FORCE-CLOSURE BY AIR VESSELS. 199
examine only one of them. These curves have several points in common with the centroids in the last case, the internal rolling and the ratio of their peripheral lengths 1:2.* Their forms, however, correspond to the unequal velocity-ratio of the two rotating arms, a and c. The lightly shaded and larger branch 2 A 2 of the centroids of a rolls in the arc E OF of that of c, the loop 2 B 2 in the arc E D F. In the two change-points A and C and B and D respectively come into contact. By once more using pins and gabs, placed at these points, we can here also obtain complete closure. The plan above our figure shows in outline how this may be carried out constructively, in a way which might replace with advantage the force-closure of Galloway. Otherwise the mechan- ism is not described as one which has any practical application, hitherto at any rate its interest, although great, is purely theoretical. We cannot tell, however, if it may not find in the future some useful application. It need only be further remarked that here, as in Fig. 154, the second pair of centroids might equally well have been used for the pair-closure.
Among those arrangements which have for their primary object the carrying of a mechanism over its dead points must be included the air vessels of fire-engines, steam-pumps, and hydraulic machinery, those, too, of various kinds of bellows, simple hand- bellows and those of the smithy or organ, and the wind-chests of blowing-engines. The air vessel at the same time corresponding exactly to the fly-wheel of a steam-engine acts as a means of making the motion of the out-flowing pressure-organ uniform, it acts, that is, not only at the dead points, but before and after them. Its action depends like that of the fly-wheel upon force-closure ; in which relation however it goes even further, for the organ by means of which the action takes place, the air, is itself force- closed.
It is interesting to observe how, in machine construction, chain- closure is more and more superseding force-closure as a means of passing the dead points. One whole class of engines, those which have to be reversed, are almost always made with two cylinders as coupled engines, i.e. with chain-closed dead points. This has even extended itself to the clumsiest of all our steam-engines, those for
- It will be seen further on, 70, that the two mechanisms are in reality the same.
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