At this Snowball sprang to his feet, and shouting down the sheep, who had begun bleating again, broke into a passionate appeal in favour of the windmill. At this=At hearing this,this指代前面所说的内容。放在句首作状语(介词at在这里表示动作发生的原因，翻译时宜按时间状语翻译，比如：一听到这话)。 and连接的两个并列分句，第二个分句中，shouting down……为现在分词短语作时间状语，主句是broke into a passionate appeal in favour of the windmill。