4 i 4 THE POPULAR SCIENCE MONTHLY. 



As matters stand at present, the majority of astronomers would 

 probably consider that these secular perturbations are not yet known 

 with an exactness sufficient to render this method superior to the 

 others that have been named perhaps as yet not even their rival. 

 Leverrier, on the other hand, himself puts such confidence in it that 

 he declined to sanction or cooperate in the operations for observing 

 the recent transit of Venus, considering all labor and expense in that 

 direction as merely so much waste. 



But, however the case may be now, there is no question that as 

 time goes on, and our knowledge of the planetary motions becomes 

 more minutely precise, this method will become continually and cumu- 

 latively more exact, until finally, and not many centuries hence, it will 

 supersede all the others that have been described. The parallax of 

 the sun, determined by Leverrier in this method, in 1872, comes out 

 8.86". 



The last of the methods mentioned in the synopsis given on page 

 405 is interesting as an example of the manner in which the sciences 

 are mutually connected and dependent. Before the experiments of 

 Fizeau in 1849, and of Foucault a few years later, our knowledge of 

 the velocity of light depended on our knowledge of the dimensions 

 of the earth's orbit : it had been found by astronomical observations 

 upon the eclipses of Jupiter's satellites that light occupied a little 

 more than 16 minutes in crossing the orbit of the earth, or about 8 

 minutes in coming from the sun ; and hence, supposing the sun's dis- 

 tance to be 95,600,000 miles, as was long believed, the velocity of 

 light must be about 192,000 miles per second; thus optics was in- 

 debted to astronomy for this fundamental element. But when Fou- 

 cault in 1862 announced that, according to his unquestionably accu- 

 rate experiments, the velocity of light could not be much more than 

 186,000 miles per second, the obligation was returned, and the suspi- 

 cions as to the received value of the sun's parallax, which had been 

 raised by the lunar researches of Hansen and Leverrier, were changed 

 into certainty. The most recent experimental determinations of the 

 velocity of light by Cornu in lS73-'74 fix the solar parallax between 

 8.80" and 8.85", according as we use Peters's " constant of aberration " 

 or Delambre's value of the " equation of light," which is the name 

 given to the time required for light to traverse the interval between 

 the sun and the earth. 



Collecting all the evidence at present attainable, it would seem 

 that the solar parallax cannot differ much from 8.86", though it may 

 be as much as 0.04" greater or smaller; this would correspond, as has 



by the square of the number of sidereal montlis in a year, and by the ratio between the masses 

 of the sun and earth. It is to be noted, however, that T and t are the periods of the earth 

 and moon, as they would be if wholly undisturbed in their motions, and hence differ 

 slightly from the periods actually observed- the differences are small, but somewhat 

 troublesome to calculate with precision. 



