THE EXACT MEASUREMENT OF PHENOMENA. 351 



In many instances we are indeed obliged to accept a 

 very carefully constructed instrument as a standard, as in 

 the case of a standard barometer. But it is then best to 

 treat all inferior instruments comparatively only, and 

 determine the values of their scales by comparison with 

 the assumed standard. 



Systematic Performance of Measurements. 



When a large number of accurate measurements have 

 to be effected, it is usually desirable to make a certain 

 number of determinations with scrupulous care, and after 

 wards use them as points of reference for the remaining 

 determinations. In the trigonometrical survey of a coun 

 try, the principal triangulation fixes the relative 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 after 

 wards 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. The distances 

 of the other stars from these standard points, were deter 

 mined in his early observations by the use of the quadrant P. 

 Even since the introduction of the transit telescope and 

 mural circle, tables of standard stars are formed at 

 Greenwich, the positions being determined with every 



P Baily s Account of Flamsteed/ pp. 378-380. 



