I have an odd device and I'd like to know if anybody has any idea what it is or can tell me something about it.  Your theories and opinions are of interest but if you know for certain I'd be especially interested in whatever information you can provide.  This thing weighs exactly 30 pounds.  The glass on the front is thick...I don't know how thick, but thick enough to suggest some significant pressure is involved.  It has some kind of glass piece inside that contains mercury.  There is writing on it that you can see in the picture, I'll convey some of it here:

"MERCOID SWITCH" is on the inside.  Around the outside edge it says:

"IN HAZARDOUS ATMOSPHERES THIS COVER MUST BE KEPT TIGHT WHILE CIRCUITS ARE ALIVE"

There's a website for "mercoid.com" and other places to find information, but rather than look at the back of a murder mystery and find the answer, I'm curious what existing knowledge might provide.  Below are some pictures, click on the thumbnails to view the whole pictures (total size of all pictures approximately 5MB.

Here is a photo showing the general shape

Here's a different angle

Here's a view from the top, the $10 is to provide scale

Here's the first part of the message around the edge

Here's the second part of the message around the edge

Turning these two knobs moves the hands on the inside

Here's an outside view with a $10 bill to provide scale

This is a closeup of the inside, the knobs in the prior picture move these hands

Mercoid Switch is what this says...they even have a website mercoid.com