That's an interesting question! I instinctively felt that there was a difference between "continual" and "continuous" but couldn't place it myself... This is quoted from the dictionary. "USAGE In precise usage, continual means 'frequent, repeating at intervals' and continuous means 'going on without pause or interruption': - we suffered from the continual attacks of mosquitoes; - the waterfall’s continuous flow creates an endless roar. The most common error is the use of continuous where continual is meant: : continual (that is, 'intermittent') rain or tantrums can be tolerated; continuous (that is, 'uninterrupted') rain or tantrums cannot be tolerated. To prevent misunderstanding, some careful writers use : intermittent instead of continual, and : uninterrupted in place of continuous. Avoid using continuous or continuously as a way of describing something that occurs at regular or seasonal intervals: in the sentence, : our synagogue’s Hanukkah candle-lighting ceremony has been held continuously since 1925, the word continuously should be replaced with annually."

I've never heard anyone say "continual", though I have heard used as an adverb--"continually"; for the adjective with that specific meaning, I would say "continuous". Something which is constant is both continuous and unchanging. It maintains a specific value and does not stop maintaining that value.