possible the conditions of sky-illumination which existed at the time of the eclipse. Having a very distinct recollection in respect to the brilliancy of the stars which I saw, and by observing when the approaching daylight had reduced the light of certain stars which were E. of the Sun at the time of the total eclipse, so as to be just visible in the telescope as they were then, I have been enabled to form a still more definite opinion of the relative brilliancy of Cancri, the two new objects which I observed, and Cancri. The fainter of the two planets, that near ✪ Cancri, was certainly brighter than Ŝ Cancri, and much more than a magnitude brighter than its neighbouring star. I am inclined to think that (a) should be classed as a good 4th magnitude, and that (b) should be classed as a 3rd magnitude, at the time of the observations on July 29. It is, of course, impossible to determine from these observations the planetary character of the stars observed. They did not exhibit such appearances as might be expected if they were comets near the sun; and since theory demonstrates the existence of such planets, I feel warranted in expressing the belief that the foregoing observations give places of two Intra-Mercurial planets. It is true that they were not so bright as might be expected if they were of size sufficient alone to account for the outstanding perturbations of Mercury, but it should be remembered that this expectation is based upon the assumption that the reflecting power of the surfaces of these planets is the same, or nearly the same, as that of Mercury. Now we know from actual observations that the intrinsic brilliancy of Mercury is scarcely 4th that of Venus when reduced to the same distance, and hence we cannot safely assume that the Intra-Mercurial planets must have the same relative brilliancy that they would have if their surfaces could reflect the light to the same extent as that of Mercury. I feel assured that by suitable devices these planets may be observed in full daylight near their elongations. Whether they are identical or not with moving spots which have been seen on the Sun's surface at different times it does not yet seem possible to determine m."

Swift's account of his work runs as follows:

"I reluctantly broke away from the wondrous scene [the Corona], and immediately essayed the well-nigh hopeless task which I had chosen-the finding of an Intra-Mercurial planet. To my dismay I soon found that I had forgotten to untie the string holding the pole in place, and this prevented all search E. of the Sun, as if I attempted a move in that direction the lower end would plunge into the ground and against the little tufts of buffalo-grass. It is, perhaps, to this circumstance alone that I owe the discovery of Vulcan some 5 minutes after its detection by Professor Watson, totality having terminated at his station before its commencement at mine.

"Almost the first sweep made to the westward of the Sun I ran across 2 stars presenting a very singular appearance, each having a red round disc and being free from twinkling. I at once resolved to observe these with great care. Time was precious and yet 6 questions demanded an immediate answer, viz:

m

1. What were their distances from the Sun ?

2. What from each other?

3. What direction from the Sun ?

4. What from each other?

Washington Observations, 1876, App. III, "Reports on Total Solar Eclipses," PP. 119-23.

5. What the magnitude of each?

6. What stars were they?

My telescope, though equatorially mounted, had no circles, and consequently no measurements were possible, but I endeavoured to be as accurate as existing circumstances would allow. My estimated answers were as follows:

1. About 3° from Sun's centre to midway between the stars.

2. About 8'.

3. South of West.

4. They were both on a line with the Sun's centre.

5. Equal, and of the 5th magnitude.

6. Probably, one was Theta Cancri; the other an Intra-Mercurial planet. "After completing these observations I resumed the quest, sweeping again southerly and W., but my fettered telescope behaved badly, and no regularity in the sweeps could be maintained, and I was surprised to find, in a few seconds, 2 stars in the field answering, in every particular, to the above description, and, sighting along the top of the tube on the outside, as in the first instance, I found they were the same objects. Again, I went through with the above comparisons, though I devoted only about one-fourth of the time given on the first occasion. Finding no necessity for modifying any of the above estimates, I, for the third time, renewed my sweeps, this time nearly along the ecliptic, though I feared to go too far to the W. lest I might not be able to get the glass back again to make a third and final observation of them, and also of the closing scenes of totality. I could place no dependence on the sweeps, and after a few seconds more (though it seemed longer) had them again in the field, This proved to be the last time. I again asked myself the already twice repeated questions, but found no appreciable change had taken place between the first and third observations—an interval of probably 14 minutes. Again I searched, but saw nothing, and, recollecting that I had no more time to spare, I endeavoured to refind the stars for a last observation, but unfortunately a small cloud (the only one within 50°) passed over them, and I was unsuccessful. I saw no stars but these 2, not even Delta, so near the Eastern limb of the Sun. As soon as totality was ended, I recorded in my note-book as follows: 'Saw 2 stars about 3° S.W. of Sun, apparently of 5th magnitude some 12' apart, pointing towards Sun. Red.' On my homeward journey the thought occurred to me that the distance between the stars was, according to memory, a little greater than half that between Mizar and Alcor, whatever that might be. Consulting Webb's Celestial Objects,' I found they were but 11 apart, which would make the distance of the two stars not to exceed 8', instead of 12', as hastily written at the time. While scanning them, I asked the mental question, 'What star looks at night to the naked eye as bright as do these through the telescope now?' Instantly, I answered 'The Pole-star.' That one was Theta Cancri is in the highest degree probable, and the other a planet is beyond all question, for on the morning of the 10th instant I observed Theta robbed of the companion I saw during the eclipsed Sun "."

