DESCRIPTIVE ANALYSIS. 515
In complete machines then, apart from other secondary mechanisms, which can generally be placed without any forcing in one or other of the subdivisions which we have considered, we frequently find that besides their
Driver and "Work-piece,
(a) the main-train, in which receptor and tool may exist, (&) the director, with its subdivisions supply and delivery, (c) the regulator, with its subdivision stop-gear, (rf) the gearing or transmitting-gear,
can be distinguished each as a distinct and separable mechanism. Their separation, which places before us the general purpose of the combination of mechanisms forming the machine, we may call the descriptive analysis of the machine.
The separation of machines into the two classes of form- changing and place- changing is often useful in considering their purpose as a whole, especially in those cases where the change affects the work-piece, where it concerns the driver ( 132) it is of less practical importance. It may therefore well find a place wherever machines are systematically treated. It must always be remembered, however, that the distinction is not an essential one, but strictly speaking a difference of degree only and not of kind. This fact makes it sometimes doubtful whether a machine belongs to the one or to the other class. In every case, however, it is according to the nature of its main-train that the machine is classed an additional reason for examining the charac- teristics of this mechanism separately.
In the form of analysis which we have indicated in this section, we have rather developed and systematised a method not unfre- quently made use of by practical men, and given it distinct form, than introduced a completely new idea. We often find machines explained from a point of view very nearly resembling ours. It appears to me altogether very desirable that this descriptive analysis should form the first part of the description of a machine. The complete or abstract analysis, examining its mechanisms in detail, can be added afterwards. In many cases this may be unnecessary ; as, for instance, where the particular mechanisms have already been studied by themselves. Two things must always here be borne in mind; first, that machines do not invariably divide themselves
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