WILLIAM SIEMENS, I<\R.S. 121 



value of any two degrees of heat which could be named. Such a 

 scale would also fall in with the readings of an Electrical Resist- 

 ance 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 Centi- 

 grade. 



The most interesting exhibits in this branch of measurement, 

 are, from an historical point of view, the original spirit thermo- 

 meter of the Florentine Accademia del Cimento, and the photo- 

 graphs of old thermometers ; 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 Prof. Himly ; Dr. 

 Andrews' apparatus for measuring the quantity of heat disengaged 

 in combustion ; Dr. Guthrie's diacalorimeter for measuring the 

 conductivity of liquids for heat, and a thermometric tube by Prof. 

 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 

 dynamically, 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 among thermometers, and other instruments 

 placed in the physical sections, but among the instruments re- 

 quired 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 



