394 THE PRINCIPLES OF SCIENCE. 



circumpolar star, since the refractive power of the atmo- 

 sphere which affects only the altitude would thus be 

 entirely avoided f . 



Astronomers also 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 clocks in a cellar, or other 

 apartment, where the changes of temperature may be as 

 slight as possible. At the Paris Observatory a clock has 

 been placed in the caves beneath the building, where 

 there is no appreciable difference between the summer 

 and winter temperature. 



To avoid the effect of unequal oscillations Huyghens 

 made his beautiful investigations, which resulted in the 

 discovery that a pendulum, of which the centre of oscil- 

 lation moved upon a cycloidal path, would be perfectly 

 isochronous, whatever the variation in the length of 

 oscillations. But though a pendulum may be rendered in 

 some degree cycloidal by the use of a steel suspension 

 spring, it is found that the mechanical arrangements 

 requisite to produce a truly cycloidal motion introduce 

 more error than they avoid. Hence astronomers seek to 

 reduce the error to the smallest amount by maintaining 

 their clock pendulums in uniform movement s ; and in 

 fact while a clock is in good order and has the same 

 weights, there need be little change in the length of 

 oscillation. 



1 Grant, ' History of Physical Astronomy/ p. 548. 



l.i, ' Ili^oin -do* Matln'niatifjues/ vol. ii. p. 420. 



