306 The Past and Coming Transits and Arctic Explorations. 



English plans considered the unfavoi'able weather likely to prevail 

 over most of the more suitable southern regions was a sufficient 

 reason for having feioer southern than northern stations. The Ameri- 

 can astronomers held just the contrary opinion. " From all the 

 reports," says Professor Newcomb, the Chief of the Washington 

 Observatory, "it was found that the chances of good weather were 

 much better in the northern than in the southern hemisphere ; there- 

 fore, instead of sending an equal number of parties North and South, 

 it was deterrnined 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 determining the 

 moment of internal contact with great accuracy, the doubts so 

 expressed were based chiefly on a phenomonon called usually the 

 "black-drop." It had been supposed that the greater part of the 

 error in the determination of the 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 Venus 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 

 sub's edge. The interval between the first kind of contact and the 

 •second, or between the 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 value of the sun's parallax obtained by other 

 methods. Unfortunately it so happened that many of the observers 

 in 1769 noted contacts of both kinds, and instead of finding the differ- 

 ence to be seventeen seconds, or thereabouts, they observed the 

 differences varying from twenty to forty seconds, and in one or two 

 instances attaining a yet greater value. This of itself would have 



