" is a matter of some difficulty.

The system described by Delisle in 1760, on the other hand, involves
merely noting the instant of ingress or egress (according to situation)
from opposite extremities of a terrestrial diameter; the disparity in
time giving a measure of the planet's apparent displacement, hence of
its actual rate of travel in miles per minute, from which its distances
severally from earth and sun are immediately deducible. Its chief
attendant difficulty is the necessity for accurately fixing the
longitudes of the points of observation. But this was much more sensibly
felt a century ago than it is now, the improved facility and certainty
of modern determinations tending to give the Delislean plan a decided
superiority over its rival.

These two traditional methods were supplemented in 1874 by the camera
and the heliometer. From photography, above all, much was expected.
Observations made by its means would have the advantages of
impartiality, multitude, and permanence. Peculiarities of vision and
bias of judgment would be eliminated; the slow progress of the
phenomenon would permit an indefinite number of pictures to be taken,
their epochs fixed to a fraction of a second; while subsequent leisurely
comparison and measurement could hardly fail, it was thought, to educe
approximate truth from the mass of accumulated evidence. The use of the
heliometer (much relied on by German observers) was so far similar to
that of the camera that the object aimed at by both was the
determination of the relative positions of the _centres_ of the sun and
Venus viewed, at the same absolute instant, from opposite sides of the
globe. So that the principle of the two older methods was to ascertain
the exact times of meeting between the solar and planetary limbs; that
of the two modern to determine the position of the dark body already
thrown into complete relief by its shining background. The former are
"methods by contact," the latter "methods by projection."

Every country which had a reputation to keep or to gain for scientific
zeal was forward to co-operate in the great cosmopolitan enterprise of
the transit. France and Germany each sent out six expeditions;
twenty-six stations were in Russian, twelve in English, eight in
American, three in Italian, one in Dutch occupation. In all, at a cost
of nearly a quarter of a million, some fourscore distinct posts of
observation were provided; among them such inhospitable, and al