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distance must lie between 92,000,000 and 96,000,000 miles.About fifty years afterwards, Encke, as the result of anelaborate investigation, came to the conclusion, that the mostprobable distance thus deducible was between 95 and 95 imillions of miles ; and this value was thenceforth generallyadopted by astronomers.
Some subsequent investigations have, however, indicatedthat a right understanding of the records of 1769 might havegiven a considerably smaller value ; and other methods ofsolving the problem having also favoured this supposition, thetransit of December 8th, 1874, was almost impatiently awaited.We shall be better able to estimate the value of the conclusionsobtained from it, if we pause for a few moments to explainthe exact nature of the observations which are made at suchtimes, and the especial difficulties which they involve.
It is found that different places upon the Earth may see theplanet begin to enter upon the Sun’s disc, or leave it, at momentswhich may differ by more than twenty minutes. It alsofollows that if such places be so situated that they can seeboth the beginning and the end of the transit, the whole dura-tion of it as witnessed from them may by no means be thesame. In fact, this difference of duration may exceed half anhour, as was the case in 1874. This effect is a joint result oftwo causes.
!>., at the end of 331 hours they would have both very nearly completed awhole number of rounds, except that the first would need -f f of an hour, orrather less than 6 minutes, more to finish his thirteenth round. If, there-fore, when they started, they were in a straight line, passing through thecommon centre of the two courses, they would at the end of 331 hours bewithin 6 minutes of passing across the same straight line exactly togetheragain. But the smaller the defect from an exact agreement of time in theirso passing, the longer would be the interval and the greater the numberof rounds before the agreement would be perfect. This illustration mayperhaps help to explain the somewhat similar behaviour of the Earth and"Venus , although the length of the circumferences of their orbits isimmensely greater than that of the short courses suggested for therunners ; and the actual speed of Venus would bring it slightly before,instead of slightly behind the Earth , at the end of its thirteenth revolution.Otherwise, the comparative speeds and the lengths of the courses are verynearly in the actual ratios of those of the two planets.