344 THE PRINCIPLES OF SCIENCE. [chap. 



adjusted to the temperature of the air,, as described by 

 him, so that the effect of radiation shall be nullified.^ 



When the avoidance of error is not practicable, it will 

 yet be desirable to reduce the absolute amount of the 

 interfering error as much as possible before employing the 

 succeeding methods to correct the result. As a general 

 rule we can determine a quantity with less inaccuracy as 

 it is smaller, so that if the error itself be small the error in 

 determining that error will be of a still lower order of 

 magnitude. But in some cases the absolute amount of an 

 error is of no consequence, as in the index error of a 

 divided circle, or the difference between a chronometer and 

 astronomical time. Even the rate at which a clock gains 

 or loses is a matter of little importance provided it remain 

 constant, so that a sure calculation of its amount can be 

 made. 



2. Bifferential Method. 



When we cannot avoid the existence of error, we can 

 often resort with success to the second mode by measuring 

 phenomena under such circumstances that the error shall 

 remain very nearly the same in all the observations, and 

 neutralise itself as regards the purposes in view. This 

 mode is available whenever we want a difference between 

 quantities and not the absolute quantity of either. Tlie 

 determination of tlie parallax of the fixed stars is exceed- 

 ingly difticult, because the amount of parallax is far less 

 than most of the corrections for atmospheric refraction, 

 nutation, aberration, precession, instrumental irregularities, 

 &c., and can with difficulty be detected among these pheno- 

 mena of various magnitude. But, as Galileo long ago 

 suggested, all such difficulties would be avoided by the 

 differential observation of stars, which, though apparently 

 close together, are really fai separated on the line of sight. 

 Two such stars in close apparent proximity will be sub- 

 ject to almost exactly equal errors, so that all we 

 need do is to observe the apparent change of place of 

 the nearer star as referred to the more distant one. 



" Clerk Maxwell, Theory of Heat, p. 228. Proceedingi of the 

 Manchester Philosophical iSociety, Nov. 26, 1867, vol. vii. p. 35. 



