The Science of the Weather 777 



inches high from floor to roof inside measurements. The roof is 

 double with an air-space between, and the sides of the box consist 

 of a double set of louvres, while the bottom also is so constructed 

 as to provide free ventilation to the thermometers withm, though 

 at the same time preventing sunshine from falling upon them. 

 The screen is placed over grass at a height of 4 feet, and the site 

 is so chosen that it shall be as open as possible to ensure the true 

 atmospheric temperature being recorded. The screen and stand 

 are always painted white both inside and outside as a further 

 precaution against the effects of the sun's heat. 



Inside the box there are usually mounted four thermometers, 

 two of the ordinary pattern in a vertical position on a frame ; one 

 of these has its bulb covered with thin muslin, which is maintained 

 always in a damp condition by water which is drawn by capillary 

 attraction along some threads of cotton wick from a water vessel 

 kept beside the thermometer. These two thermometers are known 

 as the Dry and Wet bulbs respectively. The dry bulb registers 

 the actual air-temperature, while the wet bulb gives a reading 

 somewhat lower. This is due to the loss of heat caused by evapora- 

 tion taking place from the wet muslin, and the difference in 

 temperature between the bulbs is greater in dry weather than in 

 damp, because evaporation is much more rapid when the air is 

 dry. Consequently, from a comparison between the readings of 

 the two thermometers we can obtain the degree of moisture of the 

 air, or the "relative humidity" as it is termed. In foggy weather 

 there is little or no difference between the two readings, but in 

 showery weather there is often a considerable difference, so that, 

 even though rain may be falling, it does not necessarily follow 

 that the air is damp. 



On other frames, but arranged horizontally, are two other 

 thermometers of somewhat different pattern; these are used to 

 determine the highest and lowest temperatures that have occurred 

 during any particular period, and are known as maximum and 

 minimum thermometers. 



