, ai9] Bradley s Observations 273 



characteristics of eye and judgment which make a first- 

 rate observer; his instruments were mounted in the best 

 known way for securing accuracy, and were constructed by 

 the most skilful makers ; he made a point of studying very 

 carefully the defects of his instruments, and of allowing 

 for them ; his discoveries of aberration and nutation 

 enabled him to avoid sources of error, amounting to a 

 considerable number of seconds, which his predecessors 

 could only have escaped imperfectly by taking the average 

 of a number of observations ; and his improved tables of 

 refraction still further added to the correctness of his 

 results. 



Bessel estimates that the errors in Bradley 's observations 

 of the declination of stars were usually less than 4", while 

 the corresponding errors in right ascension, a quantity which 

 depends ultimately on a time-observation, were less than 15", 

 or one second of time. His observations thus shewed a 

 considerable advance in accuracy compared with those of 

 Flamsteed ( 198), which represented the best that had 

 hitherto been done. 



219. The next Astronomer Royal was Nathaniel Bliss 

 (1700-1764), who died after two years. He was in turn 

 succeeded by Nevil Maskelyne (1732-1811), who carried 

 on for nearly half a century the tradition of accurate 

 observation which Bradley had established at Greenwich, 

 and made some improvements in methods. 



To him is also due the first serious attempt to measure 

 the density and hence the mass of the earth. By com- 

 paring the attraction exerted by the earth with that of 

 the sun and other bodies, Newton, as we have seen 

 (chapter ix., 185), had been able to connect the masses 

 of several of the celestial bodies with that of the earth. 

 To connect the mass of the whole earth with that of a 

 given terrestrial body, and so express it in pounds or tons, 

 was a problem of quite a different kind. It is of course 

 possible to examine portions of the earth's surface and 

 compare their density with that of, say, water; then to 

 make some conjecture, based on rough observations in 

 mines, etc., as to the rate at which density increases as 

 we go from the surface towards the centre of the earth, 

 and hence to infer the average density of the earth. Thus 



18 



