OF ARTS AND SCIENCES: MAY 12, 1SG3. 127 



quantity. Astronomers have also reason to believe that the adopted 

 value of aberration is correct within ■j'S^'s ^^ ^^^^ whole quantity. 

 Moreover, Foucault is confident of his determination of the velocity of 

 light within ^^ of the whole quantity ; nay, he expects to improve his 

 instruments so as to banish all errors larger than ^xnrtr of the whole 

 quantity. Neither the velocity of light, aberration, nor the sun's dis- 

 tance, can be suspected of an error to the extent of three or four per 

 cent, and yet one at least must be wrong to this degree, as the best 

 values of the three elements are irreconcilable with each other. Which 

 shall be changed ? 



It may excite surprise in those who have heard of the accuracy of 

 Astronomy without weighing the exact significance of the word as 

 applied to so large a subject, that there should still be a lingering 

 uncertainty to the extent of three or four miUions of miles in the sun's 

 distance from the earth. But the error, whatever it is, is propagated 

 from the solar system into the deepest spaces which the telescope has 

 ever traversed. The sun's distance is the measurino;-rod with which 

 the astronomer metes out the distances of the fixed stars, and the dimen- 

 sions of stellar orbits. An error of three per cent in the sun's distance 

 entails an error of three per cent in all these other distances and dimen- 

 sions. Trifling as three per cent may seem, the correction runs up to 

 600000 millions of miles in the distance of the nearest fixed star. 



Five hundred and tiventy-first Meeting. 



May 12, 18G3. — Monthly Meeting. 



The President in the chair. 



The Corresponding Secretary (who also acted as Recording 

 Secretary in the absence of the latter officer) read letters 

 relative to the Academy's exchanges ; also a letter from W. 

 W. Story, Esq., now at Rome, in acknowledgment of his elec- 

 tion as a Fellow. 



Mr. Treadwell read a memoir on the effect of cannon-shot 

 upon iron-clad ships and armor-plates generally ; this being a 

 sequel to his memoir " On the Practicability of constructing 

 Cannon of Great Caliber," published in the sixth volume of 

 the Academy's Memoirs (1856) ; the object of this sequel 



