330 THE PRINCIPLES OF SCIENCE. 



equals the object, as in surveying land, or determining a 

 weight by the balance. The requisites of accuracy now 

 are : (i) That we can repeat unit after unit of exactly 

 equal magnitude ; (2) That these can be joined together 

 so that the aggregate shall really be the sum of the 

 parts. The same conditions apply to subdivision, which 

 may be regarded as a multiplication of subordinate units. 

 In order to measure to the thousandth of an inch, we 

 must be able to add thousandth after thousandth without 

 error in the magnitude of these spaces, or in their con- 

 junction. 



The condenser electrometer, as remarked by Thomson 

 and Tait <1, is a good example of an instrument unfitted to 

 give any sure measure of electro-motive force, because the 

 friction between the parts of the condenser often produces 

 more electricity than the original quantity which was to 

 be measured. 



Measuring Instruments. 



To consider the mechanical construction of scientific 

 instruments, is no part of my purpose in this book. I 

 wish to point out merely the general purpose of .such 

 instruments, and the methods adopted to carry out that 

 purpose with great precision. In the first place we must 

 distinguish between the instrument which effects a com- 

 parison between two quantities, and the standard mag- 

 nitude which often forms one of the quantities compared. 

 The astronomer's clock, for instance, is no standard of the 

 efflux of time ; it serves but to subdivide, with approxi- 

 mate accuracy, the interval of successive passages of a 

 star across the meridian, which it may effect perhaps to 

 the tenth part of a second, or a 6 4 ' 00 part of the whole. 



i 'Elements of Natural Philosophy,' sect, 326, p. 108. 



