400 THE PRINCIPLES OF SCIENCE. 



weighing exist in both cases, supposing that the balance 

 has riot been disarranged, and that B must be exactly 

 equal to C, since it has exactly the same effect under 

 the same circumstances. In like manner it is a general 

 rule that, if by any uniform mechanical process we get 

 a copy of an object, it is unlikely that this copy will 

 be precisely the same in magnitude and form as the 

 original, but two copies will equally diverge from the 

 original, and will therefore almost exactly resemble each 

 other. 



Leslie's Differential Thermometer 1 was well adapted 

 to the experiments for which it was invented. Having 

 two equal bulbs any alteration in the temperature of the 

 air will act equally by conduction on each and produce 

 no change in the indications of the instrument. Only 

 that radiant heat which is purposely thrown upon one 

 of the bulbs will produce any effect. This thermometer 

 in short carries out the principle of the differential method 

 in a mechanical manner. 



3. Method of Correction. 



Whenever the result of an experiment is affected by an 

 interfering cause to an amount either invariable or exactly 

 calculable, it is sufficient simply to add or subtract this 

 calculated amount. We are said to correct observations 

 when we thus eliminate what is due to extraneous causes, 

 although of course we are only separating the correct 

 effects of several agents. Thus the variation in the height 

 of the barometrical column is partly due to the change 

 of temperature, and since the coefficient of absolute 

 dilatation of mercury has been exactly determined, as 

 already described (p. 395), we have only to make cal- 



i Leslie's 'Inquiry into the Nature of Heat/ p. 10. 



