adjective
BAD : The canals join to form one continual waterway.
GOOD : The canals join to form one continuous waterway.
BAD : A line of cars stretched continually down the motorway.
GOOD : A line of cars stretched continuously down the motorway.
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continual = happening repeatedly over a long period of time, especially in a way that you find annoying: 'That telephone has been ringing continually.'
continuous = without any break or interruption: 'Gas central heating provides a continuous supply of hot water.'