Sentence:  It was Dryden's opinion . . . that the drama required an alternation of comick and tragick scenes; and that it is necessary to mitigate, by alleviations of merriment, the pressure of <WSD>ponderous</WSD> events, and the fatigue of toilsome passions. 
Meaning of the word marked with <WSD> tags:  Serious, onerous, oppressive.