340 THE PRINCIPLES OF SCIENCE. [CHAP. 



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

 mode of neutralising the interfering cause by balancing 

 against it an exactly equal and opposite cause of unknown 

 amount. 



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

 experiment that the interfering cause may act in opposite 

 directions, in alternate observations, the mean result being 

 free from interference. 



I . Method of Avoidance of Error. 



Astronomers seek opportunities of observation when 

 errors will be as small as possible. In spite of elaborate 

 observations and long-continued theoretical investigation, 

 it is not practicable to assign any satisfactory law to the 

 refractive power of the atmosphere. Although the appa- 

 rent change of place of a heavenly body produced by 

 refraction 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 uncertainty 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 atmo- 

 spheric refraction, as was done by Hooke. 



Astronomers endeavour to render their clocks as accurate 

 as possible, by removing the source of variation. The 

 pendulum is perfectly isochronous so long as its length 

 remains invariable, and the vibrations are exactly of equal 

 length. They render it nearly invariable in length, that 

 is in the distance between the centres of suspension and 

 oscillation, by a compensatory arrangement for the change 

 of temperature. But as this compensation may not be 

 perfectly accomplished, some astronomers place their chief 

 controlling clock in a cellar, or other apartment, where 

 the changes of temperature may be as slight as possible. 



