by signs the action of machinery. 259 
4. If the movements are such as not to admit of this 
distinction, then when lines are drawn adjacent 
to an indicating line, and on opposite sides of it, 
they signify motions in opposite directions. 
5. Parallel straight lines denote uniform motion. 
( 6 . Curved lines denote a variable velocity. It is con- 
venient as far as possible to make the ordinates 
of the curve proportional to the different veloci- 
ties. 
7. If the motion may be greater or less within certain 
limits : then if the motion begin at a fixed mo- 
ment of time, and it is uncertain when it will 
terminate, the line denoting motion must extend 
from one limit to the other, and must be con- 
nected by a small cross line at its commence- 
ment with the indicating line. If the beginning 
of its motion is uncertain, but its end determined, 
then the cross line must be at its termination. 
If the commencement and the termination of any 
motion are both uncertain, the line representing 
motion must be connected with the indicating 
line in the middle by a cross line. 
8. Dotted lines imply rest. It is convenient some- 
times to denote a state of rest by the absence of 
any line whatever. 
9. If the thing indicated be a click, bolt, or valve, its 
dotted line should be on the right side if it is out 
: of action, unbolted, or open, and on the left side 
if the reverse is the case. 