These discoveries were hotly canvassed and their authenticity directly called in question, but not, I think, on fair or adequate grounds. It will be worth while, however, to examine the details

Washington Observations, 1876, App. III, “Reports on Total Solar Eclipses,"

p. 229.

of the controversy. Watson's idea of what he saw may be thus expressed. He first noticed a star which he thought was & Cancri, then Cancri, and near to an unknown body which he designated a; then a second strange object (designated b) which he saw near to the place in which he expected to find Cancri, the discovery of which, because he presumed it to be Cancri, led him to search no further in that part of his field of view.

The theory of the hostile critic, Professor C. H. F. Peters, is, that a was Cancri and was Cancri, and that some error in Watson's circles led to both his observations being vitiated in the same direction and to the same extent. This insinuation was however warmly repudiated by Watson". Peters dealt with Swift's record in a still more simple fashion: charged him with describing objects which he did not see at all, and implied that he concocted his alleged discovery after the publication of a telegram from Watson! Swift's reply to all this was as dignified as it was emphatic 9.

Swift's observations seem, in part at least, irreconcileable with Watson's, and if we assume the reality of Swift's 2 planets then Watson's a is a 3rd object and perhaps his b a 4th, so that in point of fact the 2 observers in question would seem to have discovered between them 4 Intra-Mercurial planets, which is in the highest degree improbable. Here the matter rests, except that the observers of the Total Solar Eclipse of May 6, 1883, say that they saw no object which could have been a planet, although specially searching for the purpose of finding a planet, if possible.

CHAPTER IV.

MERCURY.

Period, &c.-Phases.-Physical Observations by Schröter, Sir W. Herschel, Denning, Schiaparelli and Guiot.-Determination of its Mass.- When best seen.—Acquaintance of the Ancients with Mercury.-Copernicus and Mercury.-Le Verrier's investigations as to the motions of Mercury.-Tables of Mercury.

MER

ERCURY is, of the old planets, the one nearest to the Sun, round which it revolves in 87d 23h 15m 43·918, at a mean distance of 35,958,000 miles. The eccentricity of the orbit of Mercury amounting to 0.205, the distance may either extend to 43,347,000 miles, or fall as low as 28,569,000 miles. The apparent diameter of Mercury varies between 4.5′′ in superior conjunction, and 12-9" in inferior conjunction: at its greatest elongation it amounts to about 7". The real diameter may be about 3008 miles or less. The compression, or the difference between the polar and equatorial diameters, has usually been considered to be too small to be measureable, but Dawes, in 1848, gave it at

Mercury exhibits phases resembling those of the Moon. At its greatest Elongation (say W.) half its disc is illuminated, but as it approaches Superior Conjunction the breadth of the illuminated part increases, and its form becomes gibbous; and ultimately, when in Superior Conjunction, circular: at and near this point the planet is lost in the Sun's rays, and is invisible.

In case it should be thought that these accounts of the planets are deficient in statistical data, it may here be remarked that they are intended to be read in connexion with the tabulated statistics in the Appendix of this volume,

as it has been thought for several reasons undesirable to encumber the Text of Book I. with too many figures.

b An American observer, D. P. Todd, in 1880, put it at 2971 miles.

On emerging therefrom the gibbous form is still apparent, but the gibbosity is on the opposite side, and diminishes day by day till the planet arrives at its greatest Elongation E., when it again appears like a half-moon. Becoming more and more crescented, it approaches the Inferior Conjunction; and having passed this, the crescent (now on the opposite side) gradually augments until the planet again reaches its greatest W. Elongation.

Owing to its proximity to the Sun, observations on the physical appearance of Mercury are obtained with difficulty, and are therefore open to much uncertainty. The greatest possible elongation of the planet not exceeding 27° 45′ (and it being in general less), it can never be seen free from strong sunlight, under which conditions it may occasionally be detected with the naked eye during 1 or so after sunset in the spring (E. Elongation) and before sunrise in the autumn (W. Elongation), shining with a pale rosy hue. With the aid of a good telescope equatorially mounted, Mercury can frequently be found in the daytime.

Mercury has not received much attention from astronomers in the present day, and the observations of Schröter, at Lilienthal, and those of Sir W. Herschel, are the main sources of information. The former observer and his assistant Harding obtained what they believed to be decisive evidence of the existence of high mountains on the planet's surface: one in particular, situated in the Southern hemisphere, was supposed to manifest its presence from time to time, in consequence of the Southern horn, near Inferior Conjunction, having a truncated appearance, which it was inferred might be due to a mountain arresting the light of the Sun, and preventing it from reaching as far as the cusp theoretically extended. The extent of this truncature would serve to determine the height of the mountain occasioning it,

When Mercury's Elongation is the greatest possible, the planet's position is (in England) S. of the Sun, and therefore the chances of seeing it are not so good as when an Elongation coincides with a more Northerly position, albeit the Elongation is less considerable. The

greatest possible Elongation is a W. one which happens at the beginning of April. The least (17° 50′) an Elongation (also W.) which happens at the end of September.

d This has also been seen by Noble (Ast. Register, vol. ii. p. 106. May 1864).