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 51/2 per cent.  Now, in order to get this 51/2 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 51/2 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.

It is obvious that cost of construction and maintenance have much to do with the commercial value of an air compressor.  The hydraulic piston machine not only costs a great deal more in proportion to the power it produces, but it costs more to maintain it, and it costs more to run it.  It is not an uncommon thing to hear engineers speak of the hydraulic piston compressor as the “most economical” machine for the purpose, but that it is so “expensive” and takes up so much room, and requires such expensive foundations that, unless persons are “willing to spend so much money,” they had better take the next best thing, a high speed machine.  We hear of “magnificent air-compressing engines, the largest in the country,” and pilgrimages are made to see these artificial wonders when, not unlike the old pyramids, they represent a pile of inert matter—­a monument to moneyed kings.