ON COMETS. 



those stated in my paper on the sun in the last lecture, 

 will not fail to observe that they are materially smaller 

 by one thirtieth part of their respective amounts. The 

 numbers there stated are in accordance with the state of 

 our knowledge accepted at the time when that lecture 

 was delivered, which rested for its basis on observa- 

 tions made upon Venus at the time of her transit across 

 the sun's disc in the year 1769 observations by which 

 the nearest distance of the orbits of Venus and the earth 

 was concluded in terms of the earth's diameter, on the 

 same general principle, though by a somewhat more re- 

 fined and circuitous process, as that from which the 

 least distance of Mars has just now been derived. As 

 the circumstances of this earlier determination (delicacy 

 of instruments and means of observation alone excepted) 

 were much more favourable to exactness, astronomers 

 would have hesitated in accepting the more recent con- 

 clusion in preference to the former, were it not for the 

 support and corroboration it derives from another deter- 

 mination, also quite recent (though somewhat prior in 

 point of date), depending on a direct measurement of 

 the velocity of light by a peculiarly ingenious and delicate 

 process invented and executed by M. Foucault. To ex- 

 plain the nature of this process here would lead me too 

 far away from the immediate object of this discourse, from 

 which, indeed, the whole of what is above said on the 

 distance of the sun and planets would be justly con- 

 sidered as a digression were it not in some sort obliga- 

 tory on every one to account for a departure from 

 numerical statements once made. Suffice it therefore 



