【正文】 nd outbreaks of cholera still occurred in London. A Royal Commission on the Prevention of River Pollution was established in 1857, and eventually the first preventive river pollution legislation was passed in 1876 and 1890. However, there was little significant improvement in pollution until after the First World War, and the condition of rivers had deteriorated again by the end of the Second World War. Even today, a number of British and continental coastal towns discharge almost untreated sewage into nearshore waters. The increasing pollution of land water was acpanied by air pollution. This must have begun as soon as man started to use wood fires to provide ‘space hosting’ and a means of cooking food. Later surface, soft coal was discovered and used as a fuel, and records shown that coal smokes was a nuisance in London in the thirteenth century. In 1273,Edward I made the first ever antipollution law to prevent the use of coal for domestic heating, so smoke pollution has been recognized for at least 700 years. However, smoke pollution in London continued and is 9 recorded in both the sixteenth and seventeenth centuries. In the late eighteenth and throughout the nieenth centuries there was a marked increase in air pollution, because of the greater use of coal by developing industry. From 1750, the chemical industry began to develop, and this caused the discharge of acid fumes into the smoky air of some manufacturing towns. A Royal Commission was set up in 1862 to consider air pollution and this resulted in the first Alkali Act in 1863, which set limits to the concentration of acid in discharged waste gases. However, the increasing domestic and industrial bustion of coal, and the production of piped coal gas from 1815, caused air pollution to steadily get worse. Large cities were particularly affected, and the well known 5 day smog incident in London in 1952 directly contributed to the deaths of 4000 people. As a result, the Beaver Committee on Air Pollution was established in 1953, and the Clean Air Act was passed in 1956. This was the first effective statute to provide the means of controlling atmospheric pollution. Noise pollution probably started when man first developed machines. The increase in industrial plants in the nieenth century produce indoor noise pollution of the working environment for many factory and mill workers over a 6 day week. Outdoors, the development of private and public transport bright environmental noise, as the railway services came into use during the 1830s, motor transport from 1900, and regular aero plane services from 1922. during the first half of the twentieth century environmental noise considerably increased, but it was not recognized as pollution. Industrial and outdoor noise was designated as ‘nuisance’ when the Noise Abatement Act was passed in 1960. Whereas the earlier increase in noise occurred in work places and in connection with transport, during the past thirty years noise has spread into the home and places of leisure and entertainment. Certainly the most rapid increase in environmental pollution has taken place during the last 150 years, and it has been attributed to a number of interrelating factors. THE CHARACTERISTICS OF FLUIDS A fluid is a substance which may flow, that is, its constituent particles may continuously change their positions relative to one another. Moreover, it offers no lasting resistance to the displacement, however great, of one layer over another. This means that, if the fluid is at rest, no shear force (that is a force tangential to the surface on which it acts) can exist in it. A solid, on 。 1 OVERFLOW SPILLWAY An overflow spillway is a section of dam designed to permit water to pass over its crest. Overflow spillways are widely used on gravity, arch, and buttress dams. Some earth dams have a concrete gravity section designed to serve as a spillway. The design of the spillway for tow dams is not usually critical, and a variety of simple crest patterns are used. In the case of large dams it is important that the overflowing water be guided smoothly over the crest with a minimum of turbulence. If the overflowing water breaks contact with the spillway surface, a vacuum will form at the point of separation and cavitations may occur. Cavitations plus the vibration from the alternates making and breaking of contact between the water and the face of the dam may result in serious structural damage. Cavities filled with vapor, air, and other gases will form in a liquid whenever the absolute pressure of the liquid is close to the vapor pressure. This phenomenon, cavitations, is likely to occur where high velocities cause reduced pressure. Such conditions may arise if the walls of a passage are so sharply curved as to cause separation of flow from the boundary. The cavity, on moving downstream, may enter a region where the absolute is much higher. This causes the vapor in the cavity to condense and return to liquid with a resulting implosion, or collapse, extremely high pressure result. Some of the implosive activity will occur at the surfaces of the passage and in the crevices and pores of the boundary material. Under a continual bombardment of these implosions, the surface undergoes fatigue failure and small particles are broken away, giving the surface a spongy appearance. This damaging action of cavitations is called pitting. The ideal spillway would take the form of the underside of the napped of a sharpcrested weir when the flow rate corresponds to the maximum design capacity of the spillway. More exact profiles may be found in more extensive treatments of the subject. The reverse curve on the downstream face of the spillway should be smooth and gradua