METEOROLOGICAL OBSERVATIONS. 



233 



of the bulb and part of the bore being filled with mercury it 

 usually contains alcohol, in which floats a black index. On a 

 decrease of temperature the alcohol recedes taking with it the 

 index ; on an increase of temperature the alcohol alone ascends 

 in the tube, leaving the index behind indicating the minimum 

 temperature. The lowest temperature registered at Truro was 8° 

 on January 15th, 1867. 



Aqueous Vapour. As will be seen in the table of sub- 

 stances composing the air there is always a quantity of water 

 vapour present. The proportion is never constant, but varies 

 with the temperature ; the warmer the air the more water vapour 



WET & DRY BULB HYGROMETER. 



will it hold. For measuring this quantity of moisture, an 

 instrument known as the Hygrometer (Greek hugros, moist; 

 metron, a measure) is used. It consists of two ordinary mercurial 

 thermometers, as nearly as possible identical, placed on a stand 

 the one marked dry, the other wet. The bulb of the wet 

 thermometer is covered with thin muslin, round the neck of which 

 are conducting threads of lamp cotton passing into a small 

 attached vessel of water, placed at such a distance as to allow a 

 length of thread of about 3 inches. These threads draw up the 



