=Fans.=—Very large tubes of moderate length can be operated by ordinary centrifugal fans. These fans are capable of supplying air under a pressure not exceeding ten or twelve ounces per square inch with very good efficiency. They are the simplest and most inexpensive of all blowing-machines. =Blowers.=—When tubes have a length and diameter that require a pressure from one to four pounds per square inch, some form of positive blower of the Root type can be used with economy. Their construction is familiar to nearly every one at all interested in machinery, so we need give no space to their description here. [Illustration: FIG. 42. THE STURTEVANT STEEL PRESSURE BLOWER.] [Illustration: FIG. 43. ROOT’S POSITIVE PRESSURE BLOWER.] [Illustration: FIG. 44. SECTION OF ROOT’S TRUE CIRCLE BLOWER.] [Illustration: FIG. 45. THE GREEN BLOWER.] [Illustration: FIG. 46. SECTION OF THE GREEN BLOWER.] =Air-Compressors.=—By far the greater number of our tubes require an air-pressure of more than five pounds per square inch. For such air supply we recommend some form of air-compressor, and usually this is driven by a steam-engine, which forms a part of the compressor. In making our selection we should bear in mind the conditions under which the compressor will run. Usually it must be kept in constant operation at least ten hours per day, and frequently for a much longer period. This makes it important that the compressor be substantially built and supported upon a solid and firm foundation. The bearings should be broad, of good wearing material that has a low coefficient of friction, and provided at all times with ample lubrication. If poppet valves are used in the air-cylinders, and they are most common, the speed in revolutions per minute should not be high. Duplex are better than single cylinder compressors, because they deliver the air in a more steady stream,—the pulsations are less. For constant running, economy of steam is an important item; therefore some good type of cut-off valve should be provided. The air-cylinders should not be water-jacketed unless the pressure is above twenty-five pounds per square inch. It is better to use the air as warm as possible, for it will soon be cooled after entering the tube. A speed-governor should be provided with compressors which are to run at constant speed, but usually they will be run to maintain a constant pressure in the tank, and to this end a good and reliable form of pressure-governor should be provided, together with some reliable safety device to stop the engine when the speed exceeds a safe limit. But most important of all is to have the valves of the air-cylinders large in area; otherwise the efficiency of the machine will be very low. With machines working under eighty pounds pressure, a difference in pressure of one pound on opposite sides of the valves has but little effect, but when the machine is only compressing to five or ten pounds, one pound