The two verniers on the azimuth circle having been read, the magnet is then inverted, i.e.
These may be used in either the upright or the inverted position.
These, being no longer stoved in an inverted position, as had been the habit before Shirozaemons time, were not disfigured by the bare, blistered lips of their predecessors.
It varies in colour, but a black mark on the head like an inverted V remains nearly always visible.
The principle is to have a constriction in the tube above the bulb so proportioned that when the instrument is upright it acts in every way as an ordinary mercurial thermometer, but when it is inverted the thread of mercury breaks at the constriction, and the portion above the point runs down the now reversed tube and remains there as a measure of the temperature at the moment of turning over.