The group of polymers

Synthetic polymers can be divided into three separate class - thermoplastic, heat-resistant and elastomers.
Thermoplastics, such as polystyrene and polyvinyl chloride (PVC), can be repeatedly soften when heated. This is because the chain molecules in them are moveable relative to each other. The mobility increases with increasing temperature. In heat-resistant polymers (bakelite and epoxy resin) cross-links between the chains inhibit this process, so they when heated, not softened.
Elastomers - rubber and similar polymers, natural and synthetic rubber. To obtain the necessary number of links between the molecular chains of these materials are subjected to special treatment. As a result, they gain flexibility, i.e. the ability to be stretched, and after removal of the load to take its original shape.

Wood

The demand for wood is always great: she's on hand spraying, just to give a desired form, the details of the wood are easily attached to each other, and the products have a beautiful look.
Each type of wood gives a unique fibration of the wood with its natural pattern. This pattern is evident for the longitudinal cutting of the barrel when the outcrop of the internal structure of the wood fibers and cells.
Another specific feature of the surface of the wood has labels irregular round or oval shape. The outline of each such a bitch depend on the angle at which the sawed timber. Bitches come across where the trunk of the tree to leave the branch. Every living branch over time if grows in the barrel, and educated it swirls harden in his column; conversely, each dead branch forms a knot, which during the processing is eliminated.

Hard and soft wood

The wood is hard or soft. Give soft wood conifers - pine, spruce, fir, dads, and hard - deciduous, particularly beech, birch, maple, oak and poplar. In the tropics solid wood is produced from black ebony, mahogany, teak and other valuable species. Beautiful to look at and also solid wood has always been appreciated in the circle of manufacturers and buyers of furniture. Unfortunately, the trees of firm breeds grow slowly, and in the rainforests they are becoming less. For example, in Thailand in 1945. tropical forests accounted for 70% of all land, and by 1989 this figure had fallen to 18%; therefore, the Thai government banned logging. In many industrialized countries, there are calls on the reduction of import solid wood.

Logging

Usually, the trees are felled with the aid of powerful circular saws, then cut off the branches and dumped them on the treatment site for further transportation, involving cranes, crawler tractors, bulldozers. If the wood is to be transported by land, using cranes load it on a wheeled transport, although in India and Burma, to drag felled trees, as in ancient times used elephants.
In some regions the forest is easier to float along the rivers; felled trees are to the rafts and push the water by means of cranes. Then hundreds of rafts linked together and towed to sorting and labelling. In remote areas, where rivers no traffic, just the logs floated downstream up to the destination party wood.
In many countries, especially those where the harvest solid wood, not enough powerful technique for the cutting of the forest. For timber in logs export to the industrialized countries for wood processing plants.
The timber mill deck primarily free from bark, and then mechanical saws cut into billets of a certain size. At this stage of processing, the wood still contains SAP and require drying. A natural process is possible, but takes almost a year of time, so the wood normally sent to a quick lesso drying in special dryers. As a result of such artificial drying, shrinkage occurs and sometimes even warping of the wood; therefore, wood is processed only after the complete drying, when it becomes relatively stable (assuming again wet).
Sawn timber for further processing to be used in the construction and surfaces from the outside, require proof. Often, the boards transmit to longitudinal cutting, painting or etching the surface of the boards. Attractive wood is cut into very thin sheets - a veneer facing that he then glued to the base of plain wood.
Plywood consists of several sheets of wood glued together so that the fibers of adjacent sheets formed a right angle. Plywood sheet is much stronger made of ordinary wood, it is difficult to bend and split; the probability of warping is also negligible.
In the woodworking industry wastes little man found a use for everything. The slats of wood of low quality is placed between sheets of high quality wood, producing Blockboard and residual scrap is crushed into sawdust impregnated with glue and compressed to particleboard - wood chipboard plate.

Building

Ancient crafts, such as masonry, as before play an important role during the construction of houses. However, modern methods of shotcrete (layer-by-layer sketches) of the concrete mix or the installation of prefabricated structures require new knowledge and skills.
The oldest homes were built of locally available materials: branches, turf, stones and clay. All this can be cut and trim to give the desired shape with the help of primitive tools like stone axes and knives. Adobe bricks, fashioned by hand from clay with addition of straw and dried in the sun, was first used in the Eastern Mediterranean about 6000 years BC that was a significant step forward.

