When the doors of the Asile de Nuit were closed to them, Strickland and Captain Nichols sought the hospitality of Tough Bill. This was the master of a sailors' boarding-house, a huge mulatto with a heavy fist, who gave the stranded mariner food and shelter till he found him a berth. They lived with him a month, sleeping with a dozen others, Swedes, negroes, Brazilians, on the floor of the two bare rooms in his house which he assigned to his charges; and every day they went with him to the Place Victor Gelu, whither came ships' captains in search of a man. He was married to an American woman, obese and slatternly, fallen to this pass by Heaven knows what process of degradation, and every day the boarders took it in turns to help her with the housework. Captain Nichols looked upon it as a smart piece of work on Strickland's part that he had got out of this by painting a portrait of Tough Bill. Tough Bill not only paid for the canvas, colours, and brushes, but gave Strickland a pound of smuggled tobacco into the bargain. For all I know, this picture may still adorn the parlour of the tumbledown little house somewhere near the Quai de la Joliette, and I suppose it could now be sold for fifteen hundred pounds. Strickland's idea was to ship on some vessel bound for Australia or New Zealand, and from there make his way to Samoa or Tahiti. I do not know how he had come upon the notion of going to the South Seas, though I remember that his imagination had long been haunted by an island, all green and sunny, encircled by a sea more blue than is found in Northern latitudes. I suppose that he clung to Captain Nichols because he was acquainted with those parts, and it was Captain Nichols who persuaded him that he would be more comfortable in Tahiti.

这一句：

fallen to this pass by Heaven knows what process of degradation

fallen...by...这是被动句吗？这里的fallen和介词by怎么分析呢？

另外and every day the boarders took it in turns to help her with the housework 中it怎么理解？去掉it似乎也正确吧？

encircled by a sea more blue than is found in Northern latitudes. 这里的比较句，比较分句和主句是主语相同吧？即相同的主语在不同的地方比较吗？不然is found 前缺少主语吧？

to his charges 在句中应该怎么理解含义呢？