329 



employed, might be rendered nearly exact, by 

 adding a scale of equal parts to the moveable 

 frame of the telescope of the sextant. It was 

 by taking the mean of a great number of valua- 

 tions, that I saw the relative intensity of the 

 light of the great stars decrease in the following 

 manner : Sirius, Canopus, a Centauri, Acher- 

 ner, .0 Centauri, Fomalhaut, Rigel, Procyon, 

 Betelgueuse, s of the Great Dog, I of the Great 

 Dog, a. of the Crane, & of the Peacock. These 

 experiments, of which I have published else- 

 where the numerical results # , will become more 

 interesting, when travellers shall have deter- 

 mined afresh, at intervals of forty or fifty years, 

 some of those changes, which the celestial bo 

 dies seem to experience, either at their surface, 

 or in their distances from our planetary sys- 

 tem. 



When we have observed for a long time with 

 the same instruments, in our northern cli- 

 mates, and under the torrid zone, we are sur- 

 prised at the effect produced in the latter by the 

 transparency of the air, and the less extinction 

 of light, on the clearness with which the double 

 stars, the satellites of Jupiter, or certain nebulae 

 present themselves. Beneath a sky equally se- 

 rene in appearance, it would seem as if more 



* See note C at the end of this volume, and my Astron. 

 Obs., vol. i, p. 71. 



