RESISTANCE OF FLUIDS, the opposing force given by liquids and gases to bodies resting in (m on them or moving through them. Much of the principles included are embraced in hydrostatics (q.v.). As water presses in all directions alike at any point, it follows that when a floating body is at rest the pressure which it exerts in a downward direction is exactly balanced by the upward pressure of the water beneath it, and this will he irrespective of the shape of the Lody. If, however, the body is made to move through the water, the resistance of the water, and therefore of the power which must be applied to the body, will vary according to the shape of the moving body. This is illustrated in the forms of sailing and other vessels. Those which are blunt at the bow and stern are hard and slow sailers. They may he sharp at the bow, but if blunt or square at the stern the great tendency to create a vacuum at that point will retard the motion of the vessel. 'If the lines are very sloping the water freely passes to the rear with little retarding power. The resistance of a fluid to a body moving through it is
proportional to the square of the velocity, so that in general terms a body moving 4 ft. per second would require four times as much power to propel it as if moving 2 ft. per second = 16 and 4). The resistance of air follows the samelaw, except for great velocities, as in the case of projectiles, when the condensation produced in the air increases the resistance still MOM. Many accurate experiments have been made in the French army and navy, particularly by M. Thibault and gen. Morin. From their experiments it was found that the resistance of air to the movement of a plane surface at a velocity of one yard per second is 0.1295 pounds per square yard, when the surface is perpendicular to the direction, and that the pressure increases or decreases very nearly in proportion to the square of the velocity. The shape also of the solid has very nearly the same influence as when it moves through water.