PISTON AND CRANK CONSTRUCTION.--The piston is hollow, and the crank is located as close to the head as possible. This has two or more circumferential grooves, to receive packing rings. The rings are made of very hard steel, and are turned up slightly larger than the diameter of the cylinder, and then cut across diagonally, so they may be sprung into place, and when in position they will bear against the inside of the cylinder, and thus serve to prevent the passage of the gases. CALCULATING THE EFFICIENCY.--The great problem with every beginner is to know something of the power of the engine, and how it is determined. Considering that the boy knows nothing of the terms used to designate the step we shall try to make the following description as free from technicalities as possible. In Fig. 68 a cylinder is represented, containing a piston A. B C indicate the limits of the stroke, and for convenience this space is provided with eleven marks to represent the pressure of the ignited gases at various portions of the travel of the piston. PRESSURE IN EXPLOSION.--When the explosion takes place, at B, the pressure will be, approximately, 230 pounds per square inch of the piston. When it moves to the next mark the pressure has decreased to 220 pounds, at the next mark it is 200, and so on, until, at the end of the stroke, opposite C, the pressure is only 40 pounds. [Illustration: Fig. 68. Calculating Efficiency.] EXPANSION LINE.--These figures represent the _expansion_ line. It is now necessary to get the _mean effective pressure_, which means that we must know what the average pressure of the gas is in each square inch from B to C. MEAN EFFECTIVE PRESSURE.--This is obtained by adding together the figures given in the sketch, and the result is, 1530. As eleven pressures were required to produce this sum, it should be divided by that number, making the result 148, avoiding fractions, as we shall do in all the calculations. The figures represent that the mean effective pressure of the gases on the piston is 148 pounds. If this is multiplied by the area of the piston, and this result by the stroke in feet and the number of power strokes per minute, we get what is called _foot pounds_. FOOT POUNDS.--Assuming that the diameter of the piston is 5 inches, which, figure, if multiplied by 3.1416, will give its area as a little over 15-1/2 square inches. Let us assume the crank is 4 inches. This will give a power stroke of 8 inches. To find out how many power strokes there are in a minute, we must know the revolutions, and this being taken at 800, and a power stroke at only every other revolution, would mean that we have 400 impulses, and each impulse traveled 8 inches, = 3200. This represents inches, which must be converted into feet, so that we have 266 feet of power strokes per minute.