What is a water thermometer?

The Water Thermometer is an instrument in which water is substituted for mercury, for ascertaining the precise degree of temperature at which water attains its maximum density. This is at 39.2 °F., or 4 °C., and from that point downward to 32 °F., or 0 °C., or the freezing point, it expands, and it also expands from the same point upward to 212 °F., or 100 °C., or the boiling point.