214 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME of Automotive Engineers in America. The usual method of measuring viscosities is to determine the time required for a liquid to flow through a given tube or orifice at a standard temperature. A lubricating oil needs to have a sufficient viscosity to maintain a film of oil between the lubricated surfaces. Too low a viscosity results in inadequate lubrication, while too high a viscosity results in needless loss of power due to friction in the oil film itself. Oils used in cold weather should be of lower viscosity than those used in the summer because of the fact that the viscosity increases at lower temperatures, sometimes to the extent that cars cannot even be started. The viscosity of the body fluids normally remains fairly constant. When a person has a fever, the viscosity of the blood is less, and, other things being equal, it is therefore easier for the heart to pump the blood through the circulatory system. The viscosity of a liquid has its counterpart in that of gases. Both gases and liquids offer resistance to the passage of solids through them. This resistance, or friction, increases as the velocity of the moving solid increases. It is only within recent years that attention has been paid to the question of decreasing the resistance of the air or water on a fast-moving body. Engineers first became interested in the problem when they sought to increase the velocity of steamships and airplanes. Modern airplanes have been designed to offer the minimum resistance to air. More recently this same principle of streamlining has been applied to railroad trains and automobiles. Streamlining of bicycles, toy wagons, teakettles, and houses is, of course, merely a fashionable trend in design. Engineering tests with a modern car have shown the effect of different rates of speed on the gasoline mileage: 20 miles per hour 30 miles per hour 40 miles per hour 50 miles per hour 60 miles per hour 70 miles per hour 21.7 miles per gallon 19.9 miles per gallon 18.0 miles per gallon 16.0 miles per gallon 13.8 miles per gallon 11.4 miles per gallon A portion, at least, of this decrease in efficiency is due to the increased resistance offered by the air at higher speeds. The resistance offered by liquids is, of course, greater than that of gases, so that steamships have long been so designed as to ofi'er the least underwater resistance. The resistance to the flow of liquids is well illustrated in a waterdistribution system. The height to which water will rise when in motion depends on the resistance offered by pipes of different diameters. The velocity of the
THE PROPERTIES OF LIQUIDS 215 liquid in the smaller pipes is greater than that in the larger pipes, and the resistance is therefore greater. Surface Tension. Liquids also exhibit a property called surface tension. This is a force acting at the surface of a liquid which tends to cause the liquid to expose the least possible surface. Water gathers into droplets on dirty or greasy surfaces because of surface tension. In the same way mercury forms drops when it is poured on a surface to which it will not adhere. When liquids are placed in capillary tubes, surface tension causes them to rise or sink in the tube above or below the level of the surrounding liquid, depending on whether they wet or do not wet the inner surface of the tube. Surface tension may, therefore, be measured by determining the rise (or fall) of liquids in capillaries. The slender capillaries in plants account to a certain extent for the rise of sap in plants, although evaporation at the leaf end of the capillary is quite important. The fibrous nature of blotting paper makes it absorbent because the spaces between the fibers act as capillaries. Paper intended to be used with ink must be sized to prevent the spread of the ink due to this capillary action. Sponges and towels absorb water because of capillary action. Soils are cultivated in order to increase the size of the capillaries and at the same time decrease the number of capillaries which would draw water to the surface where it would be lost by evaporation. Careful cultivation is therefore very essential to plant growth during dry periods. Liquid Pressure. The pressure exerted by a liquid is equal to the sum of the external pressure on it and the pressure due to its own weight. Pressure is defined as the force per unit area. The transmission of external pressure throughout a liquid was studied by the brilliant French experimentalist, Blaise Pascal (1623- 1662). Pascal's law is stated as follows: Whenever pressure is exerted at any part of a liquid, this pressure is transmitted to all parts of the liquid, and the transmitted pressure is the same at all points as the original pressure.^ Pascal also showed how this law could be applied to obtain large forces in the hydraulic press. In this press there is a multiplication of the force in the ratio of the areas of two pistons. Thus a force of one pound on one piston would produce a force of six hundred pounds on a second piston having six hundred times the area of the first one. ' This law also applies to gases.