Brick house

A lot of people around the world live in brick houses. Typical house of brick stands on a strip Foundation of concrete, poured into the pits. The Foundation covered with a layer of so-called concrete preparation. Flooring on the ground level can be concrete or wooden floorboards, laid across the ceiling beams. The ends of these beams rest on the outer walls of the house. Support joists of the first floor also serve as intermediate low brick wall built on a concrete preparation, over which is placed a wooden beam - lag. The floorboards are fastened by nails at right angles to the joists.
The ceiling usually consists of beams between the walls to which the bottom with nails to fasten the sheets of drywall. The boardwalk top floor nailed on top of the ceiling beams of the first floor. In older homes first nailed wooden slats (lath) across the ceiling beams. Between the strips leave gaps when attached to the plaster it could seep between them. After hardening of the mortar, the plaster firmly against the rail.
The wooden frame of the roof rests on the walls of the capital, the top is covered with a waterproof material such as roofing material. Across the roof lay the pieces of wood and nailed them to the inclined bars. Slate or tiles are fastened with nails or staples to Latham.
Partitions divide the building into rooms, but do not support design. They can be built of brick, lined with concrete blocks or stucco, nailed to a wooden frame.
Door frame is usually made of wood and window frames of wood, steel or aluminum. Over door and window openings shall be beams (lintels). They serve the upper leg of masonry which might otherwise SAG and collapse. Nowadays, jumpers are made of steel or concrete. In some houses put a wooden lintel.

Wood-frame house

While most of the work during the construction of a brick house, you need to perform on the building site, wooden frame houses can be mounted from prefabricated structures. Such standard elements as the frame units and wall panels are manufactured in factories. This greatly reduces the time that is spent on house construction, as most of the work on site is producing large prefabricated structures.
In areas where not enough timber frame cased with boards. In other cases, a wooden frame can carry the wall panels of concrete.

The use of concrete

Now instead of fixing pre-cast concrete panels to the frame is applied layer by layer throwing him on the concrete with subsequent retention. In some cases, the concrete had been sprayed on the lightweight precast frame, which plays a role of reinforcement. In another system used shotcrete concrete contains tiny fibres that give it even more strength. Concrete has become one of the most important materials for the manufacture of prefabricated structures such as beams and columns, floor slabs and roofs, staircases, cladding wall panels and load-bearing structure of the walls. In some structural systems, such wall elements support the weight of the entire building and there is no need in the bearing frame of columns and beams. Concrete panel exterior walls come from the factory ready with a decorative veneer and is often inserted by Windows.

The reinforcement of fiberglass

The individual elements are made on-site by pouring concrete into wooden forms (formwork) reusable. Recently appeared encasement of glass-fiber-reinforced concrete. This formwork becomes an integral part of the design of the house. It simply sets in the desired location and pour concrete.
One of the most common ways in high-rise construction is a method of the console Assembly from the Central hollow of the trunk of reinforced concrete, which are often available Elevator shaft. The Foundation for such a home is usually done in the form of massive concrete mortar. Concrete barrel built on this base plate, and a steel or concrete frame then mounted on massive beams projecting from the barrel.
Steel load-bearing frame elements are preferably I-beams. They provide greater stiffness of beams at their relatively low weight compared to expensive steel beams. In addition to high-rise buildings, steel frames are used for the construction of small industrial and commercial buildings, low-rise apartment buildings and ordinary houses.

Road

Many of the core elements of modern road construction developed by the Romans, who were famous for their engineering and build roads with solid Foundation, good drainage and a smooth surface.
Road network the Romans had been carefully planned, and on the main lines were equipped with stop like a modern service stations on motorways. After the decline of the Roman Empire the roads are not maintained in proper condition and gradually collapsed. However, a lot of trails that were used by the Romans, make today a basis of a considerable part of the European network of highways.
In England the science of road building flourished in the early nineteenth century. Thanks to the efforts of the Scottish engineers Thomas Telford and John Macadam. Telford supervised the construction of more Books roads and 1,200 bridges. And macadam, except for roads, published works on their building and invented a road surface known as tarmacadam, or tar concrete.
The expansion of the Railways interrupted the development of road construction in England, but interest resumed in the late nineteenth century. In connection with the invention of the car. The increase in the number of vehicles demanded the creation of a new road network. Therefore, in the early 1950s, the British began the construction of the motorway network.

Road works

On some sites you will first need to demolish houses and other structures that impede construction. After that, proceed to one of the main processes - the excavation and moving huge amounts of soil and rocks, and up to tens of millions of tons.
That the line lay on a horizontal surface, large masses of soil from the raised territories pour in low-lying areas. This method is called working with the bookmark. Taken while routing the top layer of soil is used to cover road shoulders, embankments, and the Central dividing strips, and then planted with a grass and other vegetation.
To drain water to the edges of the road surface has a convex shape (curvature profile). At the beginning of construction for water collection laid for a powerful drainage system.
There are three major components of the pavement: the underlying surface, the base and cover, all together, they determine the stiffness of the coating. A hard surface made of concrete slabs, the joints between them filled with plastic sealant, which provides elasticity in the case of fluctuations of loads on the track. Not hard - cover "black gravel", i.e. a mixture of crushed stone or sand with coal tar or petroleum bitumen. Such coatings are called "tarmacadam" or "macadami with bitumen impregnation". Road surface bitumen also known as asphalt, and both versions (with Degteva and bitumen impregnation) is called the asphalt.
The coatings of both types of the underlying surface consists of compacted crushed stone or gravel. The basis of roads that are paved, usually concrete, and with unsteady from the "black gravel".

Agriculture

The modern agricultural machinery plays the important role in the production of food. Using these mechanisms one man can in a day to accomplish the scope of work, with which before mechanisation introduction could handle only whole brigade for a week.
Perhaps the most known representative of agricultural cars is the tractor created for the first time in the late nineteenth century to replace a horse. He didn't protect the tractor operator from the weather, and all conveniences were reduced to presence of an emphasis for feet and a rigid metal seat. Later there were seats on springs and the guards, which in some measure protected the driver from mud and dust, flying from under big driving wheels. But then nothing could save him from death or serious injury if the tractor overturned on a sloping plot of land.

The new design of tractors

In the beginning of 19b0 th years of the last century on tractors there were seats with pillows and similarity of cabins for creation of more comfortable working conditions of tractor operators. However, such cabs have been linked to another kind of inconveniences - noise. The covering from sheet metal like a sound pickup strengthened noise of the engine, transmission and hydraulic system to such an extent that it affected the health of the tractor operator.
Since then the cabin has changed significantly. New tractor cab protects the driver from weather, noise and overturning consequences. Almost all of them are Soundproofed, comfortable seats, heating and ventilation, windscreen wipers and a quick control.
The majority of tractors the engine and transmission form uniform rigid skeleton of all car and consequently there is no need for a separate frame. The engine power is from 15 to 190 kW and above, however, in tractors is approximately from 22 to 90 kW.
Usually on tractors the diesel set engines providing a drive on back or on all four wheels. In addition, engines rotate shaft of selection the capacities which are the reason for trailed implements or those that are mounted on the tractor, and also for the hydraulic systems that control these guns.
The basic agricultural tool, the plow. Its purpose is much like a shovel - flip to, to loosen and grind the top layer of soil, at the same time burying crop residues and weeds. In the modern mechanised agriculture a plough cling to a tractor though in many countries, as before, use a simple plough to which put horses or bulls.
Before to sow grain in the ploughed earth, you need to perform the final stage of the preparatory work. Just as the gardener uses the rake to prepare the ground for planting, the farmer uses a harrow for alignment of the ploughed earth and destruction of weeds.
One more kind of cars for preparation of soil and destruction of weeds - a cultivator supplied with steel teeths, or ploughshares for loosening the earth. The milling cultivator, whose tines or the blade are at the leading of the shaft, can be used for preparation of preliminary not ploughed soil.
Today many cultures sow by means of cars which loosen soil, put in it seeds on the set depth and through certain intervals and then covered it with soil. Such cars sow at once some numbers.

The harvest

Now harvesting a variety of crops, including root crops (potatoes) and fruit (grapes and wine grades of apples) mechanized. One of the first and the most important machines of this type was the harvester, which is used during the harvesting of grain crops (wheat). This is a self-propelled combine which mows, separates grain from straw, removes the husk, the grain falls asleep in the truck that accompanies it, and dumps straw on the earth.
Straw is often classified as waste and believe that it fit only on a laying cattle or as the rough fodder additive for it.
Usually for transportation and storage straw is Packed in bales. This is done using solomokopnitel machines, towed by a tractor and pick up the straw left combines, press it in bales and knit a twine. The grass the same way you can knit in bales, and the received hay to store as a forage for livestock.
An important achievement in the field of processing of raw materials in agriculture in recent years was the introduction of "big bales", weighing about 500 kg. Machine can easily cope with them, allowing you to spend much less time cleaning straw from the fields. Solomokopnitel collects it, loads it into the chamber where it is Packed into bales, and then this bale ties with a twine and throws out.

Crop processing

After harvesting for its processing and storage require different machines, such as trailers, Vilkaste forklifts, conveyors and machines for separating potatoes from soil and stones. Potatoes are often harvested when the ground is wet, and if it is grown on rocky soil, the crop gets a large number of stones and lumps of earth. Wet soil can lead to rotting potatoes, and stones can beat, flaked or cut.
Potatoes can be separated from the earth by the sieve, through which sifted the earth. The stones can be removed manually, however, this transaction is expensive, given the costs of labor.
The possible decision is connected with use of x-rays. Harvest moving conveyor and, falling, passes through bunches of the x-rays that the difference in density between stones and potatoes. Upon detection of stones pnevmoshchup catches them, skipping the potatoes. Such technologies that save labour is widespread, as farmers try to maintain low prices 5 their products in a competitive environment.

Mechanical watch

The first mechanical clock appeared in Europe around 1275, however, the name of the inventor is unknown to us. Suspended on a cord cargo spun the wheel, it was put in action the mechanism of hours, which each hour the ting. Soon, in hours came hands and dial. Which showed time.
The important detail of hours - escapement device, which limits the speed of the mechanism through step-by-step rotation of the toothed wheel. A speed controller operates a timer that ensures the desired speed of the clock.
The course of antique mechanical clocks were regulated by a balance wheel, which consisted of a bar with counterweights on both ends, which was attached to a vertical rod. The rocker swayed in front of back, its vibrations are transmitted to the speed controller, and that regulated the speed of rotation of the mechanism.

Clock

About 1330 in Italy was a striking clock in which the number of blows corresponded to each hour. And around 1475, was invented by a clock-spring plant. This led to the development of portable watches that were impossible for machines with weights.
Until the XVI century. Watches had only one hour, to an arrow. Share hours can be determined only approximately. It worked for most people. However, the astronomers needed more accurate recording of their observations. The result is a watch with minute, and some even with a second hand. Watch, which was regulated by the folio, was inaccurate, but the accuracy of the computation time increased significantly with the appearance in the XVII century the pendulum clock.

Modern clock

Some modern stationary and watches are still manufactured with mechanical drive, however the electric and electronic watches have become the norm. In many electric clock move arrows provides a motor which works on AC current with frequency of 50-60 Hz. A similar principle of work and some digital electronic clock, but they show the time on a digital light Board. The electronic wristwatch needs its own regulator, and this role often plays the quartz crystal which provides a nearly constant frequency of vibration under the action of electricity. The vibrating crystal generates precise time the electrical impulses that control the display of electronic figures or movement of mechanical arrows. Most inpatient hours also work on quartz crystals.
Quartz watch gives a very high accuracy up to 1 /30 seconds per day, but that's yesterday's news compared to atomic clocks that use the frequency of radiation of atoms of some elements (e.g., cesium) and provide an electronic indication of time with an error of less than one second per thousand years. Such clocks are used in experiments that require accurate timing, and to determine the time: today the second is defined as the length of time for which an atom of cesium-133 emit of 9 192 631 770 impulses to certain specific conditions.

Accuracy of the pendular mechanism

In 1580-e G. Italian physicist and astronomer Galileo Galilei (1564-1642) discovered, enhancing the isochronism of pendulum oscillations. A year before his death, he drew up a plan to use a pendulum to regulate clocks, and only in 1649 by his son Vincenzo and the mechanic at the name Balestri did finally watch Galileo. Subsequently, the perfect design of such a clock was developed by Dutch scientist Christiaan Huygens in the late 1650's, so the pendulum has proven itself reliable and accurate speed controller, and the daily error of standard hours has decreased from several minutes to ten seconds.
Another view of the regulator of the watch was created in 1658 when Robert hook invented the spring balance wheel or hair-spring. This thin coil spring was attached to the so-called balancer. Spring turns twisted and untwisted depending on the direction of motion of the balancer. This movement is allowed to operate a speed controller. The main advantage of the balance weight in comparison with the pendulum was the fact that his work was influenced much less by moving the watch. So it became possible to manufacture more smaller and more accurate pocket watches.
At the beginning of the XVIII century, new geographical discoveries and expansion of world trade resulted in a sharp increase in the number of long voyages. For successful sailing sailors needed an accurate method of determining the location of the own ship. The hardest part was to define a longitude - number of degrees East or West of the Greenwich Meridian. It needed an accurate timepiece showing the time in Greenwich mean time (England). But pitching courts stopped the pendulum and the clock with balance escapement was inaccurate in the extreme temperatures that adversely affected the work of the delicate hair springs.

Gregorian calendar

First, the Julian calendar had no complaints, but gradually found that year lasts for 11 minutes less, than counted. This error was corrected in 1582 by Pope Gregory XIII, who decreed the day 5 October 15 Oct. Otherwise the calendar date more and more would not coincide with certain seasons. To exclude such error in the future, he also gave the order century years (at the turn of the century) to be leap-year only when they are divided by 400 without a remainder. This Gregorian calendar is still used in Western countries.
Night and day was the shortest periods of time known to our ancestors. A more accurate calculation of time appeared about 4000 p. n. e., when ancient Egyptians divided the time of day on the clock. The first timepiece was the sundial, consisting mainly of the column which is stuck into the ground so that it cast a shadow on the dial with hour divisions. When the sun moved across the sky, the shadow of the column moved on a scale and showed time.