398 THE PRINCIPLES OF SCIENCE. 



of error are too fluctuating to allow of correction, so that 

 the only accurate mode of procedure is that devised by 

 Dr. Joule, of surrounding the thermometer with a copper 

 cylinder ingeniously 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 cannot be carried into 

 effect, 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 im 

 portance provided it remains constant, so that a sure 

 calculation of its amount can be made. 



2. Differential Method. 



When we cannot avoid the entrance of error, we can 

 often resort with great success to the second mode of 

 measuring phenomena under such circumstances that the 

 error shall remain nearly or quite the same in all the 

 observations, and neutralize 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. The determination of the parallax 

 of the fixed stars is exceedingly difficult, because the 

 amount of paraUax is far less than most of the corrections 



Clerk Maxwell, Theory of Heat, p. 228. Proceedings of the 

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



