I don't understand the phrase
The glass still keeps very high
in the adventure The Boscombe Valley Mystery in The Adventures of Sherlock Holmes, by Arthur Conan Doyle http://www.gutenberg.org/ebooks/1661.
Here is the whole paragraph:
"The glass still keeps very high," he remarked as he sat down. "It is of importance that it should not rain before we are able to go over the ground. On the other hand, a man should be at his very best and keenest for such nice work as that, and I did not wish to do it when fagged by a long journey. I have seen young McCarthy."