Dr. Young's Refractions, the Barometer being at 30 inches, and the internal Thermometer at 50, or the external at 47, degrees; with the corrections for + one inch in the barometer, and for one degree in the thermometer of Fahren

heit. From page 19 of Vol. 1st of Pearson's Practical Astronomy.

The correction for an increase of altitude of one inch in the barometer, or for a depression of one degree in the thermometer, is to be added to the tabular refraction; but when the barometer is lower than 30 inches, or the thermometer higher than 47 degrees, the correction becomes subtractive.

When great accuracy is required, 0,003 inch should be deducted from the observed height of the barometer, for each degree that the thermometer near it is above 50 degrees, and the same quantity added for an equal depression.

Logarithm for converting Sidereal into Mean Solar Time +9.9988126. Mean Solar into Sidereal Time +0.0011874.

A second of time, at the Equator, contains 1521 feet.

THE OBSERVATORY OF PARIS.

[The following lively and interesting article I have translated, with some abridgment, from a report made by M. Arago to the Chamber of Deputies, and afterwards published in the Annuaire du Bureau des Longitudes for 1844. Those who have taken so much interest in the establishment of American observatories at Cambridge, Washington, and Cincinnati, will learn with pleasure from this sketch, that new zeal for the progress of astronomical discovery seems now to be animating the governments and learned societies of Europe. The history of the Paris Observatory affords valuable hints for the conduct of our own establishments; and the eloquent view of the results to be expected from the use of improved instruments will show our astronomers to what objects their attention must be directed, and what they may hope to accomplish in an atmosphere that is far less frequently obstructed with clouds and mists, than that of England and France.]

WHEN Louis XIV. resolved, at the request of Colbert, soon after the foundation of the Academy of Sciences, to create the Observatory of Paris, there was no national establishment of the kind in all Europe. The astronomers of different countries, left to their own resources, were obliged to use inferior instruments, and to place them in inconvenient positions, usually without firm supports; they could not undertake any regular and systematic labor. The plan for establishing the national observatory was drawn up as early as 1667. The mason-work was commenced in 1668, and the edifice was completed on the 14th of September, 1671. It had cost over $400,000.

After this enormous expense had been incurred, who would not believe that France was provided with an observatory worthy of the country and of the science to which it was dedicated? But it was not so. The architect had drawn up the plan of the building without having sufficiently consulted the observers; they protested against it, but their complaints came too late, or were not heeded. Claude Perrault, though he had not yet built the colonnade of the Louvre, found himself alone more powerful than all the French astronomers united. He obstinately and haughtily rejected arrangements which Colbert himself had acknowledged to be useful; he resisted the great minister, in order, as he said, not to break the architectural lines, or to injure the harmony and regularity of the masses. These idle, æsthetical considerations unluckily triumphed over the foresight and the well-founded remarks of men eminent in science. The same thing, we are told, sometimes happens now-a-days.

But it should be mentioned in excuse of the architect, that the art of observing was then undergoing an actual revolution; that astronomers were not yet all of the same opinion respecting the uncertainty of the measurements of altitudes obtained by means of gnomons; that an opinion from a source already of high authority in Italy had been procured in favor of these instruments, and even recommending the erection in the interior of a colossal sun-dial; so that the great halls which are now useless in the observatory,

and the heaviness, so much censured, of the northern façade, are not to be imputed to the architect alone.

The eastern tower, left uncovered, and the vast room called the Hall of the Meridian, served for setting up or sheltering telescopes, not achromatic, from 50 to 60 feet in length, such as observers used at the end of the seventeenth century, when they wished to study the physical constitution of the planets and their satellites. Excited by the remarkable discoveries with which the science was enriched by means of these great instruments, astronomers and opticians attempted to manufacture them of a still larger size; and they soon succeeded. One of these telescopes had a focal distance of over three hundred feet. As the new edifice could no longer contain or support them, it became necessary to erect masts of a prodigious height in the open air, and even to transport into the garden a colossal tower of wood, used a short time before for the water-works of Versailles. The objectglasses were fixed at the summit of these masts or of the immense tower, while the observer held the eye-glass in his hand; the telescope, confined to these two extreme pieces, had no tube. Difficulties that might have been foreseen caused the failure of these efforts, the most gigantic that are enrolled in the annals of science. It was evident, a priori, that the observer could not with sufficient precision adjust the centres of two lenses thus isolated in space, and not connected by the intervention of any rigid body. With such instruments, also, the impossibility of observing, except for a few minutes before and after the passage of stars over the meridian, was enough to prevent any continuous and prolonged labor.

But the inherent defects of Perrault's edifice became most glaring, when the necessity was perceived of applying instruments in the plane of the meridian, to the observation of stars. Thus, in 1732, no place could be found in the great building for establishing a mural quadrant of six feet radius; in an enclosure covered by roofs entirely closed, and resting upon walls of great thickness and considerable height, no continuous opening could be made in the line of the meridian, through which all the stars, from the horizon to the zenith, might be seen at the moment of their culmination. Thus compelled to give up the great edifice, the Academy caused an out-house to be built, attached to the eastern tower. On a similar occasion, in 1742, a second little building was erected by the side of the former one. About 1760, a little tower with a revolving roof was constructed to the south of the two former buildings, to facilitate the observation of equal altitudes, and thus determine the exact time of the phenomena. These three little out-buildings, erected at a trifling cost and likely to endure but a short time, formed for many years the true, the only royal observatory of Paris. The sumptuous edifice of Perrault, towered majestically over them; but, to use an expression of that day, it was only a parade observatory.

Besides, this great building, like the other monuments of the capital, felt the effects of the carelessness and indolence which characterized the later

years of the reign of Louis XV. In 1770, it was falling into ruin. It was no longer safe to enter the large halls, especially during a thaw; the walls and vaulted ceilings, undermined by the rains, were dropping piece by piece. The incessant petitions of Cassini, strengthened by reports from the Academy, were at last heeded by the minister; and it was decided that the repairs of the edifice should be immediately commenced. As the vicious arrangements of the interior had given much trouble to astronomers, Cassini proposed that the whole upper part of the building should be taken down. But Louis XVI. was unwilling to destroy an edifice erected by his grandfather, the imposing size and severe style of which placed it among the chief architectural ornaments of the metropolis. Its demolition might also have been opposed by the recollection of the brilliant scientific achievements of which it had been the theatre.

It was here, for example, that Picard, giving up the old sight vanes, applied telescopes furnished with micrometers to the graduated instruments, and thus laid a foundation for the exactness of modern observations. Here, also, was doubled, if we may so speak, the duration of an astronomer's life, by the discovery that the stars might be observed in the day-time. In this building, too, Picard and Auzout, using the hair micrometer of their invention, determined the angular diameters of the heavenly bodies, and thus overcame the difficulties with which the genius of Archimedes had striven in vain. Within these walls, now threatened with destruction, were made the trials, the minute preparations that were necessary before undertaking, with any chance of success, the celebrated measurements executed in France, in Peru, and in Lapland, to determine the dimensions and shape of the earth. Richer watched the vibrations of his pendulum here both before and after his voyage to Cayenne, and by these comparisons established the capital fact, that the weight of terrestrial bodies is diminished as they approach the equator. Under this roof, also, Cassini established the laws of the libration of the moon, discovered four of the satellites of Saturn, the rotary movement of these new bodies, and those of the satellites of Jupiter, and the zodiacal light. Finally, in these vast halls, science began to suspect that the transmission of light was not instantaneous; and it was by observing, through the large windows of the Paris observatory, the eclipses of the satellites of Jupiter, that Roemer first determined by approximation the quickness of a ray of light; after a century and a half of further inquiry, the rate has been definitively fixed at 191,000 miles in a second.

In every country where the love of science exists at all, such recollections would have sufficed to save the most defective edifice from destruction. In France they had their full weight; and, though great repairs and improvements in the observatory were made during the last quarter of the last century, the main building preserved all its chief architectural features. In 1831, the Chamber of Deputies, wishing that our national observatory should be made equal to the most celebrated observatories in Europe, voted