between the pump and the water supply.  It is not absolutely necessary that the first check be a vertical one, but a check of some kind must be so placed.  As the water is lifted up to the boiler it is more convenient to use a vertical check at this point.  Just ahead and a few inches from the pump barrel is a horizontal check valve.  Following the course of the water toward the point where it enters the boiler, you will find another check valve.  This is called a “hot water check.” just below this check, or between it and where the water enters the boiler, you will find a stop cock or it may be a globe valve.  They both answer the same purpose.  I will tell you further on why a stop cock is preferable to a globe valve.  While the cross head pumps may differ as to location and arrangement, you will find that they all require the parts described and that the checks are so placed that they bear the same relation to each other.  No fewer parts can be used in a pump required to lift water and force it against steam pressure.  More check valves may be used, but it would not do to use less.  Each has its work to do, and the failure of one defeats all the others.  The pump barrel is a hollow cylinder, the chamber being large enough to admit the plunger which varies in size from 5/8 of an inch to I inch in diameter, depending upon the size of the boiler to be supplied.  The barrel is usually a few inches longer than the stroke of the engine, and is provided at the cross head end with a stuffing box and nut.  At the discharge end it is tapped out to admit of piping to conduct water from the pump.  At the same end and at the extreme end of the travel of the plunger it is tapped for a second pipe through which the water from the supply reaches the pump barrel.  The plunger is usually made of steel and turned down to fit snug in the chamber, and is long enough to play the full stroke of engine between the stuffing box and point of supply and to connect with the driver on the cross head.  Now, we will take it for granted, that, to begin with, the pump is in good order, and we will start it up stroke at a time and watch its work.  Now, if everything be in good order, we should have good water and a good hard rubber suction hose attached to the supply pipe just under the globe valve.  When we start the pump we must open the little pet cock between the two horizontal check valves.  The globe valve must be open so as to let the water in.  A check valve, whether it is vertical or horizontal, will allow water to pass through it one way only, if it is in good working order.  If the water will pass through both ways, it is of no account.  Now, the engine starts on the outward stroke and draws the-plunger out of the chamber.  This leaves a space in the barrel which must be filled.  Air cannot get into it, because the pump is in perfect order, neither can the air get to it through the hose, as it is in the water, so that the pressure on the outside of the water causes it to flow up through the pipes through the first check valve and into the pump barrel, and fills the space, and if the engine has a I2-inch stroke, and the plunger is I inch in diameter, we have a column of water in the pump I2 inches long and I inch in diameter.