150 ON MR. EWART’S PAPER ON THE 
in motion. M. Poisson observes that the principle 
of virtual velocities gives, immediately, the con- 
ditions of the equilibrium of forces applied to 
machines, whilst that of living forces includes 
equally the theory of their motion, and furnishes 
the most direct means of calculating the effects of 
the forces which are applied to them. After 
giving the most general notions upon the subject, 
he refers for developments to M. Poncelet and 
M. Navier. 
Machines, he says, may be defined as instru- 
ments or systems of solid bodies, adapted to 
transmit the action of the forces from one part to 
another of these assemblages. 
When a machine is in motion, certain points of 
it are subjected to the action of given forces, and 
other parts press upon exterior bodies, or are 
reciprocally pressed by those bodies which the 
machine is intended to displace or change the 
form of. The first description of forces are called 
moving forces; the forces opposed to these are 
called resisting forces. 
The connexion of the parts of a machine is 
such that it can, in general, only take two mo- 
