58. The term cowl is applied in a general way to all apparatuses or fixtures which are placed over the top of chimneys or ventilating flues, etc., to aid the draft. They serve to protect the ascending current of hot gases within the flue from the influence of contrary wind currents, which might oppose or even overbalance and reverse it. They also serve to facilitate the escape of the warm gases into the area of lower pressure, which always exists upon the leeward side of a chimney or ventilating shaft while the wind is blowing. During calm weather, a cowl always obstructs the draft somewhat.

A common form of cowl is shown in Fig. 18. It is essentially composed of a conical top E, supported by three or four legs on a frustum of a cone or deflecting collar C. The diameter of the cone should be from 2 to 2 1/2 times that of the pipe to which it is attached, and the distance from the edge of the pipe to the under surface of the cone should not exceed one-half the diameter of the pipe.

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Fig. 18.

59. The wind may be utilized to drive a current of air downwards in a pipe by an arrangement similar to that shown in Fig. 19. The pipe is provided with a funnel a, and a cone b having its apex downwards. A part of the wind which strikes the cone will be deflected downwards into the pipe, as shown by the arrows. But there will be an area of low pressure at B, and a part of the contents of the pipe will escape into it, as shown by the dotted arrow; consequently, some of the air will be lost. This device is called an induction, or blowing, cowl.

60. Automatic cowls are commonly made as shown in Fig. 20. An elbow a, having a funnel mouth b, is mounted so as to turn freely upon a central spindle c, as shown. The vane v catches the wind, and operates to keep the funnel always turned away from the wind. This device for aiding the draft of chimneys is very effective, but it is difficult to maintain in good working order. The pivot corrodes so rapidly that the elbow is apt to stick fast and fail to operate. This apparatus may also be used to produce a downward current, for ventilating purposes, by changing the vane v so that it will hold the funnel towards the wind, instead of away from it.

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Fig. 19.

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Fig. 20.

61. The cowl shown in Fig. 21 is provided with a blowing cone a, which causes the cowl to operate as an ejector. The cone gathers the wind which it catches into a small current of high velocity; and, as this current emerges into the mouth of the elbow or tube b, it communicates its velocity to the gases at the top of the chimney, forcing them ahead with it, and creating a partial vacuum around the cone. The gases from below then rush upwards with an augmented velocity, owing to the increased difference of pressure. This apparatus is called an automatic eduction cowl.

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Fig. 21.