Page:The Kinematics of Machinery.djvu/609

 [I think Prof. Reuleaux is mistaken in giving this as Willis's  of a machine. Willis merely says (Preface, 1st Ed., p. xiii.)—" .... instead of considering a machine to be an instrument by means of which we may change the direction and velocity of a given motion, I have treated it as an instrument by means of which ....," etc. He nowhere gives a formal definition of a machine, but in one place (Preface, p. iv.) describes it in a sentence which more nearly agrees with Reuleaux's definition than any other I have seen, although it certainly is still incomplete:—"Every machine will be found to consist of a train of pieces connected together in various ways, so that if one be made to move, they all receive a motion, the relation of which to that of the first is governed by the nature of the connection."]

. "An arrangement which is designed to receive motion from the action of a motor, to alter this motion, and to transmit it to an instrument formed so as to execute any kind of work." This gives certain characteristics of the machine, not, however, what it actually is.

. "We give the name machine to a system of bodies which is intended to transmit the work of forces, and consequently both to alter the intensity of the forces, and to make the velocity and direction of the motion produced such as is suitable for the purpose in view." What is a "system of bodies?" is it sufficient that it should be "intended" for all this? and so on.

. "A machine is a body or a number of bodies intended to receive at one point certain forces, and at other points to exert certain forces, the latter being in general different from the former, both in intensity and direction, and in the velocity of the point at which they are exerted." Again the "intention." The whole also is a description, not a definition.

. "A machine is an apparatus which is intended to connect a motor with the material to be worked upon." Apparatus, motor, material to be worked on, connection? From a logical point of view, how many riddles! The reader may fairly exclaim Davus sum, non Oedipus!

Lastly,  Universal Lexikon (? Hulsse). "Machine—an arrangement by which a motion, i.e., a change of place or of form, may be given to a body, by which, that is, in general, some work may be done or mechanical effect obtained." This is only descriptive, and suits a multitude of things which are not machines.

[The definition I have quoted from Willis is certainly more accurate than any of those which have followed it, it is indeed so obviously better than most of them that it is matter for wonder that it has not been generally adopted. In order simply to shew to what extent indefinite definitions have passed current here also, and not from any desire to criticise, I may add the following to Prof. Reuleaux's catalogue:—

Hart (1844). "A machine is defined to be an instrument, by means of which a given force is caused to make equilibrium with a resistance to which, it is either unequal, or not directly opposed."

Goodwin (1851). "Any contrivance by which force is transmitted from one point to another, or by means of which force is modified with respect to direction or intensity, is called a machine." Goodwin expressly includes as machines an oar, a poker, etc.

Galbraith and Haughton. "A machine is an instrument, by means of