Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/568

552 in streamlets, carries down with it air drawn in through sloping holes near the top of the pipe. The wind chest below has an opening for escape of the water, and the air passes out from another part, in a regular stream, by a nozzle pipe. To facilitate separation of the water and the air, it is found advantageous to fix a small platform under the bottom of the pipe, on which the water may impinge in its fall. The tension of the blast is determined by the height from which the water falls ; but this height seldom exceeds 27 feet, which gives a pressure of from 1 to 2 ft&amp;gt; to the square inch. While the blast obtained is very equable, there is the serious drawback that the air supplied is always more or less laden with moisture. The action of the trompe has been investigated by Mr Rodwell (Philoso phical Mag., 1864, 18G7). Another kind of blowing engine, in -which water is employed, is that invented by Mr Street ; in its simpler form it consists of a barrel-shaped vessel, supported hori zontally by the two ends of its axis. The cylinder is divided longitudinally by a plane extending from the middle of the internal surface above (the barrel being in its position of rest) to near the opposite side. Suppose the cylinder partly filled with water and made to turn a little way round on its axis, the air on one side will be compressed by the water, while that on the other will be rarefied. A valve opening outwards from the condensed side admits the air to a cavity from which a nozzle pipe proceeds, while a valve opening inwards on the rarefied side admits external air. With additional and correspond ing valves, the process is repeated on the reverse oscillation of the cylinder. Thus by swinging the cylinder from side to side, by a crank and rod connected with the engine, alternate puffs of air are propelled into a regulative air chest of special construction, which then supplies a steady blast.

Fan-blast machines are frequently employed, especially to urge the fire of steam boilers, and in puddling and reheating, and in the cupola furnaces where anthracite is burnt, or coke used for remelting pig-iron in foundries. In one common form the fan consists of four spokes of a rimless wheel, tipped with vanes and made to rotate in a cylindrical chest, in which it has often a slightly eccentric position. There are openings on both sides round the spindle for admission of air, which, sucked in by the centrifugal action of the fan as it quickly rotates, flows towards the vanes, and is driven through an exit pipe attached to another part of the cylinder. {{ti|1em|There are numerous varieties of these engines. An American machine, introduced into England a few years ago by Mr Ellis, has found considerable favour. It is represented in section in fig. 5. It consists of an iron cylindrical casing A, open about a fourth part of its circumference (a to 1} for admission of air, and an exit pipe B. Inside the casing is another cylinder, placed eccen trically to it, and which always fits close up against the wooden packing C. This cylinder acts as driver for the three fan blades or pistons D, which are capable of passing out and in through longitudinal slits in its circumference. There is a shaft passing through the small cylinder, and concentric with it at the ends, but cranked in the middle part so as to become concentric with the casing. The inner cylinder revolves round the axis of the ends of the shaft, mid on the cranked part revolve the fan blades or pistons, driven by the cylinder. The outer extremities of the fan blades follow closely the inside face of the casing. The crank is placed opposite to the point where the inner cylinder touches the inside of the casing, always retaining, it must be remembered, the same position ; when passing this point, the blades are wholly withdrawn inside the cylinder, but when passing the opposite point they are tKrust out to the fullest extent, and are always working into or out of the inner cylinder as it revolves. The air is thus continually being drawn in at the upper opening, compressed, and delivered by the lower one.}} Via. 5. Section of au American Blowing-Machine. The rotary blower, invented by Messrs Root of Con- nersville, Ind., is one which has of late years found extensive use both in America and Europe. The arrange ment differs in some essential features from that of the ordinary fan ; it acts by regular displacement of the air at each revolution, as shown in fig 6. A pair of horizontal shafts geared together at both ends traverse a case of the form of two semi-cylinders separated by a rectangle equal in depth to the diameter of the semi-cylinders, and in width to the distance be tween the centres of the shafts. These shafts carry a pair of solid arms, each hav ing a section somewhat re sembling a figure of eight ; the action of which, as they revolve, takes the air in by an aperture at the bottom of the machine, and expels it with considerable pressure, if required, at the top. The gearing outside serves merely to keep the revolving pieces in their proper position, and the power is applied directly to each shaft. One of these machines, employed to give the blast in a pneumatic railway under Broadway, New York, delivers, when worked to maximum speed, a volume of 100,000 cubic feet of air per minute. The engine is also much used in the Bessemer steel-works of this country. FIG. 6. Rotary Blower. Among the exhibits at a recent exhibition of the Franklii? Institute in America, was shown a new form of blower, acting much on the same principle as the Root blower, but, according to the report of the committee, offering certain advantages over the latter. From a cross section of thc- chambcr it appears that three drums of equal size are enclosed in it, two in a line below and one above ; the upper one is provided with wings, and the two lower have wide slots along their entire length, allowing the wings to enter in the course of rotation. The function of the two lower drums is to supply alternately abutments to prevent the escape of the air. They are caused to revolve in proper relation with the motion of the upper drum by spur-wheels on the journals, which mesh into another spur-wheel on the shaft of the upper drum. In the moving parts of this machine there are no parts that come into actual contact except the teeth of the spur-wheels. The report allows the 