The following brief discussion of these various types of compressors is based on the concise practical discussion of Mr. W. L. Saunders, M. Am. Soc. C. E., in “Compressed Air Production.” The highest isothermal results are obtained by the injection of water into the cylinders, since it is plain that the injection of cold water, in the shape of a finely divided spray, directly into the air during compression will lower the temperature to a greater degree than simply to surround the cylinder and parts by water jackets which is the means of cooling adopted with dry compressors. A serious obstacle to water injection, and that which condemns this type of compressor, is the influence of the injected water upon the air cylinder and parts. Even when pure water is used, the cylinders wear to such an extent as to produce leakage and to require reboring. The limitation to the speed of a compressor is also an important objection. The chief claim for the water piston compressor is that its piston is also its cooling device, and that the heat of compression is absorbed by the water. Water is so poor a conductor of heat, however, that without the addition of sprays it is safe to say that this compressor has scarcely any cooling advantages at all so far as the cooling of the air during compression is concerned. The water piston compressor operates at slow speed and is expensive. Its only advantage is that it has no dead spaces. In the dry compressor a sacrifice is made in the efficiency of the cooling device to obtain low first cost, economy in space, light weight, higher speed, greater durability, and greater general availability. Air compressors are also distinguished as double acting and simple acting. They are simple acting when the cylinder is arranged to take in air at one stroke and force it out at the next, and they are double acting when they take in and force out air at each stroke. In form compressors may be simple or duplex. They are simple when they have but one cylinder, and duplex when they have two cylinders. A straight line or direct acting compressor is one in which the steam and air cylinders are set tandem. An indirect acting compressor is one in which the power is applied indirectly to the piston rod of the air cylinder through the medium of a crank. Mr. W. L. Saunders writes in regard to direct and indirect compression as follows:--