The system of roads for mobility has been begun early decades. It was started since the Roman Empire conquered the world. The roads in Roman era were known as the best construction.
The Romans built roads are for the purpose of mobility, especially for the army. Having the best road, the army could march from one place to another. They built the roads as straight as possible, so that the army could take the shortest route.
It is interesting to know how the roads were built, which some of them are still in use today. First, the Romans builders would clear the ground of rocks and trees. Then they dug a trench where the road was to go and filled it with big stones. Next, they put in big stones, pebbles, cement and sand which they packed down to make a firm base. After that, they added another layer of cement mixed with broken tiles. On top of that rough construction, then they put paving stones to make the flat surface. These stones were cut so that they fitted together tightly. As the finishing touch, kerb stones were put at each sides of the road to hold in the paving stones. It was also used to make a channel for the water to run away.
Rabu, 05 Agustus 2009
Cancer
What is cancer? It is actually a group of more than one hundred separate diseases. Most of us are fear from cancer It is reasonable because next to heart disease, cancer is the second leading cause of death.
Cancer cells come from normal cells because of mutations of DNA. Those mutations can occur spontaneously. The mutations may be also induced by other factors such as: nuclear and electromagnetic radiation, viruses, bacteria and fungi, parasites, heat, chemicals in the air, water and food, mechanical cell-level injury, free radicals, evolution and ageing of DNA, etc. All such factors can produce mutations that may start cancer.
Cancer cells are formed continuously in the organism. It is estimated that there are about 10,000 cancer cells at any given time in a healthy person. Why do some result in macroscopic-level cancers and some do not? First, not all damaged cells can multiply and many of them die quickly. Second, those which potentially divide and form cancer are effectively destroyed by the mechanisms available to the immune system. Therefore cancer develops if the immune system is not working properly or the amount of cells produced is too great for the immune system to eliminate.
Cancer cells come from normal cells because of mutations of DNA. Those mutations can occur spontaneously. The mutations may be also induced by other factors such as: nuclear and electromagnetic radiation, viruses, bacteria and fungi, parasites, heat, chemicals in the air, water and food, mechanical cell-level injury, free radicals, evolution and ageing of DNA, etc. All such factors can produce mutations that may start cancer.
Cancer cells are formed continuously in the organism. It is estimated that there are about 10,000 cancer cells at any given time in a healthy person. Why do some result in macroscopic-level cancers and some do not? First, not all damaged cells can multiply and many of them die quickly. Second, those which potentially divide and form cancer are effectively destroyed by the mechanisms available to the immune system. Therefore cancer develops if the immune system is not working properly or the amount of cells produced is too great for the immune system to eliminate.
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