$217,218] Bradley V Minor Work: his Observations 271 



satellites by determining, in accordance with Galilei's 

 method (chapter vi., 127), but with remarkable accuracy, 

 the longitudes of Lisbon and of New York. 



217. Among Bradley's minor pieces of work may be 

 mentioned his observations of several comets and his 

 calculation of their respective orbits according to Newton's 

 method ; the construction of improved tables of refraction, 

 which remained in use for nearly a century ; a share in 

 pendulum experiments carried out in England and Jamaica 

 with the object of verifying the variation of gravity in 

 different latitudes ; a careful testing of Mayer's lunar tables 

 ( 226), together with improvements of them ; and lastly, 

 some work in connection with the reform of the calendar 

 made in 1752 (cf. chapter IL, 22). 



218. It remains to give some account of the magnificent 

 series of observations carried out during Bradley's adminis- 

 tration of the Greenwich Observatory. 



These observations fall into two chief divisions of unequal 

 merit, those after 1749 having been made with some more 

 accurate instruments which a grant from the government 

 enabled him at that time to procure. 



The main work of the Observatory under Bradley con- 

 sisted in taking observations of fixed stars, and to a lesser 

 extent of other bodies, as they passed the meridian, the 

 instruments used (the " mural quadrant " and the " transit 

 instrument ") being capable of motion only in the meridian, 

 and being therefore steadier and susceptible of greater 

 accuracy than those with more freedom of movement. 

 The most important observations taken during the years 

 1750-1762, amounting to about 60,000, were published long 

 after Bradley's death in two large volumes which appeared 

 in 1798 and 1805. A selection of them had been used 

 earlier as the basis of a small star catalogue, published in 

 the Nautical Almanac for 1773; but it was not tUl 1818 

 that the publication of Bessel's Fundamenta Astronomiae 

 (chapter xin., 277), a catalogue of more than 3000 stars 

 based on Bradley's observations, rendered these observations 

 thoroughly available for astronomical work. One reason 

 for this apparently excessive delay is to be found in 

 Bradley's way of working. Allusion has already been 

 made to a variety of causes which prevent the apparent 



