Hydraulic systems have many advantages as power sources  for operating various aircraft units; they combine the  advantages of light weight, ease of installation, simplification  of inspection, and minimum maintenance requirements.  Hydraulic operations are also almost 100 percent efficient,  with only negligible loss due to fluid friction. Hydraulic system liquids are used primarily to transmit and  distribute forces to various units to be actuated. Liquids  are able to do this because they are almost incompressible.  Pascal’s Law states that pressure applied to any part of a  confined liquid is transmitted with undiminished intensity  to every other part. Thus, if a number of passages exist in a  system, pressure can be distributed through all of them by  means of the liquid. Manufacturers of hydraulic devices usually specify the type  of liquid best suited for use with their equipment in view of  the working conditions, the service required, temperatures  expected inside and outside the systems, pressures the liquid  must withstand, the possibilities of corrosion, and other  conditions that must be considered. If incompressibility  and fluidity were the only qualities required, any liquid that  is not too thick could be used in a hydraulic system. But a  satisfactory liquid for a particular installation must possess  a number of other properties. Some of the properties and  characteristics that must be considered when selecting a  satisfactory liquid for a particular system are discussed in  the following paragraphs. One of the most important properties of any hydraulic fluid is  its viscosity. Viscosity is internal resistance to flow. A liquid  such as gasoline that has a low viscosity flows easily, while  a liquid such as tar that has a high viscosity flows slowly.  Viscosity increases as temperature decreases. A satisfactory  liquid for a given hydraulic system must have enough body to  give a good seal at pumps, valves, and pistons, but it must not  be so thick that it offers resistance to flow, leading to power  loss and higher operating temperatures. These factors add to  the load and to excessive wear of parts. A fluid that is too thin  also leads to rapid wear of moving parts or of parts that have  heavy loads. The instruments used to measure the viscosity of  a liquid are known as viscometers or viscosimeters. Several  types of viscometers are in use today. The Saybolt viscometer  measures the time required, in seconds, for 60 milliliters of  the tested fluid at 100 °F to pass through a standard orifice.  The time measured is used to express the fluid’s viscosity,  in Saybolt universal seconds or Saybolt FUROL seconds.  [Figure 12-1] Liquids may break down if exposed to air, water, salt, or  other impurities, especially if they are in constant motion or  subject to heat. Some metals, such as zinc, lead, brass, and  copper, have an undesirable chemical reaction on certain  liquids. These chemical processes result in the formation  Liquids with a high viscosity have a greater resistance  to heat than light or low-viscosity liquids that have been  derived from the same source. The average hydraulic liquid  has a low viscosity. Fortunately, there is a wide choice of  liquids available for use within the viscosity range required  of hydraulic liquids. Chemical stability is another property that is exceedingly  important in selecting a hydraulic liquid. It is the liquid’s  ability to resist oxidation and deterioration for long periods.  All liquids tend to undergo unfavorable chemical changes  under severe operating conditions. This is the case, for  example, when a system operates for a considerable period  of time at high temperatures. Excessive temperatures have  a great effect on the life of a liquid. It should be noted that  the temperature of the liquid in the reservoir of an operating  hydraulic system does not always represent a true state of  operating conditions. Localized hot spots occur on bearings,  gear teeth, or at the point where liquid under pressure is  forced through a small orifice. Continuous passage of a liquid  through these points may produce local temperatures high  enough to carbonize or sludge the liquid, yet the liquid in the  reservoir may not indicate an excessively high temperature.  Figure 12-1.  Saybolt viscosimeter. Chemical Stability Viscosity Hydraulic Fluid 12-2 Heating unit Container Reservoir Cork Liquid bath Thermometer Oil 60 c.c.