ASTRONOMY OF THE NINETEENTH CENTURY. 437 



at the Cape of Good Hope, verified in 1840 the existence of a sensible 

 parallax in this star, fixing it at a somewhat less value namely, 0-9128", 

 which more recent observations have raised to 0-9187". The parallax 

 of a Centauri corresponds with a distance of about 20 billions of miles. 

 Or, to represent this immense distance in another way, it may be stated 

 that light, which travels with the velocity of 190,000 miles per second, 

 takes no less than 3^ years in reaching us from a Centauri. The dis- 

 tance of 6 1 Cygni is about 60 billions of miles, and if this star were 

 suddenly extinguished, its image would, after that event, continue to 

 shine in our skies with steady radiance for ten years, so long does its 

 light take to reach us. The proper motion of 61 Cygni, which has 

 been spoken of above, corresponds with a motion of translation of that 

 star through space at the rate of at least 1,333 millions ct miles per 

 annum, a velocity nearly three times as great as that of the earth m 

 its orbit round the sun. 



At the commencement of the nineteenth century the data in exis- 

 tence upon which was founded our knowledge, of the earth's figure 

 were the following arcs : 



The earth's density was deduced from various pendulum experiments, 

 especially the series instituted at Paris by Borda in 1790; the Sche- 

 hallien observations of Maskelyne (p. 256); and Cavendish's experi- 

 ments of the leaden balls (p. 257). In the years 1801, 1802, 1803, 

 the arc in Lapland which, according to the calculations of Clairaut 

 and Maupertuis, presented some unaccountable anomalies was 

 measured over again, and extended so as to embrace more than i^ . 

 This was done by Ofverbon, Svanberg, and others, and the results ob- 

 tained were more in accordance with those of other measurements. 

 About the same time an arc was measured very accurately in England 

 from the Isle of Wight to Clifton in Yorkshire. Two arcs were mea- 

 sured in India under the direction of Colonel Lambton. One of 

 these embraced nearly 10, and this was several years afterwards ex- 

 tended to 1 6. Biot and Arago extended the French arc to a small 

 island in the Mediterranean near Iviza, so that its whole length was 

 now nearly 121. Short arcs were measured by the Geveva astrono- 



