98 THE MECHANICAL FUNCTIONS. 



may be its parts, however various the forms of its v»^heels, 

 its levers, or its pulleys, and however complicated may be 

 their connexions, resolves itself into a series of intermediate 

 instruments for the transference of motion from the source 

 of power, or the point v.'here its action is impressed, to the 

 parts which are designed ultimately to receive the action of 

 the force employed. It is an established principle in physics, 

 that mere m.achinery is incapable of generating mechanical 

 force, and that such force must always be originally derived 

 from some extraneous source. Some impulse from without, 

 whether it be the pressure of the wind, the fall of a stream of 

 water, or the action of men or horses, or any other kind of fo- 

 reign agency, must be resorted to, both to set the engine in 

 motion, and to continue its movements when they are once 

 begun. Nor is the case essentially different when the source 

 of motion apparently resides in some internal part of the 

 machine itself; in a watch, for instance, which is actuated by 

 the main spring; or in a steam-engine, which is set in mo- 

 tion by the elastic vapour contained in its cylinder: the 

 spring in the one case, and the vapour in the other, although 

 they may in one sense be regarded as impelling powers, are, 

 in reality, but intermediate agents in the distribution of a 

 force orioiinatingfrom other sources. In the watch, the force 

 may be traced to the hand which coiled the spring: in the 

 steam-engine to the fire, which has imparted elasticity to 

 the vapour. 



The living body differs from inorganic machinery in con- 

 taining within itself a principle of motion not referrible, as 

 far as we can perceive, to any of the primiary forces which 

 exist in the inanimate world. This principle has been 

 termed conlractility. In animals of the simplest construc- 

 tion, every part of the substance of the body seems to be 

 equally endowed with this contractile property, although ex- 

 hibiting no distinct appearance of a fibrous structure. This 

 is the case with all the lower zoophytes, such as the Infuso- 

 ria, Polypi, Medusas, and the simpler kinds of Eniozoa, 



Among the Polypi and Infusoria we meet with a singular 

 mode of acting upon the surrounding fluid by means of very 



