﻿578 Dr. N. Campbell and Mr. B. P. Dudding on 



maintain such a position, the judicial mind might suffer one 

 of its occasional lapses into common-sense. But while we 

 would accept his view of the probable results of the action, 

 we would protest that common-sense has nothing to do with 

 the matter. It is not common-sense that tells us how to 

 measure watts; it is the wisdom of the giants of physical 

 science accumulated for more than a century. If we know 

 quite definitely how to measure watts, we know all the 

 fundamentals of electrical engineering and much of experi- 

 mental physics. All measurement depends upon laws, and 

 these laws usually include most of the important experimental 

 facts of the science concerned with the magnitudes in ques- 

 tion. Knowledge of these laws is not common property, 

 and can no more be assumed than a knowledge of the 

 customary units. 



In opposition to our contention that a statement of the 

 laws of measurement should be included with a description 

 of the units in any defining legislation (by which we refer 

 now to the decisions of scientific committees rather than to 

 those of national legislatures), two arguments may be used. 

 First, it may be said that the choice of units, being arbitrary, 

 is suitable for legal decision, while the truth of laws, being 

 wholly free from arbitrariness, is not ; the first, therefore, 

 but not the second, is suitable for inclusion in legislation. 

 But surely, even in legislation, relevance is of some import- 

 ance. Our only objection to present legislation on such 

 matters is that it is totally irrelevant to all cases of practical 

 importance ; all that we suggest is that something should be 

 added which will make it relevant. Secondly, it might be 

 said that to meet our suggestion would be to convert the 

 legislation into a complete treatise on science. But such a 

 treatise would be either too large or too small for the purpose. 

 It would be too small if we were urging that the impossible 

 task should be performed of stating all the experimental laws 

 that are concerned ; it would be too large if we are urging 

 (as we do) that the laws directly concerned should be indi- 

 cated with a reasonable amount of precision. If we are asked 

 what is meant by " directly " and " reasonable," we can only 

 reply by giving an example. And that is what we propose 

 to do. 



2. Photometry seems a suitable system of measurement to 

 choose : its measurements (even so far as units are concerned) 

 have not yet been fixed completely with international con- 

 sent ; the laws of measurement concerned present some 

 features of intrinsic interest ; and some of the attempts at 

 establishing systems of measurement that have been made 



