514 Farm Mechanics. 



symptom and should be avoided. The fact is indicated 

 by the water in the gage glass becoming muddy and un- 

 steady, rising sometimes very high and then falling 

 again as quickly. It is often caused by dirty water, es- 

 pecially when it contains alkali or grease. 



When foaming occurs it is difficult to tell just where 

 the water stands in the boiler and here is where the danger 

 lies. The tendency with foaming is to cause the heated 

 surfaces of the boiler to become uncovered and become 

 excessively hot so that when the water returns steam may 

 be suddenly generated with explosive violence. 



644. Low Water in the Boiler. If by any chance the 

 water should become too low in the boiler, cool judgment 

 and quick action are called for, because if the crown sheet 

 has become exposed it is liable to be weakened by over- 

 heating. In short, an explosion is imminent. The thing 

 to do first is to cover at once the fire in the fire-box with 

 three, or four inches of wet ashes or earth so as to shut 

 off the heat. Do not under any circumstances undertake 

 to rake out the fire, as stirring it up fresh only makes the 

 heat more intense for the moment. 



At such a time the safety valve should not be opened as 

 the sudden release of pressure which this would permit 

 may cause an explosion by the agitation . throwing water 

 onto the overheated crown plate. The thing to do is to 

 allow the engine to cool down and when cool enough to 

 refill the boiler. 



645. Soft Plug Boilers are provided with a "soft plug" 

 which screws into the crown plate and is fitted with an 

 alloy which melts at a low heat to allow the water to be 

 forced upon the fire and extinguish it before the crown 

 sheet could be injured. Such a plug, however, is not al- 

 ways reliable as the top of it may become coated with 

 lime and thus rendered ineffective. On this account the 

 plug should be removed and scraped occasionally and it 

 is prudent to put in a new one each year or refill it. 



