making, or horology, was undergoing a developmental revolution. Mechanical clocks had existed since the fourteenth century, but had remained rather primitive in their operation until Christiaan Huygens invented the weight-and-pendulum clock in One limitation was that they were totally dependent upon the earth's gravity for their operation. This meant that they could not keep accurate time at sea, and could not be adapted for portability.
Even moving them across a room would require adjustment. The Harrison brothers set to work on developing a marine chronometer in The motivating factor was money. Mariners had to rely heavily on dead reckoning to find their way, often leading to tragic results. The Harrison strategy was to design an instrument that was not only internally accurate but also externally stable.
The Harrisons made several models of marine chronometers. The fourth model proved to be the most successful. On a nine-week voyage from England to Jamaica in , the device had only a five-second error.
The Board of Longitude, apparently miffed that a common artisan had achieved the coveted goal, reluctantly gave up only half of the prize. John, minus his brother, refused to accept only half of the reward and persisted until the other half was relinquished. The Board subjected his invention to undue scrutiny and required him to design a fifth model. This time, Harrison outdid himself by designing a compact timepiece that resembled a modern day pocket watch.
It was far more convenient than the previous models, which were heavy and bulky. The Board still refused to capitulate. Finally, only a personal appeal to King George III and the King's intervention could set things right, and Harrison received the full reward in at age seventy-nine.
Harrison lived only three more years. These two early associations of sand clocks with the sea show how navigation had become a time-dependent science. Compasses and charts, developed in the eleventh and twelfth centuries, helped navigators determine bearings and direction, but time measurement was essential to estimating distance traveled.
The sand glass may have been invented —or perfected—for use at sea where equal units of time were measured to estimate distance; by contrast, on land, unequal time measurements were more important because activities depended on the length of day. The great advances in maritime science occurred in the twelfth century with the development of the magnetic compass in Amalfi, Italy.
Other Italian port cities like Genoa and Venice contributed to the astronomical advances in navigation, and, by coincidence, Venice was the world's greatest glass-blowing center. Furthermore, the fine marble dust from the quarries at Carrara was perfect for use as sand in navigational sand clocks.
As well as measuring time as distance at sea, hourglasses were used by the navies of several nations to "keep the watch" or measure the time the crew worked. The ship's boy was in charge of turning the hourglass; to get off work early, he would "swallow the sand" or turn the glass before it was empty.
The most extraordinary hourglasses were made as gifts for royalty. Charlemagne of France possessed a hour hourglass. Other sand glasses contained multiple instruments. For example, a sand glass made in Italy in the seventeenth century contained four glasses. One had one-quarter hour of sand; the second, a half-hour of sand; the third, three-quarters of an hour of sand; and the fourth contained the full hour's measure of sand.
Some glasses also had dials with pointers, so, with each turning of the glass, the number of turns could be shown with the pointer to mark the cumulative passage of time. The upper and lower globes of each glass were blown separately with open apertures or throats.
To join them so that sand could flow from the upper globe to the lower, the two halves of the glass were bound together with cord that was then coated with wax. The two-coned glass phial could not be blown as one piece until about In about , the first clocks began to appear with the invention of the coiled spring or mainspring.
Some weight-powered clocks had been made before , but their size limited their practicality. As the mainspring was improved, smaller, tabletop clocks were manufactured and the first watches were made. Mainspring-driven clocks made curiosities out of clepsydras and sand glasses, but, interestingly, the most beautiful hourglasses were made after as decorative pieces.
These are the hourglasses that are displayed in museums. By the s, many private homes had sand clocks for household and kitchen use. Sermonglasses were used in churches to track the length of the minister's sermon. Hourglasses were also routinely used in the lecture halls of Oxford University, craftsmen's shops to regulate working hours , and in England's House of Commons where bells to signal voting and lengths of speeches were timed based on sand clocks.
During the height of the sand glass, doctors, apothecaries, and other medical practitioners carried miniature or pocket sand glasses with durations of one-half or one minute to use when timing pulses; the practice of carrying these continued until the nineteenth century. Today, miniature versions containing three minutes worth of sand are sold as egg timers and as travel souvenirs.
Larger sand clocks are still made today of ornamental materials and in interesting styles for use as decoration. All of these measuring devices clock candles, water clocks, and sand clocks have the disadvantage that they must be watched carefully. Glass for hourglasses is the same material as that used for other blown glass.
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