164 GENERAL SCIENCE 



Cooling. In small and medium-size gas engines a temperature of from 

 2000 F. to 2600 F. is developed at the instant of explosion of the gases in 

 the cylinder. As the gases expand the temperature decreases. The average 

 temperature of the expanding gases is about 1400 F. during the power stroke. 



C6oling of all parts of the engine that come in contact with the hot gases 

 is necessary. Without adequate cooling, unequal expansion and distortion of 

 the overheated parts, excessive wear and piston seizure would occur. 



The cylinders of both small and medium-size horizontal gas engines are 

 nearly always water-cooled. 



Alcohol as a Fuel. It is far more economical to use denatured 

 alcohol, which is largely grain alcohol, as a fuel than ordinary wood 

 alcohol, since one part of grain alcohol will give the same amount 

 of heat as two parts of wood alcohol. 



Coal. Coal beds were once great swamps in which trees and 

 plants have fallen and turned into coal. Some of the trees were 

 very different from those of to-day. Fossil trees found in the coal 

 mines tell us the trees must have been in some cases 50 feet high and 

 4 feet in diameter. 



Coal is divided into two general classes: 



1. Anthracite coal, or hard coal, which ignites with some dif- 

 ficulty, but burns slowly with intense heat. 



2. Bituminous coal, or soft coal, which ignites readily and burns 

 easily, if there is a good draft. Soft coal should be put on the fire 

 in small quantities, as the gases escaping from the coal may not all 

 burn, causing a great deal of smoke and wasting valuable heat. 



Two gases form during the burning of coal, carbon dioxide and 

 carbon monoxide. The carbon monoxide is formed in the lower part 

 of the fire, rises to the top and is seen burning with a blue flame 

 at the top of the coal fire. If there is not sufficient air at the top 

 of the fire, some of this gas escapes up the chimney or into the room, 

 unless the drafts are properly adjusted. People have often been 

 killed by this gas escaping into a sleeping room during the night. 



Cannel coal is very rich in volatile matter. It is very compact 

 in texture, and has an oily appearance. Kentucky, Indiana, and 

 Ohio furnish a small supply of this coal. 



Peat. In Ireland and in the United States there are great bogs 

 in which trees and plants are decaying every year, forming a thick 

 black mud called peat. In wet places the peat forms very rapidly. 



