622 ADDRESS OF PROFESSOR LOVERING. 
of these relations. If the velocity of light can be known from 
experiment, the problem may be reversed and the distance of the 
sun given to the astronomer. As soon at it appeared that Fou- 
cault’s estimate of the velocity of light fell short of the astronom- 
ical valuation by about three per cent, it was’ certain that either 
the experiment was in error, or the received aberration was too 
small, or the reputed distance of the sun was too large. An error 
of three per cent. in the experiment or in the aberration was inad- 
missible. But it was conceivable that the distance of the sun 
should be at fault, even to this extent. The popular announce- 
ment that Foucault had picked a flaw in the astronomer’s work 
was not correct. Astronomers had always known what those who 
pinned 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 conclusions. 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 ques- 
tion was, which to believe. Since 1824, when Encke published 
his exhaustive computations on the last transits 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 practical 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 astronomer Stone. When Venus has nearly entered upon ke 
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 discs; caused, perhaps, by irradiation. One ob- 
server has recorded the time when this ligature began, another 
the time when it was broken. In working up the observations 
the last transits, both classes were not combined indiscriminately: 
Mr. Stone has reéxamined the documents, classified differently the 
