346 A Short History of Astronomy [Cn. xn 



tion Hercules. A motion of the sun in this direction 

 would, he found, produce in the 14 stars apparent motions 

 which were in the majority of cases in general agreement 

 with those observed.* This result was published in 1783, 

 and a few months later Pierre Prevost (1751-1839) deduced 

 a very similar result from Tobias Mayer's collection of 

 proper motions. More than 20 years later (1805) Herschel 

 took up the question again, using six of the brightest stars 

 in a collection of the proper motions of 36 published by 

 Maskelyne in 1790, which were much more reliable than 

 any earlier ones, and employing more elaborate processes 

 of calculation ; again the apex was placed in the constellation 

 Hercules, though at a distance of nearly 30 from the 

 position given in 1783. Herschel's results were avowedly 

 to a large extent speculative, and were received by con- 

 temporary astronomers with a large measure of distrust ; 

 jut a number of far more elaborate modern investigations 

 of the same subject have confirmed the general correctness 

 of his work, the earlier of his two estimates appearing, 

 however, to be the more accurate. He also made some 

 .ttempts in the same papers and in a third (published in 

 1806) to estimate the speed as well as the direction of the 

 sun's motion ; but the work necessarily involved so many 

 assumptions as to the probable distances of the stars 

 which were quite unknown that it is not worth while to 

 quote results more definite than the statement made in 

 the paper of 1783, that " We may in a general way estimate 

 that the solar motion can certainly not be less than that 

 which the earth has in her annual orbit" 



266. The question of the comparative brightness of stars 

 was, as we have seen ( 258), of importance in connection 

 with Herschel's attempts to estimate their relative distances 

 from the earth and their arrangement in space ; it also 

 presented itself in connection with inquiries into the vari- 

 ability of the light of stars. Two remarkable cases of 

 variability had been for some time known. A star in the 

 Whale (o Ceti or Mir a) had been found to be at times 



* More precisely, counting motions in right ascension and in 

 declination separately, he had 27 observed motions to deal with (one 

 of the stars having no motion in declination) ; 2^ agreed in sign with 

 those which would result from the assumed motion of the sun. 



