ately detected by its disc, under a high magnifying power.'

Such, briefly, is the well-known history of the discovery of those planets of our system which required the aid of optical resources and of persevering search among the stars for the detection of their existence; and it is interesting to find that we are indebted for our knowledge of Astræa to a similar sagacity and perseverance. The discoverer, M. Hencke, of Driessen in Prussia, is a gentleman who, at one period of his life, was employed in the post-office of that town, but who, being gifted with a taste for astronomical pursuits, has, for the last fifteen years, been rendering himself familiar with the general features of the heavens, for the express purpose of discovering such a body as has now rewarded his exertions. The circumstances which attended the discovery of the other four asteroids rendered it, antecedently, extremely probable that others yet remained to be detected; and the difficulty lay in conducting a search of such a nature. The body to be discovered would be probably of a brightness equivalent to a star of from the 8th to the 10th magnitude, and the only sensible circumstance in which it would differ from the star would be its motion. But the motion of a body can be detected only by comparisons between its situations on different days; and there would be nothing to direct the choice of the objects to be tried amongst the hundreds that one sweep of the telescope would present to the observer. Nothing, then, it is evident, but a complete familiarity with the part of the heavens under review, and a knowledge of the relative positions of all the stars in it, to the limits

of the lowest magnitude above specified, would suffice to assure the observer of the presence of an object in a particular position on one evening which did not occupy that position on a preceding. M. Hencke, while examining a portion of the heavens in the fourth hour of R.A., on December 8, was immediately aware of the presence, directly between two stars of the 9.10th magnitude, marked on the Berlin maps, and denoted by the positions.

R. A.............4h 18m 45s
N.P.D. ......77° 18'

And R. A.

4h 20m 20$

N.P.D.....77° 28" 9"}

of a star of the 9th magnitude, not marked on the maps; and, from his familiarity with this part of the heavens, he felt assured that the star did not previously exist there. He wrote immediately to Professor Encke, and soon afterwards to Professor Schumacher (the letter to Schumacher was received by him December 13), announcing his suspicions of the discovery of a new planet, and giving the position of the star in question for the time of his observation, viz.—

z = resin (C + »); log. c = 9.54802; C = 197° 1' 46"

II. Account of some circumstances historically connected with the discovery of the planet exterior to Uranus. (Read by the Astronomer Royal.)

It has not been usual to admit into the memoirs of this society mere historical statements of circumstances which have occurred in our own times. I am not aware that this is a matter of positive regulation: it is, I believe, merely a rule of practice, of which the application in every particular instance has been determined by the discretion of those officers of the society with whom the arrangement of our memoirs has principally rested. And there can be no doubt that the ordinary rule must be a rule for the exclusion of papers of this character, and that, if a positive regulation is to be made, it must absolutely forbid

the presentation of such histories. Yet it is conceivable that events may occur in which this rule ought to be relaxed; and such, I am persuaded, are the circumstances attending the discovery of the planet exterior to Uranus. In the whole history of astronomy-I had almost said in the whole history of science there is nothing comparable to this. The history of the discoveries of new planets in the latter part of the last century, and in the present century, offers nothing analogous to it. Uranus, Ceres, and Pallas, were discovered in the course of researches which did not contemplate the possible discovery of planets. Juno and Vesta were discovered in following up a series of observations suggested by a theory which, fruitful as it has been, we may almost venture to call fanciful. Astræa was found in the course of a well

conducted re-examination of the heavens, apparently contemplating the discovery of a new planet as only one of many possible results. But the motions of Uranus, examined by philosophers who were fully impressed with the universality of the law of gravitation, have long exhibited the effects of some disturbing body: mathematicians have at length ventured on the task of ascertaining where such a body could be; they have pointed out that the supposition of a disturbing body moving in a certain orbit, precisely indicated by them, would entirely explain the observed disturbances of Uranus: they have expressed their conviction, with a firmness which I must characterize as wonderful, that the disturbing planet would be found exactly in a certain spot, and presenting exactly a certain appearance; and in that spot, and with

that appearance, the planet has been found. Nothing in the whole history of astronomy can be compared with this.

The principal steps in the theoretical investigations have been made by one individual, and the published discovery of the planet was necessarily made by one in dividual. To these persons the public attention has been principally directed; and well do they deserve the honours which they have received, and which they will continue to receive. Yet we should do wrong if we considered that these two persons alone are to be regarded as the authors of the discovery of this planet. I am confident that it will be found that the discovery is a consequence of what may properly be called a movement of the age; that it has been urged by the feeling of the scientific world in general, and has been nearly perfected by the collateral, but independent labours, of various persons possessing the talents or powers best suited to the different parts of the researches.

Without pretending to fix upon a time when the conviction of the irreconcilability of the motions of Uranus with the law of gravitation first fixed itself in the minds of some individuals, we may without hesitation date the general belief in this irreconcilability from the publication of M. Alexis Bouvard's Tables of Uranus, in 1821. It was fully shown, in the introduction to the tables, that, when every correction for perturbation indicated by the best existing theories was applied, it was still impossible to reconcile the observations of Flamsteed, Lemonnier, Bradley, and Mayer, with the orbit required by the observations made after 1781: and the elements of the orbit were

adopted from the latter observations, leaving the discordances with the former (amounting sometimes to three minutes of arc) for future explanation.

The orbit thus adopted represented pretty well the observations made in the years immediately following the publication of the tables. But in five or six years the discordance again growing up became so great that it could not escape notice. A small error was shown by the Kremsmünster Observations of 1825 and 1826: but, perhaps, I am not in error in stating that the discordance was first prominently exhibited in the Cambridge Observations, the publication of which from 1828 was conducted under my superintendence.

While still residing at Cambridge, I received from the Rev. T. J. Hussey (now Dr. Hussey) a letter, of which the following is

an extract:

Rev. T. J. Hussey to G. B. Airy.

"Hayes, Kent,

17th November, 1834. "With M. Alexis Bouvard I had some conversation upon a subject I had often meditated, which will probably interest you, and your opinion may determine mine. Having taken great pains last year with some observations of Uranus,

was led to examine closely Bouvard's tables of that planet. The apparently inexplicable discrepancies between the ancient and modern observations suggested to me the possibility of some disturbing body beyond Uranus, not taken into account because unknown. Subsequently, in conversation with Bouvard, I inquired if the above might not be the case: his answer was, that,

as might have been expected, it had occurred to him, and some correspondence had taken place between Hansen and himself re

specting it. Hansen's opinion was, that one disturbing body would not satisfy the phenomena; but that he conjectured there were two planets beyond Uranus."

My answer was in the following

terms:

tion of the astronomers of Paris was directed to Uranus.

Although no allusion is made in the last letter (M. E. Bouvard's) to the possible disturbing planet, it would be wrong to suppose that there was no thought of it. In fact, during the whole of these efforts for reforming the tables of Uranus, the dominant thought was, "Is it possible to explain the motions of Uranus, without admitting either a departure from

[EXTRACT.]

"Observatory, Cambridge,
23rd November, 1834.

"I have often thought of the irregularity of Uranus, and since. the receipt of your letter have looked more carefully to it. It is a puzzling subject, but I give it as my opinion, without hesitation, that it is not yet in such a state as to give the smallest hope of making out the nature of any external action on the planet. Flamsteed's observations I reject (for the present) without ceremony: but the two observations by Bradley and Mayer cannot be rejected. Thus the state of things is this-the mean motion and other elements derived from the observations between 1781 and 1825 give considerable errors in 1750, and give nearly the same errors in 1834, when the planet is at nearly the same part of its orbit. If the mean motion had been determined by 1750 and 1834, this would have indicated nothing: but the fact is, that the mean motions were determined (as I have said) independently. This does not look like irregular perturbation."

The astronomer royal then read a letter from M. Eugène Bouvard, showing how vigorously the atten

or the existence of a disturbing planet?" I know not how far the extensive and accurate calculations of M. Eugène Bouvard may have been used in the subsequent French calculations, but I have no doubt whatever that the knowledge of the efforts of M. Bouvard, the confidence in the accuracy of his calculations, and the perception of his failure to reconcile in a satisfactory way the theory and the observations, have tended greatly to impress upon astronomers, both French and English, the absolute necessity of seeking some external cause of disturbance.

Several months before the date of the last letter quoted, I had received the first intimation of those calculations which have led to a distinct indication of the place where the disturbing planet ought to be sought.

Professor Challis to G. B. Airy. [EXTRACT.]

"Cambridge Observatory, February 13th, 1844.

“A young friend of mine, Mr. Adams, of St. John's College, is working at the theory of Uranus, and is desirous of obtaining errors of the tabular geocentric longitudes of this planet, when near opposi

Professor Challis, in acknowledging the receipt of my answer, used the following expressions:

Professor Challis to G. B. Airy. [EXTRACT.]

Cambridge Observatory,
February 16th, 1844.

"I am exceedingly obliged by your sending so complete a series of tabular errors of Uranus. * * * The list you have sent will give Mr. Adams the means of carrying on in the most effective manner the inquiry in which he is engaged."

The next letter shows that Mr. Adams had derived results from these errors.

Mean Distance (assumed nearly in accordance with Bode's law)...... 38.4 Mean Sidereal Motion in 365.25 days Mean Longitude, 1st of October, 1845 Longitude of Perihelion Eccentricity

Mass (that of the Sun being unity)

323 34

315 55

0.1610.

0.0001656.

For the modern observations I have used the method of normal places, taking the mean of the tabular errors, as given by observations near three consecutive oppositions, to correspond with the mean of the times; and the Greenwich observations have been used down to 1830: