But the water piston fraternity promptly brings forward the question of speed. They say that, admitting that the cooling surfaces are equal, we have in one case _more time_ to absorb the heat than in the other. This is true, and here we come to an important class division in air compressing machinery--_high speed and short stroke_ as against _slow speed and long stroke_. Hydraulic piston compressors are subject to the laws that govern piston pumps, and are, therefore, limited to a piston speed of about 100 feet per minute. It is quite out of the question to run them at much higher speed than this without shock to the engine and fluctuations of air pressure due to agitation of the water piston. The quantity of heat produced, that is, the degree of temperature reached, depends entirely upon the conditions in the air itself, as to density, temperature and moisture, and is entirely independent of speed. We have seen that it is possible to lose 21.3 per cent. of work when compressing air to five atmospheres without any cooling arrangements. With the best compressors of the dry system one-half of this loss is saved by water jacket absorption, so that we are left with about 11 per cent., which the slow moving compressor seeks to erase. We are quite safe in saying that the element of _time alone_ in the stroke of an air compressor could not possibly effect a saving of more than half of this, or 5½ per cent. Now, in order to get this 5½ per cent. saving, we reduce the speed of an air-compressing engine from 350 feet per minute to 100 feet per minute. We must, therefore, in one case have a piston area _three and one-half_ times that of the other in order to get the _same capacity of air_, and in doing this we build an engine of enormous proportions with heavy moving parts. We load it down with a large mass of water, which it must move back and forth during its work, and thus we produce a percentage of friction loss alone equal to twice or even three times the 5½ per cent. heat loss which is responsible for all this expense in first cost and in maintenance, but which really is not saved after all unless water injection in the form of spray also forms a part of the system.