late Professor of Practical Astronomy in Glasgow. 15 



comprehended in it, might depend upon periodical motion 

 round some grand centre of general gravitation. This con- 

 ception, besides appearing to them warranted by every view 

 they could take of the nature of gravity, seemed moreover to 

 receive some support from the discoveries which, since the time 

 of the great Halley, have been made of what has been called 

 the " proper motions of the fixed stars," and particularly from 

 the opinion entertained by that excellent astronomer, Dr Mas- 

 kelyne, " that, probably, all the stars are continually chang- 

 ing their places by some slow and peculiar motions through- 

 out the mundane space." 



Soon after this view had arisen, out of the familiar conver- 

 sation above-mentioned, it was pubhshed in a very short ano- 

 nymous tract, entitled, " Thoughts 07i general Gravitatmi^ 

 and Views thence arising as to the state of the U'ni verse.'''' 

 The chief inducement to so early a publication was the hope 

 of drawing immediate attention to so interesting a point, 

 which might possibly lead to the discovery of some way by 

 which the matter might be brought to the test of observation. 



It is quite obvious, that the foregoing suggestions necessa- 

 rily imply a motion of the solar system, as one of that im- 

 mense host, which, for what we yet know, may be subjected 

 to the laws of periodical revolution. Accordingly, it early oc- 

 curred, that perhaps the most advantageous way of advancing 

 in this investigation, might be to try to find out, if possible, 

 symptoms of such a law as afifecting that system to which we 

 ourselves belong. 



It sometimes struck him, when looking over the progress of 

 philosophical discovery, that many things of high moment ap- 

 pear to have lain long wrapped up in embryo, by our not 

 employing ourselves more frequently in what may be called a 

 *' direct search,'''' and in filling up with more attention and bold- 

 ness the list of desiderata. Between this last step, and the ac- 

 complishment of a profound discovery, he conceived that the 

 transition might sometimes be made with no great effort of in- 

 vention, by only sifting carefully such principles as are already 

 known and familiar to us, and availing ourselves of them in 

 their full extent. 



It was by proceeding in this way, ^id when considering the 



