METEOROLOGICAL INSTRUMENTS. 221 



TEMPERATURE. 



The instruments for measuring this element are termed Thermo- 

 meters. The invention of the instrument is generally ascribed to 

 Galileo. It consists of a glass tube, with a bulb at one end con- 

 taining a quantity of liquid. The liquid with which it was first 

 filled was spirit of wine, but Halley (1695) proposed the employ- 

 ment of mercury for this purpose, and this suggestion has been 

 since very generally adopted, excepting for thermometers destined 

 to register low temperatures,~for which spirit is still necessarily 

 employed. 



The best instrument for the accurate measurement of tempera- 

 ture is the air thermometer, which consists of a portion of air 

 confined in a hollow globe connected with a long tube, in which 

 the volume of the air is measured. There is much difference of 

 opinion as to the proper method of exposing thermometers so as 

 to show the true temperature of the air. Various forms of thermo- 

 meter screens actually in use will be found in the Exhibition. 



The main improvement in thermometers of late years has been 

 the graduation of the actual stem of the instrument. It is obvious 

 that if the graduation be on a separate slab, there can be no 

 security that the individual scale refers to the special instrument 

 to which it is attached. 



Thermometers may be made self-registering in various ways. 

 Six's is one of the oldest patterns, and admits of the registration 

 of both the maximum and minimum temperatures. It is a spirit 

 thermometer with two bulbs, of which the smaller is partly full of 

 air. The tube is U shaped, and the bend of the U is occupied 

 by mercury, separating the two portions of spirit. The readings 

 are registered by iron indices, which are moved by the change of 

 position of the mercurial plug, and the instrument is reset by the 

 use of a magnet. 



This pattern of instrument was employed by Negretti, and after- 

 wards by Casella, at Professor W. A. Miller's suggestion, for deep 



