Sidor som bilder
PDF
ePub

lated rock, and at the distance of 8000 toises from the land, and 16000 from the mountain, the action of the hill must amount to nothing; and, consequently, the difference between the amplitude of the arch of the meridian, determined by celestial observation, and inferred from terrestrial measurement, would give the effect of the attraction, uncombined with any other force either assisting or opposing it. The instruments with which the Baron was furnished, were, a repeating circle of 12 inches radius by REICHENBACH, with which he proposed to measure the distances from the zenith ; a repeating theodolite of eight inches radius by the same artist, for observing azimuths and terrestrial angles; an English sectant of nine inches radius by Troughton, for taking corresponding altitudes, to regulate four chronometers, three constructed by Josiah Emery of London, and one by Louis Berthouci of Paris. With these he began his observations at the station of Notre Dame des Anges ; and, by 874 altitudes, determined the true zenith distances of a great number of stars, all which was done between the 11th and 24th of July 1810.

The situation of this station did not allow the observation of stars on both sides of the zenith, as the mountain rose very perpendicularly to the north of the convent. Such stars might indeed have been observed with an instrument, like the sector, calculated for making observations near the zenith. The repeating circle has not that advantage; for its perpendicularity to the horizon not being very accurately ascertained, and the error arising from that source being greatest near the zenith, the instrument is ill adapted to the observations which, in such a case as the present, would have been the most eligible. The altitudes, therefore, observed, were of stars considerably distant from the zenith, and all of them to the south. Indeed, though Baron DE Zach appears to be well pleased with the advantages which both the locality already described, and the instruments that have been enumerated carried with them, we do not think that in either they were remarkable, and if the result, after all, has turned out favourable, it is more to be attributed to the skill, diligence and accuracy of the observer, than to the particular advantages which he enjoyed. In one thing, indeed, we cannot but admire the power which an astronomer derives from the fine climate of Marscilles, compared with that of our island. Dr MașKELYNE, in a residence on the side of Schehalien of four months, could hardly find the means of placing his sector in the meridian ; and, with all that patience or industry could perform, could only make 337 observations. The BARON DE ZACH, in 13 days, on the shores of the Mediterrapcan, was able to make 874 observations. That Greenwich

should afford, nevertheless, a greater number of observations to be completely depended on, than any other observatory in Europe, is a strong instance how, on some occasions, the moral causes can control the physical.

The data necessary in this way to determine the attraction of Mimet, required observations to be made for finding the dif ference of latitude between the two extreme points already mentioned; and this must be done, as has been said, not only by celestial observation, but by terrestrial measurement. For these purposes, a great number of observations were made, which the Baron has given, not only in their original state, but also as reduced and prepared for the final calculation, with a degree of order and correctness altogether exemplary. We have never seen any work of the same kind, where there is more method and order in the arrangement, more accuracy in the detail, and more fairness in striking the mean, where there is any difference among the observations. It is a book, for these reasons, which no one engaged in similar pursuits can study with too much care.

In his discussion concerning the merit of the instruments en:ployed in his observations, a fact occurs concerning the repeating circle, which is certainly of importance; and, to us who are but little acquainted with the nature of that instrument, seems difficult to be explained. It appears, that these circles are subject to certain variations or anomalies, which may extend to three or four seconds, from causes altogether unknown. Our author tells us, that he had formerly remarked, in a letter addressed to the Editors of the Bibliotheque Britannique, that one cannot answer, within three or four seconds, for latitudes inferred from a long series of observations agreeing well with one another, and made with the same repeating circle; for another circle will offer another series of observations, agreeing as well with one another, but differing constantly from the first series by 3" or 4". This remark, when first made by BARON DE ZACH, appears to have drawn upon him a good deal of animadversion, though the fact itself was not disputed.

6

6

They have made it,' says the Baron, a kind of reproach, that I had not pointed out the precise source of these variations. My answer was not ready, but will appear in due time. In the meanwhile, I have the satisfaction to think, that I have awakened and directed the attention of astronomers and of artists, to an important point which requires their attention.'

He goes on to remark, that this defect, from whatever cause it may arise, had no chance of affecting the determination of the attraction of the mountain which was the present object of research. It was not the absolute latitude, either of N. D. des

Anges, or of l'Isle de Planier, that was now required; but it was the

difference between them, which the constant error above described could have no tendency to affect.

The preceding remarks, however, cast a little uncertainty on the determinations made by these most commodious and useful instruments. We have certainly no right to offer any opinion about an anomaly, which those who are best acquainted with the subject seem hitherto at a loss to explain. It has always appeared to us, that the smallness of the telescopes with which instruments held in the hand must be provided, is a considerable defect, and may perhaps have given rise to the inconsistency just mentioned.

The second article in this Treatise, relates to the difference of longitude between the two stations, and contains several remarks of great value to those engaged in similar pursuits. Having regulated his time-keepers at N. D. des Anges, by a series of observations of equal altitudes of the sun, the Baron proposed to determine the difference of time at that point and l'Isle de Planier, by signals made at the one station, and observed at the other. He enumerates, however, before describing these, the various ways in which the problem of ascertaining the difference of longitude of places had been attempted to be resolved. One is, as is well known, the occultation of stars by the moon, which, of all the methods purely astronomical, is certainly to be considered as the best. Yet if the star is small, and if it disappears behind the enlightened part of the lunar disk, it is often lost sight of from the comparative weakness of its light, before it actually touch the limb of the moon; and so also, at the emersion, it has perhaps got to some distance from the moon, before it can be distinguished. It is also remarked, that, in occultations, it sometimes happens that the star, after having touched the luminous disk of the moon, still appears for some seconds upon the disk. At first it seems to advance, and afterwards disappears altogether. This is known to have been experienced by several of the most eminent observers; by CASSINI, DE LA HIRE, FEUILLEE, &c.

