This, the text tells me, is an “outside-spring indicator, by Messrs. Elliott Bros.” The spring is not exposed to high temperature, which makes it better for indicating. I’m not sure what, but I imagine it has to do with pressure.
As well as the wonderful Steam-engine Theory and Practice, Mr. William Ripper was apparently the author of the intriguingly titled Heat Engines. I’m imagining that one makes for a fun winter’s read.