350 THE PRINCIPLES OF SCIENCE. 



and should serve only as a means of comparison between 

 .two or more magnitudes. As a general rule, we should 

 not even attempt to make the divisions of the measuring 

 scale exact multiples or submultiples of the unit, but, 

 regarding them as arbitrary marks, should determine their 

 values by comparison with the standard itself. Thus the 

 perpendicular wires in the field of a transit telescope, are 

 fixed at nearly equal but arbitrary distances, and those 

 distances are afterwards determined, as first suggested by 

 Malvasia, by watching 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 an exact determination 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 galvano- 

 meter circle, and still more difficult to construct a galva- 

 nometer, so that each division should have a given value. 

 But this is quite unnecessary, because by placing the ther- 

 mopile 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 in- 

 versely measure the value of the indications of the ther- 

 mopile. In a similar way Mr. Joule ascertained the actual 

 temperature produced by the compression of bars of metal. 

 For having inserted a simple 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 pro- 

 duced a like deflection, in order to ascertain the temperature 

 developed by pressure . 



'Philosophical Transactions' (1859), vol. cxlix. p. 119, &c. 



