220 THE POPULAR SCIENCE MONTHLY. 



causes beyond human control ; a contingency very remote, however, 

 as affecting the entirety of the observers, and from which it is to be 

 heartily hoped all will be exempted. 



The results of the transit of 1769 were rendered uncertain, to 

 some extent, by a curious attendant phenomenon called " the black 

 drop," consisting in an apparent clinging of the planet to the limb, 

 to which it is seemingly attached by a black ligament. The exact 

 cause of this illusion is not quite agreed on, but there can be little 

 doubt that it is in part a product of bad definition and inferior tele- 

 scopes, and, as such, need be expected to give less trouble in our 

 present observations of the times when the planet is really in contact 

 with the edge. It may, however, cause an error of some seconds in 

 noting the time, and in this particular seconds are all-important. 

 Encke, who discussed these results, found from them that the parallax 

 was 8."56, a value always known to be questionable; but whence the 

 sun's distance of " 95,000,000 miles," which found a place in our school- 

 books, was derived. 



Within a few years past, it has become certain, by evidence from 

 various quarters, that this is too much. Till toward the close of the 

 last century, astronomers had no other means of finding the sun's dis- 

 tance than by observations on Venus and Mars ; though, from those 

 of the latter planet, indeed, a much closer approximation to the solar 

 parallax than Kepler's value had long been obtained. Chiefly during 

 the present century, other methods have been added, of which the 

 most remarkable is that due to the French academician, Foucault. 



Though the speed of the earth in its orbit, and that of light, were 

 both unknown, yet the ratio of these two velocities had long been ascer- 

 tained. From the assumed distance of the sun above given (95,000,000 

 miles), it was evidently possible to tell the circumference of the earth's 

 orbit, and thence to say how many miles it went in a year, or a sec- 

 ond, and, by a simple multiplication, a value for the velocity of light 

 was obtained ; since, as has just been said, the latter velocity bore a 

 known proportion to the former. In this way, the value of 192,000 

 miles per second for the speed of light was found a quantity which, 

 beino- derived from an assumed distance of the sun, could not, of 

 course, be used in turn to determine it. When, however, Foucault 

 actually measured the velocity of light by a direct physical experi- 

 ment, it became possible, by a reversal of the above process, to say 

 how far the earth moved in a second ; whence we learn how far it 

 moves in a year, or, in other words, the length of its annual path ; 

 whence, again, the distance across it and the sun's distance obvious- 

 ly follow, the latter being thus found to be 92,260,000 miles, instead 

 of 95,000,000. 



From a discussion of all the different methods, Prof. Newcomb has 

 concluded that the solar parallax cannot be far from 8". 85 ; while Mr. 

 Stone, from a rediscussion of the results of the transit of 1769, be- 



