The water from very large stationary engines is cooled by allowing it to trickle down through a cooling tower, which is built somewhat like the screen cooler only on a larger scale. The object of the cooling tower is to present the greatest possible surface of water to the air, this is accomplished by screens or baffles that turn the water over and over as it falls. The water, well cooled, finally collects in a cistern at the base of the tower from which it is pumped back to the engine and thus is used over and over again. This is an ideal system when water is expensive and when engines of considerable power are used. (126) Cooling System Troubles. Overheating caused by deposits of scale or lime in the jacket is one of the most common causes of an excessively hot cylinder. When hard water containing much lime is heated, the lime is deposited as a solid on the walls of the vessel forming a hard, dense, non-conducting sheet. When scale is deposited on the outside of the cylinder walls it prevents the transfer of the heat from the cylinder to the cooling water and consequently is the cause of the cylinder overheating. Besides acting as an insulator or heat, the deposit also causes trouble by obstructing the pipes and water passages, diminishing the water supply and aggravating the trouble. Scale interferes with the action of the thermo syphon system more than with a pump, as the pressure tending to circulate the water is much lower. Whatever system is used, the scale should be removed as often as possible, the number of removals depending, of course, on the “hardness” of the water. Large horizontal engines are usually provided with hand holes in the jacket, through which access may be had to the interior surfaces on which the scale collects. Under these conditions the scale may be removed by means of a hammer and chisel. The scale may be softened by emptying half the water from the jacket and pouring in a quantity of kerosene oil, the inlet and outlet pipes being stopped to prevent the escape of the oil. The engine should now be started and run for a few minutes with the mixture of kerosene and water in the jacket; no fresh water being admitted during this time. After the mixture has become boiling hot, stop the engine and allow it to cool; it will be found that the scale has softened to the consistency of mud, and may easily be washed out of the jacket. The work of removing the scale can be reduced to a minimum by filling the jacket with a solution of 1 part of Sulphuric Acid and 10 parts of water, allowing it to stand over night. The scale will be precipitated to the bottom of the jacket in the form of a fine powder and may be easily washed out in the morning. If the jacket water is kept at a temperature above 185° F. the amount of scale deposited will be nearly doubled over that deposited at 160° F.