One point of extreme practical importance in all types of furnace tops is the path of the main bell in its descent. It is obvious that if it be attached directly to the end of the bell lever, the latter, swinging as it does in the arc of a vertical circle, will carry the bell in a similar path and in the plane of its motion it will be either one side or the other of the center of the furnace, practically all the time. The projecting end of the bell lever is quite long, and it looks as though the curvature of the path described by it could be ignored, but this is by no means the case. The versed sine of the arc described always amounts to a couple of inches and may easily exceed this, which is enough to insure there being a considerably greater portion of the charge on one side of the furnace than on the other, and much bad furnace work has been attributable to this irregularity. The path of the bell rod must always be a straight line exactly in the center of the furnace. This straight line can be produced either by a rigid guide for the bell rod, connecting the latter to the lever with a short link; a quadrant from which a flexible suspension member unwraps always in a vertical line as the lever rises and falls, or a mechanical parallel motion similar to that of Watt's steam engine. A good illustration of the quadrant system is shown in Fig. 34, in which the bell is suspended from the triangles which are hung from chains working in grooves in the castings on the ends of the bell lever, these castings being segments of circles concentric with the center of the lever.

Fig. 54. Top of Ford-Parkes system.

The parallel motion design is well shown by Fig. 36. The bell rod is not connected directly to the end of the main bell lever but to a short link which projects above it, the upper end of which is guided by a connecting link pivoted to a projection on the fulcrum of the main bell lever. The proportions are such that the bottom point of the suspension link where it is attached to the bell rod moves in an absolutely straight line. In Fig. 36 the small bell is guided by the main bell rod and therefore is not provided with any parallel motion, only with a suspension link to connect the curved movement of the bell lever with the straight-line movement of the bell rod. But in many cases the lever of the small bell is provided with its own parallel motion also. The rigid guide is used on the Baker top, the (two) bell rods passing through guide castings riveted to the gas seal and others on the superstructure at the upper end of the rods. The bell lever is connected to the rods by a short link as shown (not very clearly) by the drawing.