DR. C. W. SIEMENS' ADDRESS. 213 



trical resistance thermometer or pyrometer, of which a specimen has 

 been added to this collection by myself. 



When temperature or intensity of heat is coupled with mass, we 

 obtain the conception of quantity of heat, and if this again is referred 

 to a standard material, usually water, the unit weight of each being 

 taken, we obtain what is known as specific heat. The standard to 

 which measurements of quantity of heat are usually referred is the heat 

 required to raise a pound of water one degree fahrenheit, or the cubic 

 centimetre of water one degree centigrade. 



The most interesting exhibits in this branch of measurement, are, 

 from an historical point of view, the original spirit thermometer of the 

 Florentine Academia del Cimento, and the photographs of old ther- 

 mometers ; the original Lavoisier Calorimeter for measuring the heat 

 disengaged in combustion, Wedgwood's and Daniell's Pyrometers. 



As illustrating modern improvement, may be instanced a long brass- 

 cased thermometer showing the variation in the readings, when the 

 bulb and when the whole thermometer is immersed ; a thermometer 

 with flat bulb to improve sensitiveness ; a thermo-electric alarum, for 

 giving notice when a given temperature is reached ; an instrument for 

 measuring the temperature of fusion by means of electric contact 

 invented by Professor Himly ; Dr. Andrews' apparatus for measuring 

 the quantity of heat disengaged in combustion ; Dr Guthrie's diacalo- 

 rimeter for measuring the conductivity of liquids for heat, and a ther- 

 mometric tube by Professor Wartmann for determining the calorific 

 capacities of different liquids by the process of cooling. 



Finally, Joule has taught us how to measure the unit of heat dyna- 

 mically, and the interesting apparatus employed by him from time to 

 time in the various stages of the determination of this most important 

 constant in applied mechanics, are to be found, rightly placed, not 

 iimong thermometers, and other instruments placed in the physical 

 sections, but among the instruments required in the determination of 

 three great natural standards of length, time, and mass, and their 

 combinations. 



Another branch of the general subject is the Measurement of Light, 

 which may be divided into two principal sections, that including the 

 measurement of the wave-length of light, of different colours, and the 

 angle of polarization, which belongs purely and entirely to physical 



