The transit of Mercury over the Sun enabled me to verify the place of the aphelion, and by the result there appears to be no necessity to change the tables of this planet.' The transits of Mercury take place much oftener than those of Venus; but on account of his greater distance from the Earth, and the smallness of his parallax from the Sun, they are not susceptible of equal utility with those of Venus.

In viewing the progress of human knowledge, and especially in tracing the steps by which the noble science of astronomy has attained its present advancement, the scientific mind not only acquires fresh strength, but derives peculiar elevation from frequently contemplating the exertions of genius, and witnessing the industry and ardour with which she overcomes difficulties, insurmountable to all but genius herself. Nothing is perhaps more animating to the youthful student than frequently reflecting on the old maxim,-whatever man has done, man_ may do-and the sensible mind can scarce ly recollect that the vast magnitude of the Earth has now become the subject of actual measurement; that all the stars that are visible to the naked eye have been accurately numbered; and that even the immense distance of the Sun can now be submitted to a rigorous calculation, without deriving fresh courage from the recollection, and feeling itself reanimated in the pursuit of truth. The two transits of Venus, above referred to, have enabled astronomers to resolve this problem with an accuracy which those of former ages durst not even hope to attain. The person who first suggested this excellent method of solution was Dr. Edmund Halley, whose mathematical and philosophical labours will justly entitle him to the veneration of posterity, at a period when many whose ambition has either inflamed the passions or deepened the miseries of man will have sunk into everlasting oblivion. A few extracts from the Dissertation on this subject, which he presented

to the Royal Society, of which he was a distinguished member, will afford the reader the best information on the manner in which he was led to it, and the clearest views of the impression it made on his mind at the time of the discovery. He observes,

There are many things that appear extremely paradoxical, and even quite incredible to the illiterate, which yet, by means of mathematical principles, are easily solved. Scarcely any thing will be thought more hard and difficult than that of determining the distance of the Sun from the Earth; but this, when we are made acquainted with some exact observations, taken at places fixed upon, and chosen beforehand, for that purpose, may, without much labour, be easily effected. And this is what I am more desirous to lay before this illustrious Society, that I may explain to young astronomers, who may perhaps live to observe these things, the method by which the immense distance of the Sun from the Earth may be truly determined, to within at least a five hundredth part of what it really is.'

Dr. Halley then states the opinions of various astronomers relative to this distance, with his own reasons for doubting their conclusions, and then says,

About forty years ago, when I was in the Island of St. Helena, taking a catalogue of the stars near the south pole, I had an opportunity of observing the pas→ sage of Mercury over the Sun's disc, which succeeded better than I could have expected; for, by means of a telescope twenty-four feet long, I determined the very moment when Mercury, entering upon the Sun, seemed to touch his inward limb; and also when, in going off, it struck the limb of the Sun's disc, forming the angle of interior contact; by which means I found the interval of time during which Mercury appeared upon the Sun, even without an error of a single second.

For the lucid line, intercepted between the dark limb of the planet and the bright limb of the Sun,

although exceedingly fine, may be easily seen by the eye; and the small dent made in the Sun's limb, by Mercury's entering or leaving the disc, appears, in the first case, to vanish, and, in the latter, to begin almost instantaneously. When I perceived this, it came immediately into my mind, that the Sun's parallax might be accurately determined by such kind of observations as these, provided Mercury were nearer the Earth, and had a greater parallax from the Sun. But the difference of these parallaxes is always less than the solar parallax which we seek; and therefore Mercury, though he may frequently be seen apparently on the Sun, is not to be regarded as fit for our purpose.

There remains then the transit of Venus over the Sun's disc, whose parallax, being almost four times as great as the solar parallax, will cause very sensible differences between the times in which Venus will seem to be passing over the Sun from different parts of the Earth. And from these differences, if they be properly observed, the Sun's parallax may be determined, even to a small part of a second. Nor are any other instruments required for this purpose, than common telescopes, and clocks, which are good of their kind; and in the observers, nothing more is necessary than fidelity, diligence, and a moderate skill in astronomy.

For there is no need that the latitude of the place should be scrupulously observed, nor that the time itself should be accurately determined with respect to the meridian: it is sufficient that the clocks be regulated according to the motion of the heavens, provided the times be accurately reckoned from the total ingress of Venus into the Sun's disc, to the beginning of her egress from it; that is, when the dark globe of Venus first begins to touch the bright limb of the Sun within; which moments I know, by my own experience, may be observed to within a second of time.

But, on account of the very strict laws by which the motions of the planets are regulated, Venus is seldom to be seen within the Sun's disc; and during the course of one hundred and twenty years it could never be once observed; namely, from the year 1639 (when this most pleasing sight happened to that excellent youth Horrox, our countryman, and to him only since the creation) to the year 1761; in which year, according to the theories that have been hitherto found agreeable to the celestial motions, Venus will again pass over the Sun on the 26th of May (old style) in the morning; so that at London, about six o'clock in the morning, we may expect to see her near the middle of the Sun's disc, and not above four minutes of a degree south of his centre.

"The whole duration of this transit will be almost eight hours; namely, from two o'clock in the morning to a litle before ten, and therefore the ingress will not be visible in England; but as the Sun will, at that time, be in the 16th degree of Gemini, having nearly twenty-three degrees of north declination, it will be seen without setting in almost every part of the northern frigid zone.'

The doctor then specifies the places where Venus might be seen to enter the Sun's disc, and observes, that the mouth of the Ganges would be a favourable place for the observation; as the Sun would be nearly four hours to the east when the planet entered his disc, and as many towards the west when it leaves it; and adds, the apparent motion of Venus over the solar disc will be accelerated by almost double the horizontal parallax of Venus from the Sun; because Venus, at that time, is carried with a retrograde motion from east to west, while a spectator, placed upon the Earth's surface, is turned the contrary way, from

west to east.

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Supposing, therefore, the Sun's parallax to be twelve seconds and a half, as I have before conjectured, the parallax of Venus will be forty-three se

conds; from which, if the former be subtracted, there will remain thirty seconds and a half for the horizontal parallax of Venus from the Sun: and, therefore, at those places which lie near the tropic, the motion of Venus will be increased by that parallax forty-five seconds at least, whilst she passes over the Sun's disc; and still more so at places which are situated in the neighbourhood of the equator. Now Venus, at that time, will move on the Sun's disc very nearly at the rate of four minutes of a degree in an hour, and therefore eleven minutes of time, at least, are to be allowed for the forty-five seconds of a degree abovementioned; which, therefore, is the space of time the duration of the eclipse, caused by Venus, will, on account of the parallax, be shortened.'

In a question of this moment, however, the doctor does not conceive it adviseable to depend solely upon one place of observation for the result, and therefore fixes upon Port Nelson, on the shores of Hudson's Bay, as another convenient station for the purpose; because there the parallax of Venus will increase the duration of the transit by at least six minutes of time; for whilst the Sun from his setting to his rising seems to pass under the pole, those places on the Earth's surface will be carried from east to west, or with a motion conspiring with that of Venus and therefore she will seem to move more slowly on the Sun, and be longer in passing over his disc. If therefore it happens that this transit should be properly observed, by skilful persons, at both these places, it is clear that the duration of it will be seventeen minutes longer as seen from Port Nelson, than as seen from the East Indies.'

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Dr. H. points out Madras, Bencoolen, Pondicherry, and Batavia, as proper places for making the requisite observations, of an occurrence which none of the inhabitants then living could expect to see more than another time; and evinces his earnestness by adding,