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and the outlet for the discharge of air is by means of a series of pipes or tubes of equal crosssectional area from inlet to outlet, and curved backwards. The Hopton open-running fan, which has quite recently been introduced, has an inlet on each side, a central diaphragm, and the blades curve backwwards and run into the circumference practical tangential. It would he scarcely possible to design or construct a simpler fan than the one identified with the name of Hopton. The revolving portion, consisting of the arms and the blades, works between two brick-walls, and I can well imagine that the cost of construction and erection will be small. In dealing with closed-running fans : The Guibal has always been a more or less massive structure of considerable breadth as well as of diameter, and receives air on both sides, or on one only. The blades are inclined backwards, and curved at the extremities, so that the tips are radial. This fan revolves in a fixed easing at the sides and for most of the circumference, the clearance being as little as possible, except for a certain distance at the bottom, through which the air is discharged, the amount of opening being regulated by an adjustable shutter into a gradually enlarging chimney. The Cockson fan is virtually a Guibal fan, with a modification that the blades contract in width towards the tips, and the passage for air is uniform through the fan. The inventor and patentee (Mr. Cockson), after very careful consideration and elaborate experiment, satisfied himself that the proportioning of the blades according to his design enabled the Guibal fan to work at a higher velocity and increases the useful effect. The special improvement giving rise to the name of the Walker-Guibal was in the shutter, which, instead of the original rectangular form with the upper edge horizontal, was made after the manner of an inverted elongated V —thus _, In the ordinary Guibal each blade has the maximum resistance to face in approaching the upper edge of the shutteropening, and has the least resistance immediately on passing that opening. The result is an injurious rebound for every blade in every revolution. The anti-vibration shutter identified with the name of Messrs. Walker Brothers avoids this. The result was that vibration ceased, and practically a silent fan was produced, enabling a higher speed and less wear-and-tear. In the Schiele type of centrifugal ventilator we have a backward inclination of the blades; which, in the modern designs, has assumed a curvature, also backwards. The blades are contracted in width from inlet to outlet, and the fan (that is, its revolving part) is surrounded by a spiral casing, into which the air discharges all round the circumference, the space constantly increasing until it reaches the chimney. The Walker indestructible fan has blades curved backward from within the inlet to the circumference. The air is received on both sides, and there is an especially strongly constructed central diaphragm built up with separate layers of plates, and altogether a very strongly-made appliance. The spiral casing, which forms a portion of this arrangement, does not entirely envelop the revolving part, and the improved shutter referred to forms part of the arrangement. This fan, although recently introduced, has already established itself in various parts of the United Kingdom, and promises, in addition to the excellent results obtained, to fulfil the name it bears. In the Capell ventilator we have a combination. The central portion is a drum which receives the air on both sides. Within this drum we have blades curved backwards, and apertures in the circumference between each pair of blades, through which the air passes into the outer chamber, in which we also have blades curved backwards. The revolving portion of the arrangement includes the sides of the fan, and the air is discharged all round the circumference through a spiral casing into an enlarging chimney. The Johnston, which is a very recent form of ventilator, is the result of numerous experiments made as to the form of the blades and the design of the fan generally. The width is much less than in other fans, and the air is received on both sides. The blades are straight, and inclined backwards. They revolve within a fixed casing at each side, and discharge into a spiral casing which rapidly increases in area. The chimney is a very large one, simply clearing the top of the fan, and a considerable portion of this chimney is immediately over the fan. In the Bateau centrifugal ventilator we have the usual spiral casing and the enlarging chimney, which seems to rise much higher, in proportion to the size of the fan, than in any other arrangement. The general outline is not unlike that of the Waddle fan, but the blades are much more numerous, and these blades extend practically to the centre of the fan, and have a peculiar curvature slightly forward, and also a curvature in the line of the fan shaft. Immediately in front of the blades there is a cone, terminating in a point. Open Running and Closed Bunning. —Having briefly and generally described a considerable number of the best known types of centrifugal ventilators of the two great classes of open-running and closed running, a few remarks may be made as to the aims and principles of both classes. They agree as regards the endeavour to admit the air from the upcast shaft into the fan easily, to pass the air through the fan with as little resistance as possible, and also to discharge the air from the fan easily; in these points lie the whole object of the fan. As to the inlet, that may be and should be the same in both. Eegarding the passage of the air through the fan there is an important difference. An open-running fan must, to be efficient, discharge the air at a very low velocity, because, as is well known, the energy of bodies in motion increases, not merely as the velocity, but as the square of the velocity, and the energy in the discharged air is so much useless work. To overcome this, the passages in the more correct open-running fans, in my opinion, are curved backwards. The difficulty that I have always felt is that the air cannot be discharged from a fan without possessing considerable velocity, and in that must rest to that extent a defect in all open-running fans. As regards the outlet, my opinion is that as exactly the same quantity of air must leave the fan that enters it, the passages from inlet to outlet should be of uniform capacity throughout. Now, we consider the closed running fan, and the, 1 special advantages of this class, when properly designed, are, to my mind, very great. They have equal facilities with open running as to inlet. They are equal to open running in regard to free passages through the fan, and blades can be curved backwards. They possess the enormous advantage that the air may be discharged from the fan at a high velocity without any material loss of energy. The gradually increasing capacity into which they discharge reduces the velocity, utilises the energy of the motion of the air,