PAST AND COMING- TRANSITS AND ARCTIC EXPLORATION. 265 
that the official astronomers responsible for the English plans 
considered the unfavourable weather likely to prevail over most 
of the more suitable southern regions was a sufficient reason for 
having fewer southern than northern stations. The American 
astronomers held just the contrary opinion. “From all the re- 
ports,” says Professor Newcomb, the chief of the Washington Ob- 
servatory, 66 it was found that the chances of good weather were 
much better in the northern than in the southern hemisphere ; 
therefore, instead of sending an equal number of parties north 
and south, it was determined to send three to the northern and 
five to the southern hemisphere.” 
But it was in the actual observation of the phenomena of the 
transit that circumstances were noted which most significantly 
affect the value of the various methods. These circumstances 
I proceed now to consider, as on them must not only depend 
the opinion we are to form respecting the arrangements which 
should be made for the transit of 1882, but also the value we 
are to attach to the results secured last December. 
In the first place, it will be remembered that though doubts 
were expressed in many quarters as to the possibility of deter- 
mining the moment of internal contact with great accuracy, 
the doubts so expressed were based chiefly on a phenomenon 
called usually the “ black drop.” It had been supposed that the 
greater part of the error in the determination of solar parallax 
from the transit of 1769, had arisen from the difficulty caused 
by the “ black drop.” Some observers were assumed to have 
taken for the moment of true internal contact the instant when 
the edge of Yenus seemed to separate from the sun’s at ingress, 
or to join the sun’s at egress — a sort of dark ligament suddenly 
breaking in the first case, and as suddenly forming in the second 
case. Other observers were assumed to have judged when the 
outline of the undisturbed part of the planet’s disc belonged to 
a circle which, if complete, would have just touched the sun’s 
edge. The interval between the first kind of contact and 
the second, or between real contact and apparent contact, was 
assumed to have a constant value — seventeen seconds. This 
done, and the observations passed through what Leverrier has 
called the “ grist-mill ” of the method of least squares, there 
came out a result agreeing very well with the values of the sun’s 
parallax obtained by other methods. Unfortunately it so hap- 
pened that many of the observers in 1769 noted contacts of 
both kinds, and instead of finding the difference to be seventeen 
seconds, or thereabouts, they observed differences varying from 
twenty to forty seconds, and in one or two instances attaining 
a yet greater value. This of itself would have sufficed to 
deprive the explanation of all real value ; but it was further 
