What do you think of the first paragraph of a chapter in my book?

I agree that the first sentence is too long. It should be two sentences, the first one ending with "Downy Street." But then the second sentence doesn't quite make sense. The end of the sentence, "...stood far from the...buildings that shielded the people an absolute panorama of a vast and well-polluted river" sounds like something is missing after the word "people." The part of the sentence about the river seems unnecessary or needs to be included elsewhere. The second-to-last sentence would be stronger if you changed "it" to "the museum." It's an interesting beginning because the reader wants to know more about the museum and its secret visitors, but I think it would be stronger if you removed some of the descriptive phrases and just wrote it more directly. For instance, the paragraph would have a stronger beginning if you said, "Five miles from the Asylum..." instead of "A mere five miles..."