xxv.J ACCORDANCE OF THEORIES. 563 



In this question the theoretical relations between the 

 velocity of light, the constant of aberration, the sun's paral- 

 lax, and the sun's mean distance, are of the simplest 

 character, and can hardly be open to any doubt, so that 

 the only doubt was as to which result of observation was 

 the most reliable. Eventually the chief discrepancy was 

 found to arise from misapprehension in the reduction 

 of observations, but we have a satisfactory example of the 

 value of different methods of estimation in leading to the 

 detection of a serious error. Is it not surprising that 

 Foucault by measuring the velocity of light when passing 

 through the space of a few yards, should lead the way 

 to a change in our estimates of the magnitudes of the 

 whole universe ? 



Selection of the best Mode of Measurement. 



When we once obtain command over a question of 

 physical science by comprehending the theory of the sub- 

 ject, we often have a wide choice opened to us as regards 

 the methods of measurement, which may thenceforth be 

 made to give the most accurate results. If we can measure 

 one fundamental quantity very precisely we may be able 

 by theory to determine accurately many other quantitative 

 results. Thus, if we determine satisfactorily the atomic 

 weights of certain elements, we do not need to determine 

 with equal accuracy the composition and atomic weights of 

 their several compounds. Having learnt the relative 

 atomic weights of oxygen and sulphur, we can calculate the 

 composition by weight of the several oxides of sulphur. 

 Chemists accordingly select with the greatest care that 

 compound of two elements which seems to allow of the 

 most accurate analysis, so as to give the ratio of their 

 atomic weights. It is obvious that we only need the ratio 

 of the atomic weight of each element to that of some com- 

 mon element, in order to calculate that of each to each. 

 Moreover the atomic weight stands in simple relation to 

 other quantitative facts. The weights of equal volumes of 

 elementary gases at equal temperature and pressure have 



.200,000, a sum which, wisely expended on scientific investigations, 

 would give a hundred important results. 



o o 2 



