300 THE PEINCIPLES OF SCIENCE. [CHAP. 



equal but arbitrary distances, and those distances are after- 

 wards determined, as first suggested by Malvasia, by watch- 

 ing the passage of star after star across them, and noting 

 the intervals of time by the clock. Owing to the perfectly 

 regular motion of the earth, these time intervals give exact 

 determinations of the angular intervals. In the same way, 

 the angular value of each turn of the screw micrometer 

 attached to a telescope, can be easily and accurately 

 ascertained. 



When a thermopile is used to observe radiant heat, it 

 would be almost impossible to calculate on a priori grounds 

 what is the value of each division of the galvanometer 

 circle, and still more difficult to construct a galvanometer, 

 so that each division should have a given value. But this 

 is quite unnecessary, because by placing the thermopile 

 before a body of known dimensions, at a known distance, 

 with a known temperature and radiating power, we measure 

 a known amount of radiant heat, and inversely measure 

 the value of the indications of the thermopile. In a 

 similar way Dr. Joule ascertained the actual temperature 

 produced by the compression of bars of metal. For having 

 inserted a small thermopile composed of a single junction 

 of copper and iron wire, and noted the deflections of the 

 galvanometer, he had only to dip the bars into water of 

 different temperatures, until he produced a like deflec- 

 tion, in order to ascertain the temperature developed by 

 pressure. 1 



In some cases we are obliged to accept a very carefully 

 constructed instrument as a standard^ as in the case of a 

 standard barometer or thermometer. But it is then best 

 to treat all inferior instruments comparatively only, and 

 determine the values of their scales by comparison with 

 the assumed standard. 



Systematic Performance of Measurements. 



When a large number of accurate measurements have 

 to be effected, it is usually desirable to make a certain 

 number of determinations with scrupulous care, and after- 

 wards use them as points of reference for the remaining 



1 Philotophical Transactions (1859), vo '- cxlix. p. 119, &c. 



