THE LEVER AND WHEELWORK. 243 



THE LEVEE AND HEELOEK. 





A MACHINE is an instrument by which force or motion may be transmitted 

 and modified as to its quantity and direction. There are two ways in which 

 a machine may be applied, and which give rise to a division of mechanical 

 science into parts denominated statics and dynamics ; the one including the 

 theory of equilibrium, and the other the theory of motion. When a machine is 

 considered statically, it is viewed as an instrument by which forces of deter- 

 minate quantities and directions are made to balance other forces of other 

 quantities and other directions. If it be viewed dynamically, it is considered 

 as a means by which certain motions of determinate quantity and direction may 

 be made to produce other motions in other directions and quantities. It will 

 not be convenient, however, in the present treatise, to follow this division of 

 the subject. We shall, on the other hand, as hitherto, consider the phenomena 

 of equilibrium and motion together. 



The effects of machinery are too frequently described in such a manner as 

 to invest them with the appearance of paradox, and to excite astonishment at 

 what appears to contradict the results of the most common experience. It will 

 be our object here to take a different course, and to attempt to show that those 

 effects which have been held up as matters of astonishment are the necessary, 

 natural, and obvious results of causes adapted to produce them in a manner 

 analogous to the objects of most familiar experience. 



In the application of a machine there are three things to be considered : 

 1. The force or resistance which is required to be sustained, opposed, or over- 

 come. 2. The force which is used to sustain, support, or overcome, that re- 

 sistance. 3. The machine itself, by which the effect of this latter force is 

 transmitted to the former. Of whatever nature be the force or the resistance 

 which is to be sustained or overcome, it is technically called the weight, since, 

 whatever it be, a weight of equivalent effect may always be found. Tl^ 

 force which is employed to sustain or overcome it is technically called the 

 power. 



