206 THE POPULAR SCIENCE MONTHLY. 



their scientific faith on text-books did not expect that the problem 

 of finding the sun's distance was an exceedingly delicate case, and 

 that an ominous cloud of uncertainty hung over their wisest con- 

 clusions. Whenever it is possible to interrogate Nature in more ways 

 than one, Science is not satisfied with a single answer, nor with all the 

 answers unless they agree. The transit of Venus, the parallax of Mars, 

 and the tables of the moon, each can tell the sun's distance. But 

 their testimony was contradictory, and neither one at all times repeated 

 the same story. The question was, which to believe. Since 1824, 

 when Encke published his exhaustive computations on the last tran- 

 sits of Venus, the distance which they assigned to the sun has been 

 acquiesced in as the most probable. But the moon, as has been said, 

 has always been a thorn in the sides of mathematicians. While prac- 

 tical and theoretical astronomers have been reducing its motions to 

 stricter discipline, the suspicion has been steadily gaining strength in 

 their minds that the distance adopted from the transits was too large. 

 The effect of Foucault's experiment was, to intensify the doubt. The 

 case of the twin transits of the last century, thought to have been 

 closed forever by Encke, has recently been opened again by the as- 

 tronomer Stone. When Venus has nearly entered upon the sun, the 

 moment of interior contact is preluded by the formation of a slender 

 ligature (called the black drop) between the nearest parts of the two 

 disks caused, perhaps, by irradiation. One observer has recorded 

 the time when this ligature began, another the time when it was 

 broken. In working up the observations of the last transits, both 

 classes were not combined indiscriminately. Mr. Stone has re- 

 examined the documents, classified differently the materials, and ex- 

 tracted from them two new and independent values for the sun's 

 parallax. The reconciliation which he has suddenly brought about 

 between the experiments of Cornu and Foucault, the motions of the 

 moon, and the transits of Venus, is as perfect as it is surprising. 

 Nevertheless, the approaching transits of Venus, the earliest of which 

 is close upon us, will be welcomed, if not as the only possible way of 

 solving a hard problem, at least for the confirmation which is demanded 

 by a solution ah'eady reached ; for able astronomers have dissented 

 from the interpretation put upon the records by Stone. The minds of 

 observers have been prepared for what their eyes are to see, in Decem- 

 ber, 1874, by the experimental rehearsal of the black drop, and the 

 photographer's box will arrest the planet in the very act. 



The consequences of Foucault's experiment, substantiated as it 

 may be by the best astronomical evidence, are as far-reaching as the 

 remotest stars and nebula?. The sun's distance is the astronomer's 

 metre, through which masses, diameters, and distances, are propor- 

 tioned out to planets, comets, and stars. If the sun's distance is cut 

 down by three per cent., there must be a general contraction in all 

 the physical constants of the universe. The earth only is immediately 



