VISCOSITY, that property of a fluid or which resists rapid change of shape or arrangement of parts; the indisposition of a body, by reason of internal function, to yield to the torce of gravity, pressure, etc., and flow readily. All substances, ranging from gases to solids, are supposed to possess this property in a greater or less degree. A note worthy instance of viscosity is exhibited by sealing wax; for while it is quite rigid in re sisting forces quickly applied, it will change shape greatly nnder the action of a small force (its own weight, for example) applied con tinuously for a long time. Molasses,. tar, asphalt and many other substances also illus trate this property in a striking way; while water, alcohol, air and other liquids and gases, as shown by suitable experiments, possess the same property in a very slight degree. Bal four Stewart defines viscosity as 4111 property whidi prevents freedom of vibration and which ultiniately converts vibrations into heat.° In
practice, the hard metals exhibits no noticeable viscosity, but the fact that a long bar of the hardest steel will bend of its own weight shows the property. The viscosity of a medium is measured by the quotient of the tangential stress developed along any plane in the medium, by relative motion of its parts, divided by the rate at which the velocity of the mediutn is changing with distance perpen dkular to that plane. More briefly, this measure, which is commonly called the co efficient of viscosity, may be defined as the quotient of the resultant tangential stress at any point of the medium divided by the result ant angular velocity of the medium at the same point.