Page:The Kinematics of Machinery.djvu/472

 450 KINEMATICS OF MACHINERY.

firmly and carry with us the apparently elementary notion that the fixed part of the machine is really a portion of its kinematic linkage. It is only too easy to forget that the masonry, timber, flooring, and so on, upon which the pedestals, guides or framing- are fixed, have by that very fact become a link of the kinematic chain of a machine. I have already remarked ( 58) how often the fixed link is omitted from engravings. Unquestionably this omission has arisen from indistinctness as to its nature, and it reacts j in a similar direction upon those for whose A Jl use the engraving has been made. There is // nothing whatever to help the latter to omitted is the one which must be fixed. Who would imagine, for instance, from the accom- panying figure of an oscillating engine, taken from a modern kinematic text-book, that the bearings A and B must be rigidly connected ? They are apparently quite without connection of any kind. The example I have given is, Fro 311 however, only one among many. We cannot
 * realise the fact that the important link

be surprised, therefore, that this connection

has often been carried out incompletely in actual machine con- struction. Those engineers who are old enough will remember the noise made about the form in which Penn constructed his oscillating marine engines, simply because he employed cross-shaped frames to give special rigidity to the frame of his machine. And yet Penn did nothing more than carry out the simple requirement which we recognised at the very beginning of our investigations, as belonging to that link of the kinematic chain.

We see an exactly similar improvement now being carried out in the horizontal steam-engine, in the introduction (in America first) of a straight heavy frame directly connecting the cylinder and the plummer-block. This bed-plate of the Corliss and Allen engine, of Tangye's engines and others is nothing else than the frame d of our turning slider-crank (g'P- L ) d Fig. 312. It is difficult to believe that this special form of construction has been so recently introduced that the " improvement " embodied in it is still more or less a subject of remark, while it seemed to develop itself as a matter of course from our first propositions. But the way in