Another cause, he adds, which may render such observations defective, is the moon's parallax; in consequence of which, the immersions and emersions of stars are made at different points of the limb for observers placed in different situations. Now, it is certain that the surface of the moon is unequal, and that there are mountains on it which, according to the observations of Messrs HERSCHEL and ScHRÔDER, are not less than 4000 toises in height. With good telescopes, one may see the little asperities which their summits form on the limb of the moon.

'It was thus that, in observing an occultation of a Piscium on the 8th of September 1786, the star appeared to sink in the interval between two of these summits on the moon's limb, and disappeared in the hollow or valley between them. An observer, in another place of the earth, might have seen it hid behind one of these summits; and the two occultations supposed to have been made by the same part of the moon might have differed by several seconds. A singular observation, made by M. KocH, an astronomer of Dantzig, on the 7th of March 1794, on occasion of an occultation of Aldebaran, shows the effect which the mountains in the moon may produce in such cases. The star which just grazed as it were along the limb of the moon, was three times eclipsed by the mountains, before it totally disappeared behind the real disk. The immersion was near the superior horn; the star first disappeared, and in 10" appeared again in all its brightness; after some seconds, it disappeared, and re-appeared a second time. It was soon after concealed for the third time by a mountain, but appeared once more before the real immersion behind the true disk of the moon.'

However excellent, therefore, the method of occultations may be for great distances, it is insufficient for affording the necessary accuracy when the distances are small. An error of 1" or 2", which, on an arch of the meridian of several degrees, might be counted as nothing, would become very considerable for a difference of longitude which was only a few seconds. In such cases, the celestial signals must be abandoned, and we must have recourse to such as we can make ourselves on the surface of the earth. The first person who attempted this was Picard, in determining the difference of meridians between the observatory at Copenhagen, and the ruined observatory of Tycho Brahe in the island of Huena. He kindled a fire on the tower at Copenhagen; but he does not tell by what means he made it disappear suddenly. Other methods have been since followed. A trial of the method of finding the difference of time, by means of signals, was made near London in 1775, with great success. The signals were made by the explosion of rockets in the air, which were thrown up from Loampit hill, near London, where Mr AUBERT, a well-known lover of astronomy, had his observatory. Dr MASKELYNE observed the explosion from the Royal Observatory; Mr WOLLASTON from Chislehurst in Kent; Mr HEBERDEN from Pall-Mall in London; and Mr ELICOTT from Horseley Lane. The longitudes of these five places were thus determined with the greatest precision; the differences at any one place not exceeding a fraction of a second. The greatest distance, however, between these places, was not more than 6 English miles, or 3 French leagues. In the case of greater distances, the same method probably could not be practised with equal

success.

* It seems singular,' the Baron observes, that for making such signals they have not long since had recourse to the most natural and simple expedient, and the most easy of execution withal, that of kindling a small quantity of gunpowder in the open air. This signal is the most visible and the most instantaneous that can be conceived. It is seen at all seasons, and across rain and fog, even by the naked eye. The sudden flash of the gunpowder strikes the eye, though it be not directed precisely to the point from which the light comes, and even when the place from which the signal is made is under the horizon of the observer. It is not only during the night that these signals may be made, but they may be seen in broad day, with telescopes directed to the place where the signal is made, as I have often experienced ; and have, by that means, been relieved from the necessity of passing the night in bevouac, in the open air, as I must otherwise have done. The first use that was made of this method was in the year 1740, by Cassini and La Caille, in measuring two degrees of longitude near Jet, in Languedoc, and Aix in Provence. These two stations are distant about 40 leagues. Towards the middle of that distance they took a station on the sea side, near the mouth of one of the branches of the Rhone. There, from a terrace on the roof of a church, they set fire, evening and morning, to 10 lib. of powder. The flashes were seen distinctly at both extremities of the line; and the difference of longitude concluded accordingly. Cassini proposed to do the same for determining the difference of meridians between Paris and Vienna; but his proposal has never been executed. The quantity of gunpowder which these academicians used, was much too great; and, beside the useless expense, the signals so made were more uncertain and less instantaneous. Even with a single pound of powder, I have observed that the flame lasted for 2" or 3" ; and on that account I have never used above 6 or 8 ounces. In 1803, I made these signals on the Brocken, one of the highest mountains of the Harts, 535 toises above the level of the sea; and the signals were seen at the distance of more than 50 French leagues all round. What is most extraordinary is, that they were seen at the distance of nearly 55 French leagues on the small hill of Keylenberg, not more than 200 toises in height; and from which the Brocken itself could not be seen, on account of the curvature of the earth. The light therefore seen at Keylenberg was nothing but the repercussion of the light of the signal from the clouds, of which it is known that there are many instances.'

These remarks may be very useful to those who are engaged in similar operations, and particularly in the measurement of the degrees of longitude, or for the measurement of arches perpendicular to the meridian.

All this is followed by a table of the observations made by help of these signals, for determining the difference between the time at Notre Dame des Anges, and the Imperial Observatory

1

« FöregåendeFortsätt »