ANALYSIS OF QUANTITATIVE PHENOMENA. 393 



(4) The Method of Compensation. He may invent some 

 mode of neutralizing the interfering force by balancing 

 against it an exactly equal and opposite force of unknown 

 amount. 



(5) The Method of Reversal. He may so conduct the 

 experiment that the interfering force may act in opposite 

 directions, in alternate observations, the mean result being 

 free from interference. 



1. Method of Avoidance of Error. 



Astronomers always seek opportunities of observation 

 when errors will have the smallest effect. In spite of 

 elaborate observations and long continued theoretical 

 investigation, it is not found possible to assign any 

 satisfactory law to the refractive power of the atmo- 

 sphere. Although the apparent change of place of a 

 heavenly body thus produced, may be more or less 

 accurately calculated, yet the error depends upon the 

 temperature and pressure of the atmosphere, and, when 

 a ray is highly inclined to the perpendicular, the un- 

 certainty in the refraction becomes very considerable. 

 Hence astronomers always make their observations, if 

 possible, when the object is at the highest point of its 

 daily course, i.e. on the meridian. In some, kinds of 

 investigation, as, for instance, in the determination of the 

 latitude of an observatory, the astronomer is at liberty 

 to select one or more stars out of the countless number 

 visible. There is an evident advantage in such a case, 

 in selecting a star which passes close to the zenith, 

 so that it may be observed almost entirely free from 

 atmospheric refraction, as was done by Hooke. It 

 was ingeniously suggested by Wallis that the parallax 

 of the fixed stars might perhaps be detected by ob- 

 servations of the greatest azimuth east and west of some 



