5 8 SCIENTIFIC APPARATUS. 



bodies at the same place, are measured by means of a balance ; 

 and at first sight this mode of measurement seems different from 

 those which we have hitherto considered. For we put the body 

 to be weighed in one scale, and then put known weights into the 

 other until equilibrium is obtained or the scale turns, and then we 

 count the weights. But in a steelyard the weight is determined 

 directly by means of a length ; and in a balance which is accurate 

 enough for scientific purposes, both methods are employed. We 

 get as near as we can with the weights, and then the remainder is 

 measured by a small rider of wire which is moved along the beam, 

 and which determines the weight by its position ; that is, by the 

 measurement of a length. 



For the measurement of weight in different places a spring- 

 balance has to be used, and the weight is determined by the 

 alteration it produces in the length of the spring ; or else the 

 length of the seconds pendulum is measured, from which the 

 force of gravity on a given mass can be calculated. This last is 

 an example of a very common and useful mode of measuring 

 forces called into play by displacement or strain; namely, by 

 measuring the period of the oscillations which they produce. 



It seems unnecessary to consider any further examples, as all 

 other quantities are measured by means of some simple geo- 

 metrical or dynamical quantity which is proportional to them ; as 

 temperature by the height of mercury in a thermometer, heat by 

 the quantity of ice it will melt (the volume of the resulting water), 

 electric resistance by the length of a standard wire which has 

 an equivalent resistance. It only remains to show how, when a 

 length has been found proportional to the quantity to be mea- 

 sured, this .length itself is measured. 



For rough purposes, as for example in measuring the length of 

 a room with a foot-rule, we apply the rule end on end, and count 

 the number of times. For the piece left, we should apply the rule 

 to it and count the number of inches. Or if we wanted a length 

 expressed roughly for scientific purposes, we should describe it in 



