5 6 SCIENTIFIC APPARATUS. 



quantity, and estimating that in the same way. Every quantity is 

 measured by finding a length proportional to the quantity, and 

 then measuring this length. This will, perhaps, be better under- 

 stood if we consider one or two examples. 



The measurement of angles occurs in a very large majority of 

 scientific instruments. It is always effected by measuring the 

 length of an arc upon, a graduated circle; the circumference of this 

 circle being divided not into inches or centimeters, but into 

 degrees and parts of a degree that is, into aliquot parts of the 

 whole circumference. 



As a step towards their final measurement, some quantities, of 

 which work is a good instance, are represented in the form of 

 areas ; and there seems reason to believe that this method 

 is likely to be extended. Instruments for measuring areas . are 

 called Planimeters ; and one of the simplest of these is Amsler's, 

 consisting of two rods jointed together, the end of one being fixed 

 and that of the other being made to run round the area which is 

 to be measured. The second rod rests on a wheel, which turns as 

 the rod moves ; and it is proved by geometry that the area is pro- 

 portional to the distance through which the wheel turns. Thus 

 the measurement of an area is reduced to the measurement of a 

 length. 



Volumes are measured in various ways, but all depending on 

 the same principle. Quantities of earth excavated for engineering 

 purposes are estimated by a rough determination of the shape of 

 the cavity, and the measurement of its dimensions, namely, certain 

 lengths belonging to it. The contents of a vessel are sometimes 

 gauged in the same way ; but the more accurate method is to fill 

 it with liquid and then pour the liquid into a cylinder of known 

 section, when the quantity is measured by the height of the liquid 

 in the cylinder, that is, by a length. The volumes of irregular 

 solids are also measured by immersing them in liquid contained 

 in a uniform cylinder, and observing the height to which the 

 liquid rises j that is, by measuring a length. An apparatus for 



