ASTRONOMY. 15 



occur in ascertaining the exact length of the pendu- 

 lum: 



The various expansion of metals, respecting which 

 scarcely two pyrometers agree ; the changeable na- 

 ture of the atmosphere; the uncertainty as to the 

 true level of the sea ; the extreme difficulty of mea- 

 suring accurately the distance between the point of 

 suspension and the centre of oscillation, and even of 

 finding that centre. Besides these, another source of 

 error has been lately discovered, which seems to re- 

 move all hope of perfect accuracy ; viz. the variety 

 of terrestrial attraction, from which cause the motions 

 of the pendulum are also liable to variation, even in 

 the same latitude. In pursuing his researches Capt. 

 Kater discovered that the motions of the pendulum 

 are affected by the nature of the strata over which it 

 vibrates. 



The length of the seconds' pendulum may be de- 

 monstrated to be exactly in proportion to the force of 

 gravity at the place of observation. 



i Vibration and Length of the Pendulum. 



It had long been a desideratum to ascertain the 

 number of vibrations which would be made by a cer- 

 tain pendulum, within a given time, were it placed at 

 the level of the sea in vacuo, and at a certain tem- 

 perature. This was established in 1820 and subse- 

 quent years, and the following statement resulted: 

 viz. 



Length of a pendulum vibrating seconds in vacuo in 



51 31' 8" 4 N., the latitude of London, at the Inches. 



temperature of 62 Fahr 39,13929. 



Ditto ditto at Brassa 39.16929. 



Ditto ditto at Hare Island - - - 39.1984. 



Ditto ditto at Melville Island, Polar Sea - 39.207. 



Ditto ditto at the Galapagos Isles, lat. 0.32 N. 39.01719. 



Ditto ditto at St. Bias, lat. 21.30 N. 39.00904. 



Ditto ditto at Rio Janeiro, lat. 22.55 S. 39.01206. 



