The pressure of the atmosphere is disregarded, and all steam-gauges used on locomotives are graduated in that way. In speaking of steam pressure in future, therefore, unless otherwise specified, we shall mean _effective_ and not _absolute_ pressure. [Illustration: _Fig. 8._ Scale ¹⁄₄.] QUESTION 18. _What is meant by the expansion of steam?_ _Answer._ In all gases a repulsion is exerted between the various particles, so that any gas, however small in quantity, will always fill the vessel in which it is held. Steam possesses this same property, and if placed in any vessel the particles in endeavoring to separate from each other will exert a force on all its sides. This force we call the steam pressure. To illustrate this we will suppose that the cylinder _A_ in fig. 8 is half filled with steam of 30 lbs. pressure. If now the supply of steam is shut off, the steam in the cylinder will expand so as to push the piston upward, but with a somewhat diminishing force, the nature of which we will explain hereafter. QUESTION 19. _What is meant by the volume of steam?_ _Answer._ It means the space which the steam occupies. QUESTION 20. _What is the proportion which exists between the volume and the pressure of steam?_ _Answer._ If the temperatures remain the same they are INVERSELY PROPORTIONAL TO EACH OTHER; that is, the one increases in the same proportion as the other diminishes. If we admit steam of 30 lbs. pressure per square inch into the cylinder _A_, fig. 8, and then cut off the supply by closing the cock _C_ and allow the steam in the cylinder to expand to double its volume by pushing the piston to the end of the cylinder, the steam pressure will then be only 15 lbs.; if it should expand to three times its volume its pressure would be only one-third, or 10 lbs. per square inch. This method for calculating the pressure of steam after it has expanded is correct only for the _absolute_ and not for the _effective_ pressures of steam. In order to ascertain the effective pressures of steam after expansion, it is only necessary to make the calculation with the absolute pressure and deduct the atmospheric pressure from the result. If, after being thus expanded, the piston be pushed down again so as to compress the steam into its original space, its pressure will again be 30 lbs., providing no heat has been lost in any way. QUESTION 21. _With a cylinder of any given stroke_[4] _how can we determine approximately the pressure of the steam after expansion for any given point of cut-off?_[5] [4] The _stroke_ of a piston is the distance it moves in the cylinder, and in ordinary engines is always twice the length of the crank measured from center to center of the shaft and crank-pin. [5] The steam is said to be _cut off_ when the steam-port by which steam is admitted to the cylinder is closed by the valve.