The thermometer, known to have been used by alchemists by 1610, may have been invented by Galileo, but is known to have been improved by him. The earliest type, the water thermometer or thermoscope, was a helix shaped glass tube with an air filled bulb at the bottom to push a water column up the tube when heated. More effective liquid thermometers were made around 1640 with wine. Marking freezing and boiling points on thermometers became practice in the late 1600s with mercury thermometer, beginning with the Fahrenheit scale then Celsius scale in 1740. The ‘metallic’ thermometer, which appeared about 1910, is a circular wall mount in a wooden frame with a dial and indicator needle that resembles a clock and popular in the domestic market. Most early simple mercury thermometers were mounted to ivory, brass, copper, wood, porcelain or agate, marked with graduated scales; metal was typically preferred for outdoor use. Thermometers that leave an indicator where temperature reaches a low were developed to record minimum temperatures and are usually mounted alongside regular thermometers, the pair called maximum-minimum thermometers. A solar radiation thermometer measures maximum temperature of solar rays in open air and dates to the late 1800s. It consists of an outer tube about a foot long with a large 2-inch diameter glass bulb vacuum chamber at one end with a mercury thermometer suspended inside the outer tube. An instrument developed in 1820 consisted of a ‘U’ shaped glass tube with bulbs on each end and filled with ether with a thermometer attached alongside. The temperature at which the ether evaporates from one end of the tube and condenses on the other determines the ‘dew point’. Another method of measuring humidity is by comparing wet and dry bulb thermometers. One thermometer is connected to a tube with water and kept wet, so the difference in temperature between the two indicates humidity. The thermograph is a small cabinet-housed, pen-operated, temperature recording devise that graphs temperature changes on graph paper.