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PROCEEDINGS OF THE AMERICAN ACADEMY 



Foucault's experiment on the velocity of light has been popularly 

 announced as making a " revolution in astronomical science." But it 

 appears from the preceding sketch, that it has raised no new question 

 in Astronomy, though it may have attracted popular attention to an old 

 difficulty, and possibly given a solution to it. The three astronomical 

 methods present solar distances, which, even if we select the most 

 trustworthy decision of each, differ by three or four millions of miles ; 

 that is, by three or four per cent of the whole quantity. Though the 

 best products of the first and second methods were at one time within 

 a million of miles of each other, an increase of lunar observations, and 

 especially improvements in the lunar tables, have now carried that 

 difference up to four millions of miles. If Foucault's experiment were 

 allowed to give the casting vote, it would decide in favor of the third 

 method ; thus making the reflection of Laplace, which I have already 

 quoted, still more memorable. 



In regard to the commonly received distance of the sun, which is 

 based upon Encke's profound discussion of all the observations made 

 at the last two transits of Venus, the case stands thus : Encke decides 

 from the weights of the observations, discussed in the light of the math- 

 ematical principle of Least Squares, that the probable error of the sun's 

 distance, as given by the transits, does not exceed ^\-^ of the whole 



