Hold the compass flat with the glass uppermost and the lid open and facing away from you. Then slowly rotate it through 360deg around the long axis still keeping it flat. The bubble will should then be trapped in a groove that runs unseen around the inside of the bowl. It will of course pop out again when you put the compass down.
Alternative method. Open the compass lid, place the whole thing on your palm face up lid pointing away. Rotate your hand palm down and then slowly bring in back to palm up. Practice makes perfect.
Very simple to do, very difficult to explain without a diagram.
As Peter has said refilling the bowl is relatively simple, full immersion is best, but I've achieved it using a syringe and a lot of patience. You can (or could) get replacement seals from the Francis Baker.