284 THE PRINCIPLES OF SCIENCE. (CHAP. 



Measuring Instruments. 



To consider the mechanical construction of scientific 

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

 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 5-5^^ part of the whole. 

 The moving globe itself is the real standard clock, and the 

 transit instrument the finger of the clock, while the stars 

 are the hour, minute, and second marks, none the less 

 accurate because they are disposed at unequal intervals. 

 The photometer is a simple instrument, by which we com- 

 pare the relative intensity of rays of light falling upon a 

 given spot. The galvanometer shows the comparative 

 intensity of electric currents passing through a wire. 

 The calorimeter gauges the quantity of heat passing from 

 a given object. But no such instruments furnish the 

 standard unit in terms of which our results are to be ex- 

 pressed. In one peculiar case alone does the same instru- 

 ment combine the unit of measurement and the means of 

 comparison. A theodolite, mural circle, sextant, or other 

 instrument -for the measurement of angular magnitudes 

 has no need of an additional physical unit ; for the circle 

 itself, or complete revolution, is the natural unit to which 

 all greater or lesser amounts of angular magnitude are 

 referred. 



The result of every measurement is to make known the 

 purely numerical ratio existing between the magnitude 

 to be measured, and a certain other magnitude, which 

 should, when possible, be a fixed unit or standard magni- 

 tude, or at least an intermediate unit of which the value 

 can be ascertained in terms of the ultimate standard. But 

 though a ratio is the required result, an equation is the 

 mode in which the ratio is determined and expressed. In 



