344 KINEMATICS OF MACHINERY.
for the periodic opening and closing of the passages leading to the chamber. These arrangements may be included under the name of valve-gear, or more generally, as we shall see in 135, of directing-gear.
The chamber and piston form a kinematic chain with the (liquid or gaseous) fluid, and this is united with the crank mechanism into a compound chain, which, under some circumstances, may require a very extended formula. We may, however, disregard this for the present, keeping before our minds always the forms of piston and chamber, but not considering the fluid itself. This will greatly simplify the matter, while at the same time it will be quite sufficient for our present purposes. Crank mechanisms having their links so formed, (as chamber and piston), that they are suited for the enclosure and motion of a fluid, we shall call chamber-crank trains. As the constitution of the machine itself is independent of that of its valve-gear we need say very little about the latter here; short descriptions of its general nature will always suffice. We shall come later on (Chaps. XI and XII) to the consideration of the real kinematic meaning of the valve- gear, as well as that of the pressure-organs, in such cases as those now before us.
79.
Chamber-crank Trains from the Turning Slider- crank.
(Plates XIV. and XV.)
The arrangement adopted in the table of mechanisms in 74 gives them in what appears to be their natural order. In order, however, to reduce as far as possible the difficulties unavoidably connected with our analysis I shall here consider them in a diffe- rent way, commencing with the mechanism which is probably most familiar to the reader, that used in the ordinary " direct-acting " steam-engine. We therefore begin with the chain (Cg'P- 1 -), and take first the mechanism (C" 3 'P- L ) d , the turning slider-crank. Plates XIV. and XV. show schematically eight forms of chamber-crank gear formed from this mechanism. We shall consider them in order.
PL XIV. 1. is the form familiar to us in common direct-acting
