28 SECTIONAL ADDRESSES 
the unobserved changes are supposed to be eliminated. In certain cases 
this function will be expressed as the difference of a kinetic energy and 
a potential energy function; an important case is the case where the 
unobserved changes appear in the original Lagrangian function as 
velocities only and there are no product terms which involve a velocity 
belonging to the observed and a velocity belonging to the unobserved 
changes. There are also cases where the modified function is of this 
form approximately. 
