METEOROLOGICAL INSTRUMENTS, 



THE THERMOMETER. 



AN instrument for determining the exact variations of 

 heat or temperature. The principle on which they are con- 

 structed, is the change of volume which takes place in 

 bodies when their temperature undergoes an alteration. 

 The thermometer consists of a narrow glass tube having an 

 uniform bore, with a bulb at one end, usually filled with 

 mercury, so that expansion or contraction can only take place 

 by the rise or fall of the mercury in the tube ; the upper 

 end of the tube is hermetically sealed, within which is a 

 vacuum. The tube is usually fastened to a plate of metal 

 which is graduated to form a scale ; the rising of the mer- 

 cury shows an increase of heat, and the falling a diminution 

 of heat. The usual graduation of the thermometer is accord- 

 ing to Fahrenheit ; the is called zero ; at 32 water freezes ; 

 at 98 is blood heat ; at 212 water boils. Reaumur's scale 

 is used considerably in France ; at water freezes, and at 

 80 water boils. The scale that is used by philosophers 

 in most parts of the world, and that required to be used in 

 the academies of the State of New York by law, is the 

 centigrade scale ; its is at the freezing point, and 100 at 

 the boiling point of water. This scale is frequently gradu- 

 ated on one side of the thermometer tube and Fahrenheit's 

 scale on the other. The thermometer is of extensive use 

 not only to scientific men in their experiments, but is used 

 for regulating the temperature of buildings, hothouses, baths, 

 in the arts, and by navigators in determining the tempera- 

 ture of the water, informing them by its increased cold- 

 ness of the vicinity of land, or when approaching the Gulf 

 Stream by its greater warmth. 



