150 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

fcure reached. To reset the thermometer, hold it vertically, bulb dowi
stake gently and the thread of mercury will reunite with the mercur
in the bulb.

THE MINIMUM THERMOMETER.

The minimum thermometer records the lowest reading experience
in a given interval The most common type is a spirit thermomete
having a small index immersed in the spirit in the stem. It is als

Fig. 17.—Hygrometer.
bung horizontally. As the temperature falls the index is carried
towards the bulb by the spirit, but if the spirit subsequently expands,
in consequence of a rise of temperature, it flows past the index, which is
left in position to indicate the lowest temperature reached. To reset
this thermometer, hold it vertically, bulb up, tap it gently and the
index will find its way back to the end of the thread of spirit. (Fig. 16)-
The Thermograph is a self-recording thermometer, which gives a