352 THE PRINCIPLES OF SCIENCE. 



possible accuracy, so that they can be employed for pur- 

 poses of reference by all astronomers. 



In ascertaining the specific gravities of substances, all 

 gases are referred to atmospheric air at a given tempera- 

 ture and pressure ; all liquids and solids are referred to 

 water. We require to compare the densities of water 

 and air with great care, and the comparative densities of 

 any two substances whatever can then be with ease 

 ascertained. 



In comparing a very great with a very small magni- 

 tude, it is usually desirable to break up the process into 

 several steps, using intermediate terms of comparison. 

 We should never think of measuring the distance from 

 London to Edinburgh by laying down measuring rods 

 throughout the whole distance. A base of several miles 

 in length is selected on level ground, and compared on 

 the one hand with the standard yard, and on the other 

 with the distance of London and Edinburgh, or any other 

 two points, by trigonometrical survey. It would be ex- 

 ceedingly difficult to compare the light of a star with 

 that of the sun, which would be about thirty thousand 

 million times greater ; but Sir J. Herschel * effected the 

 comparison by using the full moon as an intermediate 

 unit. Wollaston ascertained that the sun gave 801,072 

 times as much light as the fuU moon, and Herschel 

 determined that the light of the latter exceeded that of 

 a Centauri 27,408 times, so that we find the ratio be- 

 tween the light of the sun and star to be that of about 

 22,000,000,000 to i. 



The Pendulum. ' 



By far the most perfect and beautiful of all instru- 

 ments of measurement is the pendulum. Consisting 



<i HerschePs 'Astronomy/ 817, 4th. ed. p. 553. 



