opposition to the sun early in September, and if you look into the Nautical Almanac you will find three conjunctions between them given amongst the phenomena. In this diagram there is represented the apparent path of Mars in reference to Saturn considered as a fixed point. On July 27, Mars passes Saturn in right-ascension, reaches on August 1o the most easterly point of

Path of Mars relative to Saturn in 1779.

the loop, repasses Saturn again in its retrograde course on August 25, and describes then the curve laid down in the figure, and passes on November 3 a third time, and this time close to Saturn. This is the only triple conjunction of the two planets in the nineteenth century. Yet it may be considered doubtful whether

Path of Mars relative to Saturn in 1744-45

god of strictly it ought to be called such; for that depends on what is to be understood by the term "conjunction." In our modern ephemerides planetary conjunctions are reckoned in reference to right-ascension, and so it is in the present case. But, if they were reckoned, as conjunctions of the moon and planets with the

Path of Mars and of Jupiter relative to Saturn in 1742-43. The dotted line represents the path of Jupiter.

sun are still reckoned, and as formerly conjunctions generally were reckoned, in reference to "longitude," there will be no triple conjunction of Mars and Saturn this year, since the loop line,

which Mars describes, does not reach the circle of latitude passing through Saturn, but turns back while still about 21' short of it, as the next diagram shows. Accordingly, in longitude there will be only one conjunction, on November 3, and no triple conjunction in the present century. For the sake of comparison I have prepared other diagrams, showing the paths of Mars relative to Saturn on the three last occasions of triple conjunctions, viz., in 1779, when Mars described a zigzag curve, 1744-45, when it described a long flat loop, and 1742-43, when, while Mars described the large loop, Jupiter moved along the small flat curve in the figure, and the long-continued proximity of the three planets furnished a very interesting popular sight. At the third conjunction, on May 17, Mars passed at about 10' distance from Saturn, and on June 1 it was in very close conjunction with Jupiter. The planets were observed by Cassini II. and Maraldi II., but the nearest approach was not observable in Europe, as it occurred some hours after setting. An observer in America might have seen the nearest limbs only about 40" asunder.

Similarly, as in the case of Mars and Saturn, I have represented in another diagram the path of Mars relative to Jupiter on the three last occasions of their being three times in conjunction, namely in 1836-37, 1789-90, and 1646-47. The next occasion preceding this last was in 1503-4, and it was indeed a remarkable one, since during the same apparition Mars was also three times in conjunction with Saturn, so that altogether there were seven conjunctions, three between Mars and Jupiter, three between Mars and Saturn, and one between Jupiter and Saturn. The only observatory then existing, and deserving to be so called, was that of Walter, in Nuremberg, so that there seemed to be not much chance of finding any actual observations or notices of these conjunctions recorded except in Walter's observations. When I saw in the last printed list of presents received by the Society the title of Snell's edition of the Observations of the Landgrave of Hesse, one of the books formerly belonging to Mr. Sheepshanks, I remembered that Walter's observations are to be found in the appendix, and I have, therefore, looked into the book and have not been disappointed in finding some of these conjunctions recorded amongst Walter's last observations.†

We have not thought it necessary to reproduce this diagram, as the circle of latitude passing through Saturn on August 10th is given in our woodcut of the first diagram.—ED. Astronomical Register.

† [Note by Mr. Marth.] The title of the book is: Coeli et siderum in eo errantium Observationes Hassiacae ; quibus accesserunt,

So far about triple conjunctions which are past. Now a word about those to come. I find that the next triple conjunction between Mars and Saturn will take place in the year 1946, so that I cannot well wish the gentlemen present, except the youngest, that they may live long enough to see it. The next triple conjunction between Mars and Jupiter will not occur till 1980. But before these two events, a far rarer event will take place, namely, in 1940, a triple conjunction between Jupiter and Saturn, such as has only once been observed, in 1682-83 by Flamsteed* and Hevelius, which before that has not happened since 1425, and of which only one previous instance has been computed, namely, that occurring in the year 7 B.C., computed by Idelert and Encke, and also, about twenty years ago, by Professor Pritchard, in a paper in the memoirs of this Society, §

Joannis Regiomontani & Bernardi Walteri Observationes Noribergicæ. Lugduni Batav. 1618. It may, perhaps, not be out of place, if I mention a circumstance in connection with Walter's Observatory, which was recalled to my mind by a late political incident. The erection of this, the first modern observatory which deserves the name, was chiefly due to the beneficent influence of the learning of Regiomontanus, who, in 1471, had settled in the free town. The then King of Hungary, Matthias Corvinus, who appreciated learning, was bringing together a considerable library, and had, in 1469, called Regiomontanus to Buda, in order to preside over it; but the wars, in which the king got involved, frustrated his good intentions, and Regiomontanus had to leave, and was then received with open arms by the free citizens of Nuremberg, especially by Walter. The king's library was afterwards captured and carried off to Constantinople by the Turks, and it is this library (or rather the poor remnants of it still existing) which the Turks have now had the good sense to restore to Hungary. The ephemerides, extending from 1475 to 1506, which Regiomontanus published in 1474, and which were afterwards of such good service to Columbus, Vasco de Gama, and the other great navigators of the time, were dedicated by him to King Matthias.

