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POPULAR SCIENCE REVIEW. 
This is the method which is sometimes known as the direct 
method. It indicate^ the circumstance which lies at the basis 
of all the methods of observing the transit. The difficulties 
involved in this method are sufficiently manifest. Two observa- 
tions, in order to be brought into comparison, must be made 
either at the same instant of absolute time, or nearly so, the 
exact difference of absolute time being known ; and as the 
observers are at opposite parts of the earth this condition in- 
volves the necessity of the accurate determination of the longi- 
tude of each station, and of the local time of each observation. 
Moreover, the determination of the exact position of Venus on 
the sun’s disc is not only a task of considerable difficulty in 
itself, but, inasmuch as it requires time, while Venus is all the 
while travelling onwards, the position of Venus at any given 
instant has to be inferred from observations not all made at 
that instant, and is thus not in reality a direct determination. 
These considerations led Halley to devise his now famous 
method, in which an observation of duration is substituted for 
the determination of position at a particular instant. If an 
observer at a northern station times Venus as she travels along 
a certain chord, it becomes possible to determine the position 
of the chord ; so also the position of the chord along which a 
southern observer notes the transit, can be inferred from the 
observed duration.* Thence the distance between the two 
chords can be deduced ; and this is precisely what is required 
and would be obtained (as we have seen) for the direct solution 
of the problem. 
The difficulties in Halley’s method are easily recognised. 
First, the whole transit requires to be seen, since the beginning 
and end have to be observed. As a transit may last several 
hours, this circumstance introduces a double difficulty ; for the 
selection of stations is limited by the fact that, instead of half 
the earth being available, the portion of the earth where the 
whole transit is visible is only the part common to the two 
hemispheres which are in day-light at the beginning and end 
of the transit ; and, moreover, a station requires to be well 
placed (so as to produce suitable parallactic displacement of 
Venus) both at the beginning and end, and there may be very 
few stations so situated. To this may be added the circum- 
stance that it is antecedently less likely that favourable meteor- 
ological conditions will prevail both at the beginning and at the 
end of the transit, than at one or other epoch. But this, 
perhaps, is counterbalanced by the advantage that an observa- 
* I have here taken no account of the earth’s rotation, and have spoken 
only of the motion of Venus’s centre, not wishing to deal with any but the 
rough elements of the problem. 
