machines do not create or consume, but only transmit and apply power; and it is only by conceiving of power as a constant element, independent of every kind of machinery, that the learner can reach a true understanding of the nature of machines. when once there is in the mind a fixed conception of power, dissociated from every kind of mechanism, there is laid, so to speak, a solid foundation on which an understanding of machines may be built up.

to believe a fact is not to learn it, in the sense that these terms may be applied to mechanical knowledge; to believe a proposition is not to have a conviction of its truth; and what is meant by learning mechanical principles is, as remarked in a previous place, to have them so fixed in the mind that they will involuntarily arise to qualify everything met with that involves mechanical movement. for this reason it has been urged that learners should begin by first acquiring a clear and fixed conception of power, and next of the nature and classification of machines, for without the first he cannot reach the second.

machines may be defined in general terms as agents for converting, transmitting, and applying power, or motion and force, which constitute power. by machinery the natural forces are utilised, and directed to the performance of operations where human strength is insufficient, when natural force is cheaper, and when the rate of movement exceeds what the hands can perform. the term "agent" applied to machines conveys a true idea of their nature and functions.

machinery can be divided into four classes, each constituting a division that is very clearly defined by functions performed, as follows:—

first. motive machinery for utilising or converting the natural forces.

second. machinery for transmitting and distributing power.

third. machinery for applying power.

fourth. machinery of transportation.

or, more briefly stated—

motive machinery.

[29] machinery of transmission.

machinery of application.

machinery of transportation.

these divisions of machinery will next be treated of separately, with a view of making the classification more clear, and to explain the principles of operation in each division. this dissertation will form a kind of base upon which the practical part of the treatise will in a measure rest. it is trusted that the reader will carefully consider each proposition that is laid down, and on his own behalf pursue the subjects farther than the limits here permit.

(1.) to what three general objects are machines directed?—(2.) how are machines distinguished from other works or structures?—(3.) into what four classes can machinery be divided?—(4.) name one principal type in each of these four divisions.