54 THE PRINCIPLES OF SCIENCE. 



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 all 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 356 

 Fahr. having been actually observed. Such temperatures, 

 therefore, do not require very accurate measurement. All 

 meteorological measurements depending on the accidental 

 condition of the sky are of infinitely less importance than 

 physical measurements in which such accidental conditions 

 do not intervene. Many profound investigations depend 

 upon our knowledge of the radiant energy continually 

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

 measured when the sky is perfectly clear, and the absorp- 

 tion of the atmosphere at its minimum. The slightest 

 interference of cloud destroys the value of such a measure- 

 ment, except for meteorological purposes, which are of 

 vastly less generality and importance. It is seldom use- 

 ful, again, to measure such a quantity as the height of 

 a snow-covered mountain within a foot, when the thick- 

 ness of the snow alone may cause it to vary 25 feet or 

 more, when in short the height itself is indefinite to that 

 extent 6 . 



e Humboldt's 'Cosmos' (Bohn), vol. i. p. 7. 



