Types of Hourglass
Types of Hourglass
Types of Hourglass
Before the invention of mechanical clocks, timepieces used the sun's motion or simple measurement devices to track time. The sundial may be the best known ancient keeper of time, and it is still manufactured as a popular garden accessory—but for its visual interest, not for practical time measurement. Stonehenge, the giant monument built of upright stones on the Salisbury Plain of Wiltshire, England, may have been used as a sundial and for other time and calendar purposes. Sundials have obvious disadvantages; they can't be used indoors, at night, or on cloudy days.
Other simple measurement devices were used to mark the duration of time. Four basic types could be used indoors and regardless of the weather or time of day. The candle clock is a candle with lines drawn around it to mark units of time, usually hours. By observing how much of the length of a candle burned in one hour, a candle made of the same material was marked with lines showing one-hour intervals. An eight-hour candle showed that four hours had passed when it had burned down beyond four marks. The clock candle had the disadvantages that any changes in the wick or wax would alter burning properties, and it was highly subject to drafts. The Chinese also used a kind of candle clock with threads used to mark the time intervals. As the candle burned, the threads with metal balls on their ends fell so those in the room could hear the passage of the hours as the balls pinged on the tray holding the candle.
The oil lamp clock that was used through the eighteenth century was a variation and improvement on the candle clock. The oil lamp clock had divisions marked on a metal mount that encircled the glass reservoir containing the oil. As the level of oil fell in the reservoir, the passage of time was read from the markings on the mount. Like the candle clock, the oil lamp clock also provided light, but it was less prone to inaccuracies in materials or those caused by drafty rooms.
Water clocks were also used to mark the passage of time by allowing water to drip from one container into another. The marks of the sun's motion were made on the first container, and, as water dripped out of it and into another basin, the drop in water level showed the passage of the hours. The second container was not always used to collect and recycle the water; some water clocks simply allowed the water to drip on the ground. When the eight-hour water clock was empty, eight hours had passed. The water clock is also known as the clepsydra.
Sand timer hourglass (also called sand glasses and sand clocks) may have been used by the ancient Greeks and Romans, but history can only document the fact that both cultures had the technology to make the glass. The first claims to sand glasses are credited to the Greeks in the third century B.C. History also suggests sand clocks were used in the Senate of ancient Rome to time speeches, and the hourglasses got smaller and smaller, possibly as an indication of the quality of the political speeches.
The hourglass first appeared in Europe in the eighth century, and may have been made by Luitprand, a monk at the cathedral in Chartres, France. By the early fourteenth century, the sand glass was used commonly in Italy. It appears to have been widely used throughout Western Europe from that time through 1500. The hourglass or sand clock follows exactly the same principle as the clepsydra. Two globes (also called phials or ampules) of glass are connected by a narrow throat so that sand (with relatively uniform grain size) flows from the upper globe to the lower. Hourglasses were made in different sizes based on pre-tested measurements of sand flow in different sizes of globes. A housing or frame that enclosed the globes could be fitted to the two globes to form a top and bottom for the metal sand timer and was used to invert the hourglass and start the flow of sand again. Some hourglasses or sets of hourglasses were set in a pivoted mount so they could be turned easily.
The earliest writings referring to sand glasses are from 1345 when Thomas de Stetsham, a clerk on a ship called La George in the service of King Edward III (1312-1377) of England, ordered 16 hourglasses. In 1380, following the death of King Charles V (1337-1380) of France, an inventory of his possessions included a "large sea clock … in a large wooden brass-bound case."
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 1656. 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 1728. The motivating factor was money. In 1714, the English Admiralty set up an award of ￡20,000 for anyone who could provide mariners with a reliable clock that, when used with celestial sightings, could keep them informed of their longitude at sea. 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 1761, 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 1773 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 (742-814) of France possessed a 12-hour hourglass. In the sixteenth century, Holbein (1497-1543) the artist made spectacular hourglasses for Henry VIII (1491-1547) of England. 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 1800.
In about 1500, the first clocks began to appear with the invention of the coiled spring or mainspring. Some weight-powered clocks had been made before 1500, 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 1500 as decorative pieces. There are many types of hourglasses, such as metal hourglass，wood hourglass and so on.
By the 1400s, 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.
How to use a massage ball
Like a foam roller, a massage ball can also be used to help release tension in our achy muscles after long hours spent in the office or after a workout. One of the differences being that it can get to those hard to reach areas such as the upper back, buttocks and feet. “Knots” or “trigger points” can be massive sources of pain in our bodies and using self-massage techniques can be very satisfying. Before diving in, there are a few important things to know which will help you achieve the best results.
Why a massage ball
Massage balls are affordable and small and therefore they can easily fit into a suitcase or handbag to use wherever you go. They also promote self-sufficiency so there is no need to rely on anyone else. Notwithstanding, it does not always give the same results as a traditional massage delivered by an experienced therapist
Find the right ball
There are many different types of massage balls ranging from very smooth and firm like a lacrosse ball to small and soft like a squash ball. Other balls include a tennis ball and the trusty spikey massage ball. To each his own but if you’re new to using a massage ball, perhaps start with a spikey ball or a tennis ball.
Where and when
Since they are so conveniently easy to use, you can use them almost anywhere for example against a wall, the back of a chair, on the floor or use your hands. Some office workers keep them at their desks as a reminder to use them during the day to help with releasing built-up muscle tension from poor posture or stress.
Start with only a few knots at a time, the most painful area being first. The idea is to trap the knot in the muscle with the ball and apply gently to medium pressure until the painful sensation has faded. Once you have the correct spot (and you will know when), hold it there and try to relax until only about 80% of the ache remains. When pressing too firmly, the sensation can be too painful for you to relax which defeats the purpose of using the massage ball in the first place, it could also potentially irritate the area. You are looking for a “good pain”.Roll the ball around to look for more tender spots or just enjoy gently going back and forth over the tight muscle. If you feel the muscle needs it, you can repeat it twice a day. After releasing the knot, follow it up with gentle stretches to the same muscle. It’s okay to lightly exercise the muscle afterward but avoid fatiguing it for 24 hours.
The gear shift knob in a manual transmission equipped car or light truck is the large knob, usually made of plastic, that attaches to the top of the gear shift lever. Over time the surface of the knob can wear and you might wish to replace it to spruce up the interior of your vehicle. Many aftermarket manufactures offer specialty gear shift knobs. Some of these are made out of exotic materials such as leather or billet aluminum. Some companies offer novelty gear shift knobs for owners who want to personalize the interiors of their vehicles.