Dynam´ics is the science which deals with the laws of force in their relation to matter at rest or in motion, and as such it is differentiated from kinematics, which considers motion mathematically, and apart from the forces producing it. Dynamics is divided into two great branches: statics, which treats of solid bodies at rest under the action of forces; and kinetics, which treats of the action of forces in producing motion in solid bodies. Formerly the latter alone was called dynamics, and to this, in conjunction with statics, the general name mechanics was given. In the wide sense dynamics includes also hydrodynamics. It is to Newton that we owe the clear statement of the three primary laws of force on which the science of dynamics is based. These are: (1) that every body remains in a state of rest, or of uniform motion along a straight line, unless it is compelled by force to change that state; (2) that rate of change of momentum is in proportion to the force employed, and occurs along the straight line in which the force acts; (3) that, as the result of every action, there is always an equal and opposite reaction. These laws, which were formulated from experiment, involve the conception of force as a primary influence or action expressed in terms of space, time, and matter. Now, in dealing with the laws of force, a standard of measurement is required which shall be applicable to all forces at all times, and we therefore require to begin by establishing units of space, time, and mass. There are two systems of units in use, the one British, the other French. In the British system
the foot is taken as the unit of length, and the second as the unit of time. In the French the centimetre is the unit of length, the second the unit of time; the unit velocity in the one case being that of one foot per second, in the other one centimetre per second. The British unit of mass is the pound (the mass of a certain lump of platinum deposited in the Exchequer Office, London); the French the gramme; and accordingly the French units of space, mass, and time are commonly known as the C.G.S. (centimetre, gramme, second) units. As the weight of a pound (or a gramme) is not the same at all parts of the earth's surface, it cannot give us of itself an absolute or dynamical unit of force, that is, an invariable unit; but taking it in conjunction with unit time and unit velocity, we do obtain such a unit. Two absolute units of force are in common use in dynamics, the poundal and the dyne, the latter being the absolute unit in the C.G.S. system. The former is that force which, acting on the mass of one pound for one second, generates in that mass a velocity of one foot per second. The latter is that force which, acting on the mass of one gramme for one second, generates in that mass a velocity of one centimetre per second. It is important in dynamics to distinguish between mass and weight. The mass of one pound is the quantity of matter equal to a certain standard quantity (a certain lump of metal) and is quite independent of force. The weight of one pound is the force with which the mass of one pound is attracted to the earth's surface by the force of gravity. Another important term is momentum: the momentum of a body in motion at any instant is the product of the mass of the body and the velocity at that instant. See Couple; Elasticity; Energy; Force; Hydrodynamics; Kinematics; Kinetics; Statics; Thermodynamics; Waves.—Bibliography: Kelvin and Tait, Natural Philosophy; A. Gray, Dynamics; P. G. Tait, Dynamics; S. L. Loney, Mechanics and Hydrostatics for Beginners.
Dyn´amite. See Explosives.
Dy´namo. See Generator.
Dynamom´eter, an apparatus for measuring the power or rate of working of a machine. There are two types, the transmission dynamometer, which measures the power of the machine without sensibly diminishing it; and the absorption dynamometer, which measures the power by using it all. The instrument is generally employed to determine the horse-power transmitted by a shaft or by belting.—Cf. Aspinall Parr's Electrical Engineering Testing.
Dyrrhachium. See Durazzo.
Dy´sart, a royal and municipal burgh of Scotland, in Fife, on the Firth of Forth. It is an old place, and is a member of the Kirkcaldy district of parliamentary burghs. Pop. of royal burgh, 4197.
Dys´entery is a disease of an acute type, due to the action of a bacillus, characterized by pain and frequent passage of blood and mucus. Owing to improved sanitation, dysentery has become less frequent. In temperate countries sporadic cases occur from time to time, and occasional epidemics break out, but in the tropics widespread epidemics occur, and the disease is a serious menace. It is a very frequent camp disease, and has been the scourge of all armies in tropical and semi-tropical regions. The bacilli are widely spread by the fæces of infected persons, and usually the infection takes place by the mouth. The onset is rapid, and marked by fever, pain in the abdomen, and frequent stools. At first mucus only is seen in the stools, but soon blood appears. In very acute cases the patient is seriously ill in forty-eight hours, and may die on the third or fourth day. Moderate cases may go on for several weeks, with resulting convalescence. Some cases become chronic in type, and a person may have chronic dysentery for years. Bismuth in large doses is given, and morphia is a most useful drug to relieve the pain and quieten the bowel. Normal saline solution is given by rectum after the acute stage, whenever possible. Chronic dysentery requires dietetic treatment for the persistent dyspepsia and irritability of the bowel. Amœbic dysentery, due to the Amœba dysenteriæ, is a distinct disease.
Dys´odile, a yellowish or greenish foliated carbonaceous substance found in Sicily originally, and derived from the decay of minute organisms. When ignited, it burns and emits a very unpleasant smell.
Dyson, Sir Frank Watson, F.R.S., LL.D.(Edin.), British astronomer, born at Ashby 8th Jan., 1868, the son of a Baptist minister. Educated at Bradford Grammar School and Trinity College, Cambridge, he became chief assistant at the Royal Observatory, Greenwich, in 1894, and secretary of the Royal Astronomical Society in 1899. Astronomer Royal for Scotland in 1905, he was appointed Astronomer Royal for England in 1910. He was knighted in 1915.