CELESTIAL MEASURINGS AND WEIGHINGS. 177 



by a conjuncture of circumstances, which ahiiost of ne- 

 cessity obUges us to take a coup-d'oniloi the whole subject, 

 and make up our minds, not only as to the validity of 

 what is done, but of the manner in which it has been 

 done ; the methods employed ; the direction in Avhich 

 we are henceforth to proceed, and the probability of 

 further progress. 



(2.) The subject to which this lecture is devoted 

 affords an instance of a conjuncture of this kind. We 

 have already had occasion incidentally, in Lect. III. 

 9, to call attention to the change which it has 

 been found necessary to make (at present of a pro- 

 visional rather than a definitive character) in our esti- 

 mate of the distance of the sun a change, implying 

 of course the necessity of a proportionate alteration in 

 all those statements of the dimensions of our system, 

 such as the diameters of the planetary orbits ; of the sun 

 and the planets themselves ; and the distances of their 

 satellites from the primary, and even the estimate of the 

 masses of all these bodies and the dimensions of the 

 cometary orbits : all those elements, in short, which 

 assume directly or indirectly the mean distance of the 

 sun as their unit of scale. There is reason to believe, 

 too, that the distance of the moon (our knowledge of 

 which does not assume that of the sun as known) has 

 been somewhat misestimated, and that an alteration 

 (though not nearly to so great a proportional extent), 

 bringing our nearest celestial neighbour into somewhat 

 closer proximity than heretofore supposed, is required. 



(3.) The dimensions and figure of the earth itself too, 



M 



