Both have a meaning of uninterrupted time and are synonyms. Continuous also includes uninterrupted space and sequence. So you might say the skyline has a continous expanse of skyscrapers but not a continual one.
Each word also has a different second definition.
Continuous is a mathmatical term and continual could not be substituted in this case.
Continual has a definition of occurring in steady but interrupted sequence such as a continual series of wars. If you say a continuous series of wars, then the wars have no time between them, whereas the continunal series could have short breaks between the wars.
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