INSTRUMENTS USED IN MEASUREMENT. 57 



this purpose is called a Stereometer. The liquid must be so 

 chosen that no chemical action takes place between it and the 

 solid immersed, and that it wets the solid, so that no air bubbles 

 adhere to the surface. Thus mercury is used in the case of metals 

 by the Standards Department. 



Time is measured for ordinary purposes by the length of the arc 

 traced out by a moving hand on a circular clock-face. For astro- 

 nomical purposes it is sometimes measured by counting the ticks 

 of a clock which beats seconds, and estimating mentally the frac- 

 tions of a second and in cases where the period of an oscillation 

 has to be found, it is determined by counting the number of oscil- 

 lations in a time sufficient to make the number considerable, and 

 then dividing that time by the number. But by far the most accu- 

 rate way of measuring time is by means of the line traced by a 

 pencil on a sheet of paper rolled round a revolving cylinder, or a 

 spot of light moving on a sensitive surface. If the pencil is made 

 to move along the length of the cylinder so as to indicate what is 

 happening as time goes along, the time of each event will be found 

 when the cylinder is unrolled by measuring the distance of the 

 mark recording it from the end of the unrolled sheet, provided 

 that the rate at which the cylinder goes round is known. In this 

 way Helmholtz measured the rate of transmission of nerve-dis- 

 turbance. 



A very common case of the measurement of force is the baro- 

 meter, which measures the pressure of the atmosphere per square 

 inch of surface. This is determined by finding the height of the 

 column of mercury which it will support (mercurial barometer), or 

 the strain which it causes in a box from which the air has been 

 taken out (aneroid barometer). The height in the former case 

 may be measured directly, or it may first be converted into the 

 quantity of turning of a needle, and then read off as length of arc 

 on a graduated circle; in the latter case the strain is always 

 indicated by a needle turning on a graduated circle. 



The mass, and (what is proportional to it) the weight, of different 



