II. THE11MOMETRY. 223 



lO18b. Six's Thermometer with mercurial wet bulb ther- 

 mometer attached, thereby combining four instruments in one, 

 namely, maximum, minimum, hygrometer, and present tempera- 

 ture. S. G. Denton. 



1019. Long Brass-Cased Thermometer. Showing the 

 difference in length of the mercurial column after being pointed 

 and divided with the whole length of the tube immersed in water 

 at the various temperatures between 32 and 212 ; the same with 

 the bulb only in the water. Dring and Fage. 



1020. Very delicate Spiral Bulb Thermometer. Ex- 

 tremely sensitive, capable of indicating small variations of tem- 

 perature. Dring and Fagc. 



1022. Standard Thermometer, calibrated throughout, 



Dring and Fag e. 



A standard thermometer divided on the tube, used for purposes where 

 great accuracy is required. The tubes used for these thermometers are 

 selected with great care, particular attention being paid to the uniformity of 

 the bore. The method of ascertaining this is usually performed as follows : 

 A portion of mercury is introduced into the tube, and the length it occupies 

 is noted ; it is then carried a little further on, and its length compared with 

 the former length. So on all down the tube ; if the length has decreased 

 from the first measurement, it shows that the bore of the tube has increased, 

 and vice versa. The process is known as calibration. 



1023. Pour Thermometers. Showing the different scales 

 principally in use. Fahrenheit, Celsius or Centigrade, Reaumur, 

 arid De Lisle. Dring and Fage. 



Fahrenheit's scale is used principally in Great Britain, its Colonies, and the 

 United States. The zero of this scale is obtained from a mixture of salt and 

 snow ; thirty-two degrees is the point at which ice begins to melt, and 212, or 

 boiling point, from boiling water, when the barometer stands at 29 905. One 

 advantage of this scale is that temperatures may often be expressed in whole 

 degrees, whereas in other scales fractions of degrees are frequently necessary. 



1024. Celsius or Centigrade Scale. Generally used on the 

 continent. Dring and Fage. 



The zero of this scale is that point at which ice begins to melt, and 100 

 the point at which water boils when the barometer stands at 760mm. Celsius 

 is the name of the inventor of this scale ; it is called Centigrade from its 

 being divided centesimally. 



1025. Bee quer el's Thermo- Electric Thermometer. 



Conservatoire des Arts et Metiers, Paris. 

 lO25a. M. Bee quer el's Thermo-Electric Pyrometer. 



Conservatoire des Arts et Metiers. 



1026. Hodgkinson's Actinometer. Actinometer by the 

 Rev. G. C. Hodgkinson, described in the Proceedings of the 

 Royal Society, vol. XX., p. 328. 



Kew Committee of the Royal Society, Kew Observatory. 



It is a large thermometer, filled with alcohol coloured blue, and havino- a 



bore much contracted for a great part of its length, in order that the scale 



