1 42 GRAVITATION— THE EXPLANATION 



what you mean by the question; there is nothing by 

 reference to which we could expose delinquencies of the 

 standard rod, nothing by reference to which we could 

 conceive the nature of the supposed delinquencies. Still 

 the standard rod was chosen with considerable care; its 

 material was selected to fulfil certain conditions — to be 

 affected as little as possible by casual influences such 

 as temperature, strain or corrosion, in order that its 

 extension might depend only on the most essential char- 

 acteristics of its surroundings, present and past.* We 

 cannot say that it was chosen to keep the same absolute 

 length since there is no such thing known; but it was 



* In so far as these casual influences are not entirely eliminated by 

 the selection of material and the precautions in using the rod, appropriate 

 corrections must be applied. But the rod must not be corrected for 

 essential characteristics of the space it is measuring. We correct the 

 reading of a voltmeter for temperature, but it would be nonsensical to 

 correct it for effects of the applied voltage. The distinction between 

 casual and essential influences — those to be eliminated and those to be 

 left in — depends on the intention of the measurements. The measuring 

 rod is intended for surveying space, and the essential characteristic of 

 space is "metric". It would be absurd to correct the readings of our 

 scale to the values they would have had if the space had some other 

 metric. The region of the world to which the metric refers may also 

 contain an electric field; this will be regarded as a casual characteristic 

 since the measuring rod is not intended for surveying electric fields. 

 I do not mean that from a broader standpoint the electric field is any less 

 essential to the region than its peculiar metric. It would be hard to say 

 in what sense it would remain the same region if any of its qualities were 

 other than they actually are. This point does not trouble us here, because 

 there are vast regions of the world practically empty of all characteristics 

 except metric, and it is to these that the law of gravitation is applied both 

 in theory and in practice. It has seemed, however, desirable to dwell on 

 this distinction between essential and casual characteristics because there 

 are some who, knowing that we cannot avoid in all circumstances cor- 

 rections for casual influences, regard that as license to adopt any arbi- 

 trary system of corrections — a procedure which would merely have the 

 effect of concealing what the measures can teach us about essential 

 characteristics. 



