Where water is used over again in the machines (by returning the exhaust water from the machines to a reservoir), such addition of glycerine is more easily resorted to. Where moderate risks of frost have to be dealt with, the proportion of one gallon of glycerine to 300 gallons of water proves effectual. If the water is at a high pressure, such as 1,500 lbs. to the square inch, it is less liable to freeze than when used at a low pressure.
Again, it was at first feared that accidents might be frequent from the bursting of hydraulic pipes and cylinders under high pressure. Such, however, has been proved not to be the case in practice, and even where pipes or cylinders do burst, the pressure is at once dissipated, as the body of water which can escape at the opening is but slight.
It is desirable to use water which is as free as possible either from suspended matter or from chemical impurity. The former increases the wear and tear of the packing, and is otherwise inconvenient, and the latter acts injuriously on the seats and fittings of valves. Sea-water can be used for hydraulic machinery, but on account of its corrosion, fresh-water is better.
Water-pressure has sometimes been applied to operate machines which are worked continuously and not intermittently, and to continuous working rotary machines. This is unwise, for in applying hydraulic power to the continuous working of shafting or shop tools, the amount of power developed by the hydraulic engine cannot be varied to suit the work to be done, neither can the speed be regulated with sufficient nicety.
HYDRAULIC MOTORS.
Pressure or Hydraulic Motors form an interesting variety of hydraulic devices; they consist of working cylinders with valves and pistons, and resemble forcing pumps in their construction, but differ from them in their operation; the pistons not being moved by any external force applied to them through cranks, levers, etc., but by the weight or pressure of a column of water acting directly upon or against the pistons. Pressure engines or motors are applicable to locations—such as afford a suitable supply of water for the motive column; but where-ever refuse, impure, salt or other water can be obtained from a sufficient elevation, such may be used to raise a quantity of fresh water by these machines.
The stress considered in hydro-mechanics is always a pressure, as liquids are in general capable of sustaining only a slight tension without disruption: the intensity of the pressure is measured by the number of units of force per unit of area. Thus we say, one thousand pounds of pressure per square inch of piston—the pounds and the square inches are the units used in these calculations.
