750 ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1908. 



the British Association's meetings " On the electric origin of chem- 

 ical heat," at Manchester, in 1842 ; " On the calorific effects of mag- 

 neto-electricity " and " On the mechanical value of heat," at Cork, in 

 1843 ; " On specific heat," at York, in 1844 ; and " On the mechanical 

 equivalent of heat," at Cambridge, in 1845. But at that date, when 

 there was as yet no doctrine of conservation of energy, when scien- 

 tific men were not accustomed to distinguish either in language or 

 in fact between force and work, when "caloric" was classed with 

 light and sound among the "imponderables," Joule's work was 

 listened to with impatience, and his teachings fell upon deaf ears. 

 Was he not an amateur, dabbling in science, and carried away with 

 strange notions ? For the Oxford meeting, too, Joule had prepared a 

 paper. Its title was " On the mechanical equivalent of heat, as 

 determined from the heat evolved by the agitation of liquids." It 

 was relegated to an unimportant place, and would have received as 

 little notice as its predecessors but for Thomson's intervention. 



Thomson, in fact, though he at first had some difficulty in grasping 

 the significance of the matter, threw himself heart and soul into the 

 new and strange doctrines that heat and work were mutually con- 

 vertible, and for the next six or eight years, partly in cooperation 

 with Joule, partly independently, he set his unique powers of mind to 

 unravel those mutual relations. 



Thomson's mind was essentially metrical. He was never satisfied 

 with any phenomenon until it should have been brought into the 

 stage where numerical accuracy could be determined. He must 

 measure, he must weigh, in order that he might go on to calculate. 



" I often say," °' he once remarked, " that when you can measure 

 what you are speaking about and express it in numbers, you know 

 something about it ; but when you can not measure it, when you can 

 not express it in numbers, your knowledge is of a meager and unsatis- 

 factory kind; it may be the beginning of knowledge, but you have 

 scarcely, in your thoughts, advanced to the stage of science, what- 

 ever the matter may be. * * * The first step toward numerical 

 reckoning of properties of matter, more advanced than the mere ref- 

 erence to a set of numbered standards, as in the mineralogist's scale 

 of hardness, or to an arbitrary trade standard, as in the Birmingham 

 wire-gauge, is the discovery of a continuously varying action of 

 some kind and the means of observing it definitely and measuring it 

 in terms of some arbitrary unit or scale division. But more is neces- 

 S;ary to complete the science of measurement in any department, and 

 hat,. i,?, the fixing on something absolutely definite as the unit of 



'tieCttire on " Electrical units of measurements " at Institution of Civil 

 ■^eer^,-!viay 3, 1883. Reprinted in Popular Lectures and Addresses, Vol. I, 



