Eel, the popular name of fishes belonging to the teleostean sub-order Apodes. The common eel (Anguilla vulgaris) is the type of a special family (Anguillidæ) and has a very wide distribution in the fresh waters of the globe. It is snake-shaped, devoid of ventral fins, and the minute scales are embedded in the slimy skin. When five or six years old it migrates to the deep sea for spawning, after which it probably dies. Curious flattened larvæ (Leptocephalus) hatch out from the floating eggs, and undergo a metamorphosis to become young eels or elvers, which when a year old ascend rivers in vast numbers as 'eel fare'. Eels are esteemed as an article of food, and even elvers are compressed into a sort of cake. In England river eels are caught in great numbers by means of eel-bucks
or eelpots, traps consisting of a kind of basket with a funnel-shaped entrance composed of willow rods converging towards a point, so that the eels can easily force their way in but cannot return. A stocking or tube of coarse cloth hanging from an aperture of a box down into the interior is also used. In England a kind of trident called an eel-spear is used also for taking them. A fisherman wades to the shallows, and, as he strikes his spear in the mud in every direction around him, the eels reposing on the bottom are caught between the prongs. They are also taken by hooks and lines and in other ways. See Conger-eel; Muræna. Electric eels belong to another group. See Electrical Fishes.
Effen´di, a Turkish title which signifies lord or master. It is particularly applied to the civil, as aga is to the military officers of the Sultan. Thus the Sultan's first physician is called Hakim effendi, and the priest in the seraglio Imam effendi.
Efferves´cence, the rapid escape of a gas from a liquid, producing a turbulent motion in it, and causing it to boil up. It, is produced by the actual formation of a gas in the liquid, as in fermentation, or by the liberation of a gas which has been forced into it, as in aerated beverages.
Efficiency, in mechanics and engineering, the ratio of the useful energy given out by a machine to the energy supplied to it. Energy cannot be created or destroyed, but it may assume various forms, and, within limits, can be changed from any one of these forms to any other. A machine or engine is an apparatus for converting energy in some given form into energy in another assigned form. In practice it is found impossible to convert the whole of the given energy into the form wanted, there being always a residue which is not of the right kind, and is, therefore, counted as useless. The smaller the residue, the more efficient is the machine. In the machines of elementary mechanics, such as the lever or the screw, the energy supplied is work done by the power or effort, and the energy wanted is work done on the load. If E is the effort, and W the load, then if there were no friction we would have E = Wr, where r is the velocity ratio, or ratio of the velocities of the points of application of load and effort. The relation found by experiment, however, is usually of the type E = Wr + C, where C is a constant. The efficiency is the fraction Wr/E or 1 - C/E, so that it increases with the load. In heat engines, energy in the form of heat is converted into mechanical energy. Heat is taken in at the source, part of it is changed into mechanical energy, and the remainder is rejected to the condenser. According to the second law of thermodynamics, the efficiency of such an engine has a definite upper limit which it cannot exceed, this being the ratio of the difference of the temperatures of the source and the condenser to the temperature of the source, these temperatures being measured on the absolute scale, that is, from -273° C. reckoned as the zero. The efficiency of a steam-engine is usually compared with that of an ideal engine working between the same temperatures, and going through a definite periodic set of operations called the Rankine cycle. If the thermal efficiency of an actual engine is 27 per cent, and that of an ideal engine working on the Rankine cycle is 30 per cent, obviously the important figure is the ratio of 27 to 30, or 90 per cent.
The performance of a steam-engine depends, not only on its thermal efficiency, but also on its boiler efficiency and its mechanical efficiency. The boiler efficiency is the percentage of the heat obtainable from the fuel consumed which is actually used in the engine; in a good boiler it may be 75 per cent. The mechanical efficiency is the ratio of the work given out at the crank-shaft to the work done on the piston; in other words, it is the ratio of brake horse-power to indicated horse-power. It may perhaps be 80 per cent. To arrive at the over-all efficiency, the various partial or component efficiencies must be multiplied together. In comparing one type of engine with another, what is important is obviously this over-all efficiency, or ratio of energy output to the theoretical energy value of the fuel employed. Thus, to take the case of marine engines, the Diesel oil-engine is inferior to the turbine and to the reciprocator in point both of thermal and of mechanical efficiency.
But when the efficiency of the boilers is taken into account, the Diesel comes out very decidedly ahead of the others. Taking coal at 10,000 British thermal units per pound, and Diesel oil at 18,000 British thermal units per pound, Mr. T. R. Wollaston has given the following figures for the number of British thermal units consumed per brake horse-power hour: steam-engine 19,000; steam turbine 21,000; gas-engine 15,000; Diesel engine 9000. Electrical plant in general reaches a high standard of efficiency. Some figures are: transmission lines 85 to 95 per cent; motors and generators at full load 70 to 80 per cent from 1 to 5 h.p., 80 to 90 per cent from 5 to 50 h.p., and 95 per cent for large sizes. Electrical transformers are the most efficient of all machines. Their efficiency ranges from about 90 per cent in small sizes, up to perhaps 98.5 per cent for large machines at full load. See Energy; Internal-combustion Engines; Steam-engines; Thermodynamics.
Efflores´cence, the property which certain hydrated salts have of losing water when exposed to air. Thus washing-soda, Na2CO3, 10H2O, if left in air becomes opaque, loses its crystalline appearance, and finally falls to a powder by loss of water. The term is also applied in botany to the process of flowering.
Effluents, a general term applying to liquids, on being discharged, after undergoing some form of treatment. The term is more particularly applied to the purified liquid discharged into rivers and streams from sewage-works, the crude sewage having been freed of the grosser solids, and rendered clear and innocuous to animal and vegetable life.
Effodien´tia, the name proposed for a new order of mammals to include pangolins and aard-varks. See Edentata.