xv.] ANALYSIS OF QUANTITATIVE PHENOMENA. 345 



A good telescope furnished with an accurate micrometer 

 is alone needed for the application of the method. 

 Huyghens appears to have been the first observer who 

 actually tried to employ the method practically, but 

 it was not until 1835 that the improvement of telescopes 

 and micrometers enabled Struve to detect in this way 

 the parallax of the star a Lyrae. It is one of the many 

 advantages of the observation of transits of Venus for the 

 determination of the solar parallax that the refraction of 

 the atmosphere affects in an exactly equal degree the planet 

 and the portion of the sun's face over which it is passing. 

 Thus the observations are strictly of a differential nature. 



By the process of substitutive weighing it is possible 

 to ascertain the equality or inequality of two weights 

 with almost perfect freedom from error. If two weights 

 A and B be placed in the scales of the best balance 

 \ve cannot be sure that the equilibrium of the beam 

 indicates exact equality, because the arms of the beam 

 may be unequal or unbalanced. But if we take B out 

 and put another weight C in, and equilibrium still 

 exists, it is apparent that the same causes of erroneous 

 weighing exist in both cases, supposing that the balance 

 has not been disarranged ; B then 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 as the original in magnitude and form, 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. 



1 Leslie, Inquiry into the Nature of Heat, p. 10. 



