OBSERVATORY

, a place destined for observing the heavenly bodies; or a building, usually in form of a tower, erected on some eminence, and covered with a terrace, for making astronomical observations.

Most nations, at almost all times, have had their observatories, either public or private ones, and in various degrees of perfection. A description of a great many of them may be seen in a dissertation of Weidler's, De præsenti Specularum Astronomicarum Statu, printed in 1727, and in different articles of his History of Astronomy, printed in 1741, viz, pa. 86 &c; as also in La Lande's Astronomy, the preface pa. 34. The chief among these are the following:

I. The Greenwich Observatory, or Royal Observatory of England. This was built and endowed in the year 1676, by order of King Charles the 2d, at the instance of Sir Jonas Moore, and Sir Christopher Wren: the former of these gentlemen being Surveyor General of the Ordnance, the office of Astronomer Royal was placed under that department, in which it has continued ever since.

This observatory was at sirst furnished with several very accurate instruments; particularly a noble sextant of 7 feet radius, with telescopic sights. And the sirst Astronomer Royal, or the person to whom the province of observing was sirst committed, was Mr. John Flamsteed; a man who, as Dr. Halley expresses it, seemed born for the employment. During 14 years he watched the motions of the planets with unwearied diligence, especially those of the moon, as was given him in charge; that a new theory of that planet being found, shewing all her irregularities, the longitude might thence be determined.

In the year 1690, having provided himself with a mural arch of near 7 feet radius, made by his Assistant Mr. Abraham Sharp, and fixed in the plane of the meridian, he began to verify his catalogue of the fixed stars, which had hitherto depended altogether on the distances measured with the sextant, after a new and very different manner, viz, by taking the meridian altitudes, and the moments of culmination, or in other words the right ascension and declination. And he was so well pleased with this instrument, that he discontinued almost entirely the use of the sextant.

Thus, in the space of upwards of 40 years, the Astronomer Royal collected an immense number of good observations; which may be found in his Historia Cœlestis Britannica, published in 1725; the principal part of which is the Britannic catalogue of the fixed stars.

Mr. Flamsteed, on his death in 1719, was succeeded by Dr. Halley, and he by Dr. Bradley in 1742, and this last by Mr. Bliss in 1762; but none of the observations of these gentlemen have yet been given to the public.

On the demise of Mr. Bliss, in 1765, he was succeeded by Dr. Nevil Maskelyne, the present worthy astronomer royal, whose valuable observations have been published, from time to time, under the direction of the Royal Society, in several folio volumes.

The Greenwich Observatory is found, by very accurate observations, to lie in 51° 28′ 40″ north latitude, as settled by Dr. Maskelyne, from many of his own observations, as well as those of Dr. Bradley.

II. The Paris Observatory was built by Louis the 14th, in the fauxbourg St. Jaques, being begun in 1664, and finished in 1672. It is a singular but magnificent building, of 80 feet in height, with a terrace at top; and here M. De la Hire, M. Cassini, &c, the king's astronomers, have made their observations. Its latitude is 48° 50′ 14″ north, and its longitude 9′ 20″ east of Greenwich Observatory.

In the Observatory of Paris is a cave, or pit, 170 feet deep, with subterraneous passages, for experiments that are to be made out of the reach of the sun, especially such as relate to congelations, refrigerations, &c. In this cave there is an old thermometer of M. De la Hire, which stands always at the same height; thereby| shewing that the temperature of the place remains always the same. From the top of the platform to the bottom of the cave is a perpendicular well or pit, used formerly for experiments on the fall of bodies; being also a kind of long telescopical tube, through which the stars are seen at mid-day.

III. Tycho Brahe's Observatory was in the little island Ween, or the Scarlet Island, between the coasts of Schonen and Zealand, in the Baltic sea. This Observatory was not well situated for some kinds of observations, particularly the risings and settings; as it lay too low, and was landlocked on all the points of the compass except three; and the land horizon being very rugged and uneven.

IV. Pekin Observatory. Father Le Compte describes a very magnificent Observatory, erected and furnished by the late emperor of China, in his capital, at the intercession of some Jesuit missionaries, chiefly father Verbiest, whom he appointed his chief observer. The instruments here are exceeding large; but the divisions are less accurate, and in some respects the contrivance is less commodious than in those of the Europeans. The chief are, an armillary zodiacal sphere, of 6 Paris feet diameter, an azimuthal horizon 6 feet diameter, a large quadrant 6 feet radius, a sextant 8 feet radius, and a celestial globe 6 feet diameter.

V. Bramins' Observatory at Benares, in the East Indies, which is still one of the principal seminaries of the Bramins or priests of the original Gentoos of Hindostan. This Observatory at Benares it is said was built about 200 years since, by order of the emperor Ackbar: for as this wise prince endeavoured to improve the arts, so he wished also to recover the sciences of Hindostan, and therefore ordered that three such places should be erected; one at Delhi, another at Agra, and the third at Benares.

Wanting the use of optical glasses, to magnify very distant or very small objects, these people directed their attention to the increasing the size of their instruments, for obtaining the greater accuracy and number of the divisions and subdivisions in their instruments. Accordingly, the Observatory contains several huge instruments, of stone, very nicely erected and divided, consisting of circles, columns, gnomons, dials, quadrants, &c, some of them of 20 feet radius, the circle divided first into 360 equal parts, and sometimes each of these into 20 other equal parts, each answering to 3′, and of about two-tenths of an inch in extent. And although these wonderful instruments have been built upwards of 200 years, the graduations and divisions on the several arcs appear as well cut, and as accurately divided, as if they had been the performance of a modern artist. The execution, in the construction of these instruments, exhibits an extraordinary mathematical exactness in the fixing, bearing, sitting of the several parts, in the necessary and sufficient supports to the very large stones that compose them, and in the joining and fastening them into each other by means of lead and iron.

See a farther description, and drawing, of this Observatory, by Sir Robert Barker, in the Philos. Trans. vol. 67, pa. 598.

Observatory Portable. See Equatorial.

OBTUSE Angle, one that is greater than a right- angle.

Obtuse-angled Triangle, is a triangle that has one of its angles Obtuse: and it can have only one such.

Obtuse Cone, or Obtuse-Angled Cone, one whose angle at the vertex, by a section through the axis, is Obtuse.

Obtuse Hyperbola, one whose asymptotes form an Obtuse angle.

Obtuse-angular Section of a Cone, a name given to the hyperbola by the ancient geometricians, because they considered this section only in the Obtuse cone.