308 Introduction to the Study of Science 



weight of water is many times greater than the volume of that 

 weight of water. Second, the expansion of steam under decreas- 

 ing pressure suggests the compressibility of steam or its elastic- 

 ity. These two points we shall consider briefly with reference 

 to their practical application in machines. 



Volume of steam. The relative volumes of equal weights of 

 water and of steam at atmospheric pressure may be presented in 

 this way. One pound of water occupies .016 of a cubic foot 

 of space (62.5 pounds occupying one cubic foot of space). The 

 steam produced from this weight of water occupies at this 

 pressure about 26.7 cubic feet of space, or more than 1600 

 times the space occupied by a pound of water. You will find 

 it helpful to make a graphic representation of the relative 

 volumes of steam and water for a given unit of weight. 



This great difference between the volume of steam generated 

 from a given amount of water and the volume of the water 

 becomes a serious danger in certain conditions. The water in 

 a boiler may run too low, as the common expression has it, and 

 the heat remain constant, possibly heating red-hot the parts of 

 the boiler not covered by water. The intense heat concentrated 

 upon a small amount of water causes it to boil with increasing 

 violence, throwing it against the superheated parts. The water 

 vaporizes with startling rapidity. The volume of steam 

 generated from the small quantity of water is too great to escape 

 through the safety valve or to be retained by the boiler. The 

 result is an explosion, the tremendous energy generated in a 

 flash, as it were, bursting through the weakest part of the boiler. 



When the energy of steam is kept under control, it may be 

 made to perform useful work, or, as it is usually stated, it may 

 be converted into mechanical energy. This is exactly the 

 reverse of the conversion of mechanical energy or motion into 

 heat energy, as by friction or by compression of gas in a bicycle 

 pump. If admitted into a cylinder in which there is a movable 

 piston connected with a crank or driving wheel, the steam by 

 its expansion forces the piston to one end and then to the other, 



