442 THE PRINCIPLES OF SCIENCE. [CHAP. 



heated and cooled, and we cannot exactly define the state 

 in which it is at any moment, our care in measuring will 

 be wasted, because it can lead to no law. It is of little 

 use to determine very exactly the electric conductibility of 

 carbon, which as graphite or gas carbon conducts like a 

 metal, as diamond is almost a non-conductor, and in 

 several other forms possesses variable and intermediate 

 powers of conduction. It will be of use only for 

 immediate practical applications. Before measuring these 

 we ought to have something to measure of which the con- 

 ditions are capable of exact definition, and to which at a 

 future time we can recur. Similarly the accuracy of our 

 measurement need not much surpass the accuracy with 

 which we can define the conditions of the object treated. 



The speed of electricity in passing through a conductor 

 mainly depends upon the inductive capacity of the sur- 

 rounding substances, and, except for technical or special 

 purposes, there is little use in measuring velocities which 

 in some cases are one hundred times as great as in other 

 cases. But the maximum speed of electric conduction is 

 probably a constant quantity of great scientific importance, 

 and according to Prof. Clerk Maxwell's determination in 

 1868 is 174,800 miles per second, or little less than that 

 of light. The true boiling point of water is a point on 

 which practical thermometry depends, and it is highly 

 important to determine that point in relation to the ab- 

 solute thermometric scale. But when water free from air 

 and impurity is heated there seems to be no definite limit 

 to the temperature it may reach, a temperature of 180 

 Cent, having been* actually observed. Such temperatures, 

 therefore, do not require accurate measurement. All 

 meteorological measurements depending on the accidental 

 condition of the sky are of far less importance than 

 physical measurements in which such accidental con- 

 ditions do not intervene. Many profound investigations 

 depend upon our knowledge of the radiant energy con- 

 tinually poured upon the earth by the sun ; but this must 

 be measured when the sky is perfectly clear, and the 

 absorption of the atmosphere at its minimum. The 

 slightest interference of cloud destroys the value of such 

 a measurement, except for meteorological purposes, which 

 are of vastly less generality and importance. It is seldom 