* [Note by Mr. Marth.] "Whilst the Common People have admired to see the two Superior Planets Saturn and Jupiter continue to near each other this whole year, and our Astrologers have affrighted them with fearful Predictions of direful events to succeed this appearance, the more Judicious are desirous to know how often and at what time their Conjunctions happen. -Flamsteed in Phil. Tr. vol. xiii. p. 244. Hevelius' account is to be found in the same volume, p. 325.

[Note by Mr. Marth.] Ideler, Handbuch der mathem. u. technischen Chronologie. 1826. Vol. II. p. 406.

§ [Note by Mr. Marth.] Mem. vol. xxv., p. 119.—In the "Lehrbuch," Ideler had already corrected himself by suppressing the amusingly strained passage about the possibility of the two planets appearing to weak eyes as one star, and by communicating the result of Encke's computations. These results for the conjunctions of the planets are:

the contents of which, however, it is only fair to say, had been anticipated by Ideler himself twenty-five years before.

However, it is, perhaps, of more immediate interest to know what conjunctions will happen in the course of the next few years, and I have, therefore, represented here the close conjunction of Mars and Saturn on June 30, 1879, when they will pass each other at a distance of about 80", though not while visible to us in Europe. Since telescopes have been in use, there is only one instance known when the occultation or the covering of one planet by another has been observed, and that was the covering of Mercury by Venus, observed in May, 1737, by Bevis, at Greenwich.

The next conjunctions of interest will be those of Mars with Saturn and Jupiter, in July, 1881. In the preceding April Jupiter comes into conjunction with Saturn, as usually in every twentieth year, but very near to the sun and invisibly. But in July Mars will overtake both and the three superior planets, so near each other, will be for several weeks an interesting sight in the mornings. I have represented their mutual positions on five of these mornings in the next diagram. The nearest approach of Mars to Jupiter in the afternoon of July 22, will not be visible in Europe. And now to go back to a very remarkable former appearance of the three planets, I have laid down their positions for some evenings in the autumn of 1604. On September 26, Kepler and some of his disciples had observed the conjunction of Mars and Saturn, Jupiter being about 7° distant. On October 9 Mars was observed in conjunction with Jupiter, as represented in the diagram. On the following evening, October 10, one of the disciples, while looking for them, saw not three but four bright stars. Cloudy evenings intervened. On October 16 Kepler saw the four stars. On October 17 the constellation appeared as laid down in the figure. What was the fourth star? Much brighter than Jupiter and nearly approaching to the brightness of Venus when at her greatest brilliancy? It was the splendid and beautiful New Star of 1604, which though only seen low near the horizon,

had fired Kepler's imagination, as those acquainted with his works well know.*

I have prepared another diagram referring to an entirely different subject, but will not occupy your time any longer.

Dr. Royston-Pigott was called upon to read a paper On collimating reversible instruments such as the altazimuth, equatorial, and transit instruments by the destruction of parallax.

Dr. Royston-Pigott was understood to say, in explanation of the object of the method proposed, that when a very distant object was not easily attainable as a meridian mark, the employment of scales of inches, placed at convenient distances, would enable the observer to correct the collimation or test the squareness of the axis of the telescope to that of the declination circle, in the case of reversible instruments, with great precision. This is accomplished by ascertaining trigonometrically the absolute "throw" of the instrument when alternately reversed, i.e., turned over westwards and eastwards, and directed towards the same object. A spider line vertically placed in the centre of the field of view would thus be seen to cut the distant scale if the collimation were perfect exactly at points, whose distance was the absolute "throw" of the equatorial, &c.

And if two scales were employed, both visible in the same field of view, respectively placed (say) 66 and 156 yards distant from the object-glass, the spider line readings on the two scales would be absolutely the same only when the collimation is perfect. Dr. Royston-Pigott also explained his method of bringing into focus the near and distant scale in the same field of view, and at the same instant, by cutting off the aperture of the objectglass, so as to leave a vertical aperture only and about half-an-inch broad.t

The least error in the collimation developes a considerable parallax on the most distant scale on reversing the instru

ments.

Mr. Dunkin: Dr. Pigott says that by his contrivance he can determine the collimation of his instrument within two seconds of arc, which certainly is not a large quantity, but I should feel myself very much put out if the collimation determined on the

Quisquis es, qui portentum stupendum oculis tuis non adspexisti, facem imaginare tibi flagrantissimam luminis purissimi, ventis validissimis agitatam et quassatam: talis erat vibratio luminis, talis flammarum ebullitio, talis scintillatio ignivoma rapidissima."-Kepleri Opera. Ed. Frisch. Vol. iv. p. 282.

[Note by Dr. Royston-Pigott.] A second of arc is represented by an inch disc 206265 inches,, i.e., 5729 yards distant, or by a line th of an inch broad, placed 57 yards 10 inches distant.