xui] MEASUREMEXT OF PHENOMENA. 301 



determinations. In the tri!:^ononietiical survey of a coun- 

 try, the principal triaugukitiou tixes the reUitive positions 

 and distances of a few points with rigid accuracy. A 

 minor triangulation refers every prominent hill or village 

 to one of the principal points, and then the details are 

 filled in by reference to the secondary points. The survey 

 of the heavens is effected in a like manner. The ancient 

 astronomers compared the right ascensions of a few prin- 

 cipal stars with the moon, and thus ascertained their posi- 

 tions with regard to the sun; the minor stars were afterwards 

 referred to the principal stars. Tycho followed the same 

 method, except that he used the more slowly moving 

 planet Venus instead of the moon. Flamsteed was in the 

 habit of using about seven stars, favourably situated at 

 points all round the heavens. In his early observations 

 the distances of the other stars from these standard points 

 were determined by the use of the quadrant.^ Even since 

 the introduction of the transit telescope and the mural 

 circle, tables of standard stars are formed at Greenwich, 

 the positions being determined with all possible accuracy, 

 so that they can be employed for purposes of reference by 

 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 tlie densities of water and 

 air with great care, and the comparative densities of any 

 two substances whatever can then be ascertained. 



In comparing a very great with a very small magnitude, 

 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 

 Ediidjurgh by laying down measuring rods, throughout the 

 whole length. A base of several miles 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. Again, it would be exceedingly difficult to coni- 

 ])are the light of a star with that of the sun, whicli would 

 be about tlurty thousand million times greater; Init ller- 



' Buily's Accoinit of Flamsteed, pp. 378 — 380. 



