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ment expense to convey skilful observers to one of the stations which had been judged suitable for observing the phenomenon. The petition was complied with, and after some difficulty as to the choice of a leader, the ship Endeavour, of 370 tons burden, was placed under the command of Captain Cook. Continental astronomers also betook themselves to the most advantageous posts for observing the transit, and when at length Venus came between the Earth and the Sun, her arrival and passage were watched by observers at Wardhus, Kola, and Kajeneberg, at St. Petersburg, Orenberg, Yakutsk, Pekin, Manilla, Batavia, Otaheite, St. Joseph (in California), Kola, Hudson's Bay, and other well-chosen stations.

Most of the observations of the transit were well and skilfully conducted; but a circumstance, which now for the first time attracted serious attention, caused a certain difficulty in their interpretation. The observers had to determine the exact interval of time occupied by Venus in crossing the Sun's disc-or else in certain cases the exact moment of time when she began to cross the solar disc. Now, if Venus presented the appearance of a mathematical point, the observation would be simply described in the words I have just used. But as Venus has a disc of appreciable dimensions, the question arises, whether the commencement or end of transit shall be considered with reference to the moment when the disc of Venus just touches the Sun's disc on the outside, or to the moment when her disc just touches his on the inside, or, finally, to the

moment when her centre is crossing the Sun's edge. As to the last of these cases, we may dismiss it at once from consideration, because no observer, however experienced, could pretend to determine within a second or two when exactly one half of the disc of Venus was upon the Sun's. Either, then, the observer must note when Venus is, as at a (fig. 14), just touching the edge of the Sun's disc, s s', on the outside, or, as at B, just touching that edge (the limb as astronomers call it) on the outside. Now, taking the case of ingress, if the observer knew exactly where Venus would begin to

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cross the Sun's disc, as at a, and very nearly the true instant of such ingress, he might, if the weather were very favourable, determine within a second or two the moment when the uniformity of the limb ss' was marred by the encroachment of the disc of Venus. But as a matter of fact neither one condition nor the other is fulfilled, and so, even under favourable circumstances of weather, he might not detect the exact commencement of ingress. Nor is this all. By favourable weather, I mean something more than clear weather. In weather which seems perfectly clear the telescopist will often see the edge of the Sun's disc

absolutely rippled through the effects of atmospheric disturbance, and when the Sun is near the horizon, even in good observing weather, the outline of the disc is often egregiously disturbed. Under such circumstances nothing would be more difficult than to assign the exact moment when external contact took place. But these are the very circumstances to be looked for in the observations necessary for determining the Sun's distance. The Sun must be low down at all the stations most suitable (in other respects). For the essential point both in Halley's method and Delisle's-nay, in any method whatever for determining the Sun's parallax-is that two observers shall be as far as possible from each other on the illuminated hemisphere of the Earth; so that they must be near the rim of that hemisphere. In other words, they must be near that great circle of the Earth along which the Sun is seen exactly on the horizon. The Sun then will be nearly on the horizon at such stations, and will be viewed necessarily under somewhat unfavourable conditions.

It was this consideration which led, and probably will always lead, to the selection of internal contact as the proper phase to be observed. In attempting to time the moment when Venus is just within the Sun's disc, either at ingress or egress, there will, of course, be difficulties arising from atmospheric disturbances; but on the whole the observation of this phase is easier than the observation of external contact.

But, unfortunately, a certain peculiarity affects the

appearance of Venus when just within the Sun's disc. Instead of appearing circular, she assumes (just before she leaves the Sun's edge at ingress, or just as she reaches it at egress) a pear-shaped aspect, as at 1 (fig. 15), or such an aspect, as at 2 or 3, according to the nature of the telescope made use of, the conditions of the atmosphere, and the visual power of the observer. Thus the question arises, whether the assumption of this aspect is to be regarded as the true moment of internal contact, or whether that is the true moment when the circular part of Venus's apparent outline, if

FIG. 15.

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continued so as to form a complete circle, would just touch the Sun's outline. Between one moment and the other several seconds occur, and the whole question is one of seconds.

Here is a difficulty of grave importance; for it is to be noticed that a practised eye would be needed to determine the moment when the outline of Venus would just touch the Sun's if undisturbed, and even a practised eye might well be deceived on such a point. On the other hand, though an observer might time the exact moment when the outline of Venus is just clear of the Sun's (and within it), either at ingress or egress, yet as this moment depends (as already mentioned) on the qualities of the telescope and on the observer's

visual powers-probably also on the state of the weather -there is room for a considerable error to creep in.

I shall discuss further on the interpretation of the peculiarity just considered. At present the point to be attended to is the actual effect produced by this peculiarity upon the trustworthiness of the results obtained in 1769. The observers were for the most part unprepared for the serious difficulty which the peculiarity actually introduces. Although in 1761 Venus had been distorted at ingress and egress, while Halley himself had in 1753 noticed that Mercury exhibited a similar distortion, it had not occurred to astronomers to discuss the phenomenon, and so to be enabled to point out how the difficulty might be got over. In fact, the whole matter had been so completely overlooked, that, as I have said, few of the observers. in 1769 were prepared for the peculiarities which Venus presents when in interior contact with the Sun.*

The result was that much difficulty was experienced in interpreting the observations. It will be understood

* There is nothing more remarkable in the history of astronomical observations than the little preparation made for important occurrences, such as transits and eclipses, so far as the actual observation of the phenomena is concerned. Abundant preparation is made as far as instrumental means are concerned; but again and again the history of astronomy exhibits cases where the actual phenomena of transits and eclipses take the observer by surprise, and so are not observed as well as they might have been, even when abundant information has been in reality available for the instruction of those who are to take part in watching such occurrences. There is much, for example, in the history of eclipse-observations that is exceedingly painful to the real lover of science, more particularly in the wearisome repetition of observations which have already revealed all they can reveal, and in the apparent dearth of invention as respects the devisal of new modes of research.

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