CREAM. ^ 289 



Cleav.in[^. — The Interval during which a boiler requires no 

 cfeaning depends upon the quantity and the quality of water 

 evaporated. Under usual conditions, in order to obtain the 

 best results, a boiler should be cleaned every six or eight weeks. 



If a boiler is to be cleaned it should be allowed to stand until 

 it is partially cooled off. When blown out cold the metal in the 

 interior will usually be found covered with a thick coating of soft 

 deposit, which can easily be scraped off or washed off with a 

 hose and stream of water. 



If a boiler be blown off while the metal is at a high tempera- 

 ture, the deposited matter is usually baked and forms a solid and 

 hard coating, increasing rapidly if not carefully removed by the 

 process of chipping. 



Boiler Poiucr. — The manner in which the horse-power of a 

 boiler is usually calculated is far from satisfactory, depending 

 rather upon its size than its power of evaporation. 



In 1S84 the American Society of Mechanical Engineers 

 adopted the following definite standard: 



" A horse-power shall be equivalent to an evaporation of 

 thirty pounds of water into dry steam per hour from feed- 

 water at 100^ Fahrenheit, and under a pressure of 70 lbs. per 

 square inch above the atmosphere." 



Steam-engine. — The engine should be provided with a gov- 

 ernor to regulate its speed, a lubricator to oil valve and piston, 

 and a sufficient number of oil cups, so that all bearings may be 

 properly oiled. 



Stai'ting the Engine. — Before starting, all bearings should be 

 supplied with oil, and all waste pipes connected with cylinder 

 and steam-chest opened. The engine should then be started 

 slowly, so as to allow the water to escape. A quantity of steam 

 will always condense as it comes in contact with the cold 

 cylinder- walls, in addition to the water already present in the 

 steam-pipe. This water does not pass off as readily as steam, 

 neither can it be compressed to any great extent. Therefore, 

 if more water be present in the cylinder than will fill the clear- 

 ance space, and this water not be allowed to escape, the piston 

 moving towards the end of its stroke will strike the water, and 

 consequently be compelled to stop. The greater the speed of 

 the piston as it advances, the greater the force with which it 

 strikes the water, resulting in many cases in a broken cylinder 

 bead. 



