1^ On the apparent Changes 



Such are the defects of the doctrines of the astronomical 

 schools on this subject. They are not to be imputed to the 

 Plumian professor, but belong to the system, which he de- 

 livers as received. His esteemed work was among my books, 

 and in seeking to give an account of atmospheric refractions, 

 as they are called, my opinion of him led me to that. I should, 

 have found the same things in any and every treatise of astro- 

 nomy to which I could have referred. 



The different methods of ascertaining the amount of these 

 changes of place, determining it for one altitude and object by 

 direct observation, and estimating it for other altitudes, objects, 

 and places, do honour to the talents of astronomers in every 

 age. Tycho, Cassini, De la Caille, Newton, Bradley, Hawksbee, 

 Maskelyne, and others, calculated, but possessed no correct 

 conceptions of the causes of the phsenomena. 



These changes of place having been found to vary in them- 

 selves, not only at different altitudes of the luminaries, but at 

 the same elevations also, in different states of the atmosphere, 

 and changes in the barometer and thermometer having been 

 observed to be cotemporaneous with those; these, although 

 themselves depended upon other changes, have been considered 

 as connected with and influencing those, and have been taken 

 into the account in which those were estimated and calculated. 

 There are, however, conditions and changes of the atmosphere, 

 dependent upon other causes than its temperature and weight, 

 which are indicated by the hygrometer, and upon which the 

 phaenomena themselves more immediately depend. These, 

 therefore, rather than those, or perhaps together with those, 

 are to be considered ; and thus it appears how uncertain all 

 knowledge is, not founded upon a knowledge of causes. 

 Rect^ scire est per causas scire. — Bacon. 



The apparent changes of place in the luminaries, in their 

 lowest stations near the horizon, are indeed subjected to ano- 

 malies, which render confessedly, the calculations applied to 

 the higher, not to be depended upon in these last stations. 



Whether in calculations, which require these apparent vari- 

 ations of change of place to be taken into the account, and 



