The most interesting feature of this cylinder is the piston inlet valve.  It is evident that this valve being attached to the piston needs no springs or other connections, but is opened and closed exactly at the right time by its natural inertia.  With only about 1/4 of an inch throw of valve a large area is opened, through which the free air is drawn.  The valve is made of a single piece of composition metal and is practically indestructible.  Its construction is such that it fills the clearance spaces to a greater extent than is usual in air compressors.  A singular feature is that indicator cards taken on these cylinders show a free air line in some cases a little above the atmospheric line.  Poppet valve compressors almost invariably show a slight vacuum, due to several causes, mainly the duty performed in compressing the springs of the valves, but the vacuum is also influenced by insufficiency of valve area, hot air cylinders, etc.  This cylinder gives its full volume of air, and apparently a little more at times, because the air is admitted by a concentrated inlet in which free air is always moving in one direction.  After it has been started, the speed of the compressor is such that the air attains a momentum due to its velocity and density; this serves a useful purpose in piling up the free air in the cylinder before the inlet valve closes on the return stroke.

[Illustration:  FIG. 17.—­COMBINED STEAM AND AIR INDICATOR CARD: 

Taken from a 16x18 Sergeant piston inlet air compressor, meyer’s cut-off at 3/10.  Steam at 58 lb.; air pressure, 77 lb.; total engine friction, 5 per cent.]

Fig. 17 illustrates a combined steam and air indicator card taken from one of these cylinders.  It will be observed that with steam and air cylinders equal in diameter and stroke, an air pressure of 77 pounds is reached with a steam pressure of only 58 pounds.  The reason for this is plainly shown in the cards, their areas being nearly equal.  What is made up in the air card by high pressure is represented in the steam card by greater volume.  The indicated efficiency deduced from these cards is 95 per cent., that is, the area of the air card divided by the area of the steam card, representing the resistance divided by the power, results in 95 per cent.  While several cards have been taken on the cylinders showing a loss by friction of only 5 per cent., yet on the average the best practice shows a loss of 6 per cent. or an efficiency of 94 per cent.  This result indicates an almost perfect proportion between power and resistance, and good workmanship in air-compressing machinery.  It is difficult to conceive an engine of this size being worked with a less expenditure for friction than 5 or 6 per cent.  Were it possible to retain the heat which is in the air, and which is represented by the space between the dotted isothermal curve and the actual curve, we might attain high efficiency in using compressed air power, but it is evident that the power represented by the area of this space will be lost by radiation of heat before it is used in an engine situated several hundred feet